Mount Agung, located on the E end of the island of Bali, Indonesia, rises above the SE rim of the Batur caldera. The summit area extends 1.5 km E-W, with the highest point on the W and a steep-walled 800-m-wide crater on the E. Recorded eruptions date back to the early 19th century. A large and deadly explosive and effusive eruption occurred during 1963-64, which was characterized by voluminous ashfall, pyroclastic flows, and lahars that caused extensive damage and many fatalities. More recent activity was documented during November 2017-June 2019 that consisted of multiple explosions, significant ash plumes, lava flows at the summit crater, and incandescent ejecta. This report covers activity reported during April-May 2022 and December 2022 based on data from the Darwin Volcanic Ash Advisory Center (VAAC).

Activity during 2022 was relatively low and mainly consisted of a few ash plumes during April-May and December. An ash plume on 3 April rising to 3.7 km altitude (700 m above the summit) and drifting N was reported in a Darwin VAAC notice based on a ground report, with ash seen in HIMAWARI-8 visible imagery. Another ash plume was reported at 1120 on 27 May that rose to 5.5 km altitude (2.5 m above the summit); the plume was not visible in satellite or webcam images due to weather clouds. An eruption was reported based on seismic data at 0840 on 13 December, with an estimated plume altitude of 3.7 km; however, no ash was seen using satellite imagery in clear conditions before weather clouds obscured the summit.

Geologic Background. Symmetrical Agung stratovolcano, Bali's highest and most sacred mountain, towers over the eastern end of the island. The volcano, whose name means "Paramount," rises above the SE rim of the Batur caldera, and the northern and southern flanks extend to the coast. The summit area extends 1.5 km E-W, with the high point on the W and a steep-walled 800-m-wide crater on the E. The Pawon cone is located low on the SE flank. Only a few eruptions dating back to the early 19th century have been recorded in historical time. The 1963-64 eruption, one of the largest in the 20th century, produced voluminous ashfall along with devastating pyroclastic flows and lahars that caused extensive damage and many fatalities.

Information Contacts: Darwin Volcanic Ash Advisory Centre (VAAC), Bureau of Meteorology, Northern Territory Regional Office, PO Box 40050, Casuarina, NT 0811, Australia (URL: http://www.bom.gov.au/info/vaac/).

Tengger Caldera, located at the N end of a volcanic massif in Indonesia’s East Java, consists of five overlapping stratovolcanoes. The youngest and only active cone in the 16-km-wide caldera is Bromo, which typically produces gas-and-steam plumes, occasional ash plumes and explosions, and weak thermal signals (BGVN 44:05, 47:01). This report covers activity during January 2022-December 2023, consisting of mostly white gas-and-steam emissions and persistent weak thermal anomalies. Information was provided by the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and satellite imagery. The Alert Level remained at 2 (on a scale of 1-4), and visitors were warned to stay at least 1 km from the crater.

Activity was generally low during the reporting period, similar to that in 2021. According to almost daily images from MAGMA Indonesia (a platform developed by PVMBG), white emissions and plumes rose from 50 to 900 m above the main crater during this period (figure 24). During several days in March and June 2022, white plumes reached heights of 1-1.2 km above the crater.

Figure 24. Webcam image showing a gas-and-steam plume from the Bromo cone in the Tengger Caldera on 2 April 2023. Courtesy of MAGMA Indonesia.

After an increase in activity at 2114 on 3 February 2023, a PVMBG team that was sent to observe white emissions rising as high as 300 m during 9-12 February and heard rumbling noises. A sulfur dioxide odor was also strong near the crater and measurements indicated that levels were above the healthy (non-hazardous) threshold of 5 parts per million; differential optical absorption spectroscopy (DOAS) measurements indicated an average flux of 190 metric tons per day on 11 February. Incandescence originating from a large fumarole in the NNW part of the crater was visible at night. The team observed that vegetation on the E caldera wall was yellow and withered. The seismic network recorded continuous tremor and deep and shallow volcanic earthquakes.

According to a PVMBG press release, activity increased on 13 December 2023 with white, gray, and brown emissions rising as high as 900 m above Bromo’s crater rim and drifting in multiple directions (figure 25). The report noted that tremor was continuous and was accompanied in December by three volcanic earthquakes. Deformation data indicated inflation in December. There was no observable difference in the persistent thermal anomaly in the crater between 11 and 16 December 2023.

Figure 25. Webcam image showing a dark plume that rose 900 m above the summit of the Bromo cone in the Tengger Caldera on 13 December 2023. Courtesy of MAGMA Indonesia.

All clear views of the Bromo crater throughout this time, using Sentinel-2 infrared satellite images, showed a weak persistent thermal anomaly; none of the anomalies were strong enough to cause MODVOLC Thermal Alerts. A fire in the SE part of the caldera in early September 2023 resulted in a brief period of strong thermal anomalies.

Geologic Background. The 16-km-wide Tengger caldera is located at the northern end of a volcanic massif extending from Semeru volcano. The massive volcanic complex dates back to about 820,000 years ago and consists of five overlapping stratovolcanoes, each truncated by a caldera. Lava domes, pyroclastic cones, and a maar occupy the flanks of the massif. The Ngadisari caldera at the NE end of the complex formed about 150,000 years ago and is now drained through the Sapikerep valley. The most recent of the calderas is the 9 x 10 km wide Sandsea caldera at the SW end of the complex, which formed incrementally during the late Pleistocene and early Holocene. An overlapping cluster of post-caldera cones was constructed on the floor of the Sandsea caldera within the past several thousand years. The youngest of these is Bromo, one of Java's most active and most frequently visited volcanoes.

Information Contacts: Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Jalan Diponegoro 57, Bandung 40122, Indonesia (URL: http://www.vsi.esdm.go.id/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.esdm.go.id/v1); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/).

Saunders is one of eleven islands that comprise the South Sandwich Islands in the South Atlantic. The active Mount Michael volcano has been in almost continuous eruption since November 2014 (BGVN 48:02). Recent activity has resulted in intermittent thermal anomalies and gas-and-steam emissions (BGVN 47:03, 48:02). Visits are infrequent due to its remote location, and cloud cover often prevents satellite observations. Satellite thermal imagery and visual observation of incandescence during a research expedition in 2019 (BGVN 28:02 and 44:08) and a finding confirmed by a National Geographic Society research team that summited Michael in November 2022 reported the presence of a lava lake.

Although nearly constant cloud cover during February 2023 through January 2024 greatly limited satellite observations, thermal anomalies from the lava lake in the summit crater were detected on clear days, especially around 20-23 August 2023. Anomalies similar to previous years (eg. BGVN 48:02) were seen in both MIROVA (Middle InfraRed Observation of Volcanic Activity) data from MODIS instruments and in Sentinel 2 infrared imagery. The only notable sulfur dioxide plume detected near Saunders was on 25 September 2023, with the TROPOMI instrument aboard the Sentinel-5P satellite.

Geologic Background. Saunders Island consists of a large central volcanic edifice intersected by two seamount chains, as shown by bathymetric mapping (Leat et al., 2013). The young Mount Michael stratovolcano dominates the glacier-covered island, while two submarine plateaus, Harpers Bank and Saunders Bank, extend north. The symmetrical Michael has a 500-m-wide summit crater and a remnant of a somma rim to the SE. Tephra layers visible in ice cliffs surrounding the island are evidence of recent eruptions. Ash clouds were reported from the summit crater in 1819, and an effusive eruption was inferred to have occurred from a N-flank fissure around the end of the 19th century and beginning of the 20th century. A low ice-free lava platform, Blackstone Plain, is located on the north coast, surrounding a group of former sea stacks. A cluster of cones on the SE flank, the Ashen Hills, appear to have been modified since 1820 (LeMasurier and Thomson, 1990). Analysis of satellite imagery available since 1989 (Gray et al., 2019; MODVOLC) suggests frequent eruptive activity (when weather conditions allow), volcanic clouds, steam plumes, and thermal anomalies indicative of a persistent, or at least frequently active, lava lake in the summit crater. Due to this observational bias, there has been a presumption when defining eruptive periods that activity has been ongoing unless there is no evidence for at least 10 months.

Information Contacts: MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser (URL: https://dataspace.copernicus.eu/browser).

Shishaldin is located on the eastern half of Unimak Island, one of the Aleutian Islands. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. The previous eruption ended in May 2020 and was characterized by intermittent thermal activity, increased seismicity and surface temperatures, ash plumes, and ash deposits (BGVN 45:06). This report covers a new eruption during July through November 2023, which consisted of significant explosions, ash plumes, ashfall, and lava fountaining. Information comes from daily, weekly, and special reports from the Alaska Volcano Observatory (AVO) and various satellite data. AVO monitors the volcano using local seismic and infrasound sensors, satellite data, web cameras, and remote infrasound and lightning networks.

AVO reported that intermittent tremor and low-frequency earthquakes had gradually become more regular and consistent during 10-13 July. Strongly elevated surface temperatures at the summit were identified in satellite images during 10-13 July. On 11 July AVO raised the Aviation Color Code (ACC) to Yellow (the second color on a four-color scale) and Volcano Alert Level (VAL) to Advisory (the second level on a four-level scale) at 1439. Later in the day on 11 July summit crater incandescence was observed in webcam images. Observations of the summit suggested that lava was likely present at the crater, which prompted AVO to raise the ACC to Orange (the second highest color on a four-color scale) and the VAL to Watch (the second highest level on a four-level scale). The US Coast Guard conducted an overflight on 12 July and confirmed that lava was erupting from the summit. That same day, sulfur dioxide emissions were detected in satellite images.

A significant explosion began at 0109 on 14 July that produced an ash plume that rose to 9-12 km altitude and drifted S over the Pacific Ocean (figure 43). Webcam images and photos taken around 0700 from a ship SW off Unimak Island showed small lahar deposits, which were the result of the interaction of hot pyroclastic material and snow and ice on the flanks. There was also ashfall on the SW and N flanks. A smaller explosion at 0710 generated an ash plume that rose to 4.5 km altitude. Webcam images and pilot reports showed continued low-level ash emissions during the morning, rising to less than 4.6 km altitude; those emissions included a small ash plume near the summit around 1030 resulting from a small explosion.

Figure 43. Photo of a strong ash plume that rose to 9-12 km altitude on the morning of 14 July 2023. Lahar deposits were visible on the SW flank (white arrows). Photo has been color corrected. Courtesy of Christopher Waythomas, AVO.

Seismic tremor amplitude began increasing at around 1700 on 15 July; strongly elevated surface temperatures were also reported. An ash plume rose to 4.6 km altitude and drifted SSE at 2100, based on a satellite image. A continuous ash plume during 2150 through 2330 rose to 5 km altitude and extended 125 km S. At 2357 AVO raised the ACC to Red (the highest color on a four-color scale) and the VAL to Warning (the highest level on a four-level scale), noting that seismicity remained elevated for more than six hours and explosion signals were frequently detected by regional infrasound (pressure sensor) networks. Explosions generated an ash plume that rose to 4.9 km altitude and drifted as far as 500 km SE. Activity throughout the night declined and by 0735 the ACC was lowered to Orange and the VAL to Watch. High-resolution satellite images taken on 16 July showed pyroclastic deposits extending as far as 3 km from the vent; these deposits generated lahars that extended further down the drainages on the flanks. Ash deposits were mainly observed on the SSE flank and extended to the shore of Unimak Island. During 16-17 July lava continued to erupt at the summit, which caused strongly elevated surface temperatures that were visible in satellite imagery.

Lava effusion increased at 0100 on 18 July, as noted in elevated surface temperatures identified in satellite data, increasing seismic tremor, and activity detected on regional infrasound arrays. A significant ash plume at 0700 rose to 7 km altitude and continued until 0830, eventually reaching 9.1 km altitude and drifting SSE (figure 44). As a result, the ACC was raised to Red and the VAL to Warning. By 0930 the main plume detached, but residual low-level ash emissions continued for several hours, remaining below 3 km altitude and drifting S. The eruption gradually declined and by 1208 the ACC was lowered to Orange and the VAL was lowered to Watch. High-resolution satellite images showed ash deposits on the SW flank and pyroclastic deposits on the N, E, and S flanks, extending as far as 3 km from the vent; lahars triggered by the eruption extended farther down the flanks (figure 45). Lava continued to erupt from the summit crater on 19 July.

Figure 44. Photo of an ash-rich plume rising above Shishaldin to 9.1 km altitude on 18 July 2023 that drifted SE. View is from the N of the volcano and Isanotski volcano is visible on the left-hand side of the image. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Figure 45. Near-infrared false-color satellite image of Shishaldin taken on 18 July 2023 showing ash deposits on the N, E, and S flanks extending as far as 3 km from the vent due to recent eruption events. Courtesy of Matthew Loewen, AVO.

Elevated surface temperatures were detected in satellite images during 19-25 July, despite occasional weather cloud cover, which was consistent with increased lava effusion. During 22-23 July satellite observations acquired after the eruption from 18 July showed pyroclastic flow and lahar deposits extending as far as 3 km down the N, NW, and NE flanks and as far as 1.5 km down the S and SE flanks. Ash deposits covered the SW and NE flanks. No lava flows were observed outside the crater. On 22 July a sulfur dioxide plume was detected in satellite data midday that had an estimated mass of 10 kt. In a special notice issued at 1653 on 22 July AVO noted that eruptive activity had intensified over the previous six hours, which was characterized by an hours-long steady increase in seismic tremor, intermittent infrasound signals consistent with small explosions, and an increase in surface temperatures that were visible in satellite data. Pilots first reported low-level ash plumes at around 1900. At 2320 an ash plume had risen to 9 km altitude based on additional pilot reports and satellite images. The ACC was increased to Red and the VAL to Warning at 2343. Satellite images indicated growth of a significantly higher ash plume that rose to 11 km altitude continued until 0030 and drifted NE. During the early morning hours of 23 July ash plumes had declined to 4.6 k altitude. Seismic tremor peaked at 0030 on 23 July and began to rapidly decline at 0109; active ash emissions were no longer visible in satellite data by 0130. The ACC was lowered to Orange and the VAL to Watch at 0418; bursts of increased seismicity were recorded throughout the morning, but seismicity generally remained at low levels. Elevated surface temperatures were visible in satellite data until about 0600. On 24 July pilots reported seeing vigorous gas-and-steam plumes rising to about 3 km altitude; the plumes may have contained minor amounts of ash.

During 24-25 July low level seismicity and volcanic tremor were detected at low levels following the previous explosion on 23 July. Strongly elevated surface temperatures were observed at the summit crater in satellite data. Around 2200 on 25 July seismicity began to increase, followed by infrasound signals of explosions after 0200 on 26 July. An ash plume rose to 3 km altitude at 0500 and drifted ENE, along with an associated sulfur dioxide plume that drifted NE and had an estimated mass of 22 kt. Diffuse ash emissions were visible in satellite data and rose to 6.1-7.6 km altitude and extended 125 km from the volcano starting around 1130. These ash events were preceded by about seven hours of seismic tremor, infrasound detections of explosions, and five hours of increased surface temperatures visible in satellite data. Activity began to decline around 1327, which included low-frequency earthquakes and decreased volcanic tremor, and infrasound data no longer detected significant explosions. Surface temperatures remained elevated through the end of the month.

Seismicity, volcanic tremor, and ash emissions remained at low levels during early August. Satellite images on 1 August showed that some slumping had occurred on the E crater wall due to the recent explosive activity. Elevated surface temperatures continued, which was consistent with cooling lava. On 2 August small explosive events were detected, consistent with low-level Strombolian activity. Some episodes of volcanic tremor were reported, which reflected low-level ash emissions. Those ash emissions rose to less than 3 km altitude and drifted as far as 92.6 km N. Pilots that were located N of the volcano observed an ash plume that rose to 2.7 km altitude. Seismicity began to increase in intensity around 0900 on 3 August. Seismicity continued to increase throughout the day and through the night with strongly elevated surface temperatures, which suggested that lava was active at the surface.

An ash cloud that rose to 7.6-7.9 km altitude and drifted 60-75 km NE was visible in a satellite image at 0520 on 4 August. Pilots saw and reported the plume at 0836 (figure 46). By 0900 the plume had risen to 9.1 km altitude and extended over 100 km NE. AVO raised the ACC to Red and the VAL to Warning as a result. Seismic tremor levels peaked at 1400 and then sharply declined at 1500 to slightly elevated levels; the plume was sustained during the period of high tremor and drifted N and NE. The ACC was lowered to Orange and the VAL to Watch at 2055. During 5-14 August seismicity remained low and surface temperatures were elevated based on satellite data due to cooling lava. On 9 August a small lava flow was observed that extended from the crater rim to the upper NE flank. It had advanced to 55 m in length and appeared in satellite imagery on 11 August. Occasional gas-and-steam plumes were noted in webcam images. At 1827 AVO noted that seismic tremor had steadily increased during the afternoon and erupting lava was visible at the summit in satellite images.

Figure 46. Photo showing an ash plume rising above Shishaldin during the morning of 4 August 2023 taken by a passing aircraft. The view is from the N showing a higher gas-rich plume and a lower gray ash-rich plume and dark tephra deposits on the volcano’s flank. Photo has been color corrected. Courtesy of Chris Barnes, AVO.

Strong explosion signals were detected at 0200 on 15 August. An ash cloud that was visible in satellite data extended 100 km NE and may have risen as high as 11 km altitude around 0240. By 0335 satellite images showed the ash cloud rising to 7.6 km altitude and drifting NE. Significant seismicity and explosions were detected by the local AVO seismic and infrasound networks, and volcanic lightning was detected by the World Wide Lightning Location Network (WWLLN). A sulfur dioxide plume associated with the eruption drifted over the S Bering Sea and parts of Alaska and western Canada. Seismicity was significantly elevated during the eruption but had declined by 1322. A pilot reported that ash emissions continued, rising as high as 4.9 km altitude. Elevated surface temperatures detected in satellite data were caused by hot, eruptive material (pyroclastic debris and lava) that accumulated around the summit. Eruptive activity declined by 16 August and the associated sulfur dioxide plume had mostly dissipated; remnants continued to be identified in satellite images at least through 18 August. Surface temperatures remained elevated based on satellite images, indicating hot material on the upper parts of the volcano. Small explosions were detected in infrasound data on the morning of 19 August and were consistent with pilot reports of small, short-lived ash plumes that rose to about 4.3 km altitude. Low-level explosive activity was reported during 20-24 August, according to seismic and infrasound data, and weather clouds sometimes prevented views. Elevated surface temperatures were observed in satellite images, which indicated continued hot material on the upper parts of the volcano.

Seismic tremor began to increase at around 0300 on 25 August and was followed by elevated surface temperatures identified in satellite images, consistent with erupting lava. Small explosions were recorded in infrasound data. The ACC was raised to Red and the VAL to Warning at 1204 after a pilot reported an ash plume that rose to 9.1 km altitude. Seismicity peaked at 1630 and began to rapidly decline at around 1730. Ash plumes rose as high as 10 km altitude and drifted as far as 400 km NE. By 2020 the ash plumes had declined to 6.4 km altitude and continued to drift NE. Ash emissions were visible in satellite data until 0000 on 26 August and seismicity was at low levels. AVO lowered the ACC to Orange and the VAL to Watch at 0030. Minor explosive activity within the summit crater was detected during 26-28 August and strongly elevated surface temperatures were still visible in satellite imagery through the rest of the month. An AVO field crew working on Unimak Island observed a mass flow that descended the upper flanks beginning around 1720 on 27 August. The flow produced a short-lived ash cloud that rose to 4.5 km altitude and rapidly dissipated. The mass flow was likely caused by the collapse of spatter that accumulated on the summit crater rim.

Similar variable explosive activity was reported in September, although weather observations sometimes prevented observations. A moderate resolution satellite image from the afternoon of 1 September showed gas-and-steam emissions filling the summit crater and obscuring views of the vent. In addition, hot deposits from the previous 25-26 August explosive event were visible on the NE flank near the summit, based on a 1 September satellite image. On 2 and 4 September seismic and infrasound data showed signals of small, repetitive explosions. Variable gas-and-steam emissions from the summit were visible but there was no evidence of ash. Possible summit crater incandescence was visible in nighttime webcam images during 3-4 September.

Seismicity began to gradually increase at around 0300 on 5 September and activity escalated at around 0830. A pilot reported an ash plume that rose to 7.6 km altitude at 0842 and continued to rise as high as possibly 9.7 km altitude and drifted SSE based on satellite images (figure 47). The ACC was raised to Red and the VAL to Warning at 0900. In addition to strong tremor and sustained explosions, the eruption produced volcanic lightning that was detected by the WWLLN. Around 1100 seismicity decreased and satellite data confirmed that the altitude of the ash emissions had declined to 7.6 km altitude. By 1200 the lower-altitude portion of the ash plume had drifted 125 km E. Significant ash emissions ended by 1330 based on webcam images. The ACC was lowered to Orange and the VAL to Watch at 1440. Satellite images showed extensive pyroclastic debris flows on most of the flanks that extended 1.2-3.3 km from the crater rim.

Figure 47. Webcam image taken from the S of Shishaldin showing a vertical ash plume on 5 September 2023. Courtesy of AVO.

During 6-13 September elevated surface temperatures continued to be observed in satellite data, seismicity remained elevated with weak but steady tremor, and small, low-frequency earthquakes and small explosions were reported, except on 12 September. On 6 September a low-level ash plume rose to 1.5-1.8 km altitude and drifted SSE. Occasional small and diffuse gas-and-steam emissions at the summit were visible in webcam images. Around 1800 on 13 September seismic tremor amplitudes began to increase, and small explosions were detected in seismic and infrasound data. Incandescent lava at the summit was seen in a webcam image taken at 0134 on 14 September during a period of elevated tremor. No ash emissions were reported during the period of elevated seismicity. Lava fountaining began around 0200, based on webcam images. Satellite-based radar observations showed that the lava fountaining activity led to the growth of a cone in the summit crater, which refilled most of the crater. By 0730 seismicity significantly declined and remained at low levels.

Seismic tremor began to increase around 0900 on 15 September and rapidly intensified. An explosive eruption began at around 1710, which prompted AVO to raise the ACC to Red and the VAL to Warning. Within about 30 minutes ash plumes drifted E below a weather cloud at 8.2 km altitude. The National Weather Service estimated that an ash-rich plume rose as high as 12.8 km altitude and produced volcanic lightning. The upper part of the ash plume detached from the vent around 1830 and drifted E, and was observed over the Gulf of Alaska. Around the same time, seismicity dramatically decreased. Trace ashfall was reported in the community of False Pass (38 km ENE) between 1800-2030 and also in King Cove and nearby marine waters. Activity declined at around 1830 although seismicity remained elevated, ash emissions, and ashfall continued until 2100. Lightning was again detected beginning around 1930, which suggested that ash emissions continued. Ongoing explosions were detected in infrasound data, at a lower level than during the most energetic phase of this event. Lightning was last detected at 2048. By 2124 the intensity of the eruption had decreased, and ash emissions were likely rising to less than 6.7 km altitude. Seismicity returned to pre-eruption levels. On 16 September the ACC was lowered to Orange and the VAL to Watch at 1244; the sulfur dioxide plume that was emitted from the previous eruption event was still visible over the northern Pacific Ocean. Elevated surface temperatures, gas-and-steam emissions from the vent, and new, small lahars were reported on the upper flanks based on satellite and webcam images. Minor deposits were reported on the flanks which were likely the result of collapse of previously accumulated lava near the summit crater.

Elevated seismicity with tremor, small earthquakes, and elevated surface temperatures were detected during 17-23 September. Minor gas-and-steam emissions were visible in webcam images. On 20 September small volcanic debris flows were reported on the upper flanks. On 21 September a small ash deposit was observed on the upper flanks extending to the NE based on webcam images. Seismic tremor increased significantly during 22-23 September. Regional infrasound sensors suggested that low-level eruptive activity was occurring within the summit crater by around 1800 on 23 September. Even though seismicity was at high levels, strongly elevated surface temperatures indicating lava at the surface were absent and no ash emissions were detected; weather clouds at 0.6-4.6 km altitude obscured views. At 0025 on 24 September AVO noted that seismicity continued at high levels and nearly continuous small infrasound signals began, likely from low-level eruptive activity. Strongly elevated surface temperatures were identified in satellite images by 0900 and persisted throughout the day; the higher temperatures along with infrasound and seismic data were consistent with lava erupting at the summit. Around 1700 similarly elevated surface temperatures were detected from the summit in satellite data, which suggested that more vigorous lava fountaining had started. Starting around 1800 low-level ash emissions rose to altitudes less than 4.6 km altitude and quickly dissipated.

Beginning at midnight on 25 September, a series of seismic signals consistent with volcanic flows were recorded on the N side of the volcano. A change in seismicity and infrasound signals occurred around 0535 and at 0540 a significant ash cloud formed and quickly reached 14 km altitude and drifted E along the Alaska Peninsula. The cloud generated at least 150 lightning strokes with thunder that could be heard by people in False Pass. Seismicity rapidly declined to near background levels around 0600. AVO increased the ACC to Red and the VAL to Warning at 0602. The ash cloud detached from the volcano at around 0700, rose to 11.6 km altitude, and drifted ESE. Trace to minor amounts of ashfall were reported by the communities of False Pass, King Cove, Cold Bay, and Sand Point around 0700. Ash emissions continued at lower altitudes of 6-7.6 km altitude at 0820. Small explosions at the vent area continued to be detected in infrasound data and likely represented low-level eruptive activity near the vent. Due to the significant decrease in seismicity and ash emissions the ACC was lowered to Orange and the VAL to Watch at 1234. Radar data showed significant collapses of the crater that occurred on 25 September. Satellite data also showed significant hot, degassing pyroclastic and lahar deposits on all flanks, including more extensive flows on the ENE and WSW sections below two new collapse scarps. Following the significant activity during 24-25 September, only low-level activity was observed. Seismicity decreased notably near the end of the strong activity on 25 September and continued to decrease through the end of the month, though tremor and small earthquakes were still reported. No explosive activity was detected in infrasound data through 2 October. Gas-and-steam emissions rose to 3.7 km altitude, as reported by pilots and seen in satellite images. Satellite data from 26 September showed that significant collapses had occurred at the summit crater and hot, steaming deposits from pyroclastic flows and lahars were present on all the flanks, particularly to the ENE and WSW. A small ash cloud was visible in webcam images on 27 September, likely from a collapse at the summit cone. High elevated surface temperatures were observed in satellite imagery during 27-28 September, which were likely the result of hot deposits on the flanks erupted on 25 September. Minor steaming at the summit crater and from an area on the upper flanks was visible in webcam images on 28 September.

During October, explosion events continued between periods of low activity. Seismicity significantly increased starting at around 2100 on 2 October; around the same time satellite images showed an increase in surface temperatures consistent with lava fountaining. Small, hot avalanches of rock and lava descended an unspecified flank. In addition, a distinct increase in infrasound, seismicity, and lightning detections was followed by an ash plume that rose to 12.2 km altitude and drifted S and E at 0520 on 3 October, based on satellite images. Nighttime webcam images showed incandescence due to lava fountaining at the summit and pyroclastic flows descending the NE flank. AVO reported that a notable explosive eruption started at 0547 and lasted until 0900 on 3 October, which prompted a rise in the ACC to Red and the VAL to Warning. Subsequent ash plumes rose to 6-7.6 km altitude by 0931. At 1036 the ACC was lowered back to Orange and the VAL to Watch since both seismic and infrasound data quieted substantially and were slightly above background levels. Gas-and-steam emissions were observed at the summit, based on webcam images. Trace amounts of ashfall were observed in Cold Bay. Resuspended ash was present at several kilometers altitude near the volcano. During the afternoon, low-level ash plumes were visible at the flanks, which appeared to be largely generated by rock avalanches off the summit crater following the explosive activity. These ash plumes rose to 3 km altitude and drifted W. Trace amounts of ashfall were reported by observers in Cold Bay and Unalaska and flights to these communities were disrupted by the ash cloud. Satellite images taken after the eruption showed evidence of pyroclastic flows and lahar deposits in drainages 2 km down the SW flank and about 3.2 km down the NE flank, and continued erosion of the crater rim. Small explosion craters at the end of the pyroclastic flows on the NE flank were noted for the first time, which may have resulted from gas-and-steam explosions when hot deposits interact with underlying ice.

During 4 October seismicity, including frequent small earthquakes, remained elevated, but was gradually declining. Ash plumes were produced for over eight hours until around 1400 that rose to below 3.7 km altitude. These ash plumes were primarily generated off the sides of the volcano where hot rock avalanches from the crater rim had entered drainages to the SW and NE. Two explosion craters were observed at the base of the NE deposits about 3.2 km from the crater rim. Webcam images showed the explosion craters were a source of persistent ash emissions; occasional collapse events also generated ash. Seismicity remained elevated with sulfur dioxide emissions that had a daily average of more than 1,000 tons per day, and frequent small earthquakes through the end of the month. Frequent elevated surface temperatures were identified in satellite images and gas-and-steam plumes were observed in webcam images, although weather conditions occasionally prevented clear views of the summit. Emissions were robust during 14-16 October and were likely generated by the interaction of hot material and snow and ice. During the afternoon of 21 October a strong gas-and-steam plume rose to 3-4.6 km altitude and extended 40 km WSW, based on satellite images and reports from pilots. On 31 October the ACC was lowered to Yellow and the VAL was lowered to Advisory.

Activity in November was characterized by elevated seismicity with ongoing seismic tremor and small, low-frequency earthquakes, elevated surface temperatures, and gas-and-steam emissions. There was an increase in seismic and infrasound tremor amplitudes starting at 1940 on 2 November. As a result, the ACC was again raised to Orange and the VAL was increased to Watch, although ash was not identified in satellite data. An ash cloud rose to 6.1 km altitude and drifted W according to satellite data at 2000. By 0831 on 3 November ash emissions were no longer visible in satellite images. On 6 and 9 November air pressure sensors detected signals consistent with small explosions. Small explosions were detected in infrasound data consistent with weak Strombolian activity on 19 and 21 November. Seismicity started to decrease on 21 November. On 25 November gas-and-steam emissions were emitted from the vent as well as from a scarp on the NE side of the volcano near the summit. A gas-and-steam plume extended about 50 km SSE and was observed in satellite and webcam images on 26 November. On 28 November small explosions were observed in seismic and local infrasound data and gas-and-steam emissions were visible from the summit and from the upper NE collapse scarp based on webcam images. Possible small explosions were observed in infrasound data on 30 November. Weakly elevated surface temperatures and a persistent gas-and-steam plume from the summit and collapse scarps on the upper flanks. A passing aircraft reported the gas-and-steam plume rose to 3-3.4 km altitude on 30 November, but no significant ash emissions were detected.

Satellite data. MODIS thermal anomaly data provided through MIROVA (Middle InfraRed Observation of Volcanic Activity) showed a strong pulse of thermal activity beginning in July 2023 that continued through November 2023 (figure 48). This strong activity was due to Strombolian explosions and lava fountaining events at the summit crater. According to data from MODVOLC thermal alerts, a total of 101 hotspots were detected near the summit crater in July (11-14, 16-19, 23-24 and 26), August (4, 25-26, and 29), September (5, 12, and 17), and October (3, 4, and 8). Infrared satellite data showed large lava flows descending primarily the northern and SE flanks during the reporting period (figure 49). Sulfur dioxide plumes often exceeded two Dobson Units (DUs) and drifted in different directions throughout the reporting period, based on satellite data from the TROPOMI instrument on the Sentinel-5P satellite (figure 50).

Figure 48. Graph of Landsat 8 and 9 OLI thermal data from 1 June 2024 showing a strong surge in thermal activity during July through November 2023. During mid-October, the intensity of the hotspots gradually declined. Courtesy of MIROVA.

Figure 49. Infrared (bands B12, B11, B4) satellite images show several strong lava flows (bright yellow-orange) affecting the northern and SE flanks of Shishaldin on 18 July 2023 (top left), 4 June 2023 (top right), 26 September 2023 (bottom left), and 3 October 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 50. Strong sulfur dioxide plumes were detected at Shishaldin and drifted in different directions on 15 August 2023 (top left), 5 September 2023 (top right), 25 September 2023 (bottom left), and 6 October 2023 (bottom right). Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Geologic Background. The symmetrical glacier-covered Shishaldin in the Aleutian Islands is the westernmost of three large stratovolcanoes in the eastern half of Unimak Island. The Aleuts named the volcano Sisquk, meaning "mountain which points the way when I am lost." Constructed atop an older glacially dissected edifice, it is largely basaltic in composition. Remnants of an older edifice are exposed on the W and NE sides at 1,500-1,800 m elevation. There are over two dozen pyroclastic cones on its NW flank, which is covered by massive aa lava flows. Frequent explosive activity, primarily consisting of Strombolian ash eruptions from the small summit crater, but sometimes producing lava flows, has been recorded since the 18th century. A steam plume often rises from the summit crater.

Information Contacts: Alaska Volcano Observatory (AVO), a cooperative program of a) U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508-4667 USA (URL: https://avo.alaska.edu/), b) Geophysical Institute, University of Alaska, PO Box 757320, Fairbanks, AK 99775-7320, USA, and c) Alaska Division of Geological & Geophysical Surveys, 794 University Ave., Suite 200, Fairbanks, AK 99709, USA (URL: http://dggs.alaska.gov/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard, Maryland, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Ioto (Iwo-jima), located about 1,200 km S of Tokyo, lies within a 9-km-wide submarine caldera along the Izu-Bonin-Mariana volcanic arc. Previous eruptions date back to 1889 and have consisted of dominantly phreatic explosions, pumice deposits during 2001, and discolored water. A submarine eruption during July through December 2022 was characterized by discolored water, pumice deposits, and gas emissions (BGVN 48:01). This report covers a new eruption during October through December 2023, which consisted of explosions, black ejecta, discolored water, and floating pumice, based on information from the Japan Meteorological Association (JMA), the Japan Coast Guard (JCG), and satellite data.

JMA reported that an eruption had been occurring offshore of Okinahama on the SE side of the island since 21 October, which was characterized by volcanic tremor, according to the Japan Maritime Self-Defense Force (JMSDF) Iwo Jima Air Base (figure 22). According to an 18 October satellite image a plume of discolored water at the site of this new eruption extended NE (figure 23). During an overflight conducted on 30 October, a vent was identified about 1 km off the coast of Okinahama. Observers recorded explosions every few minutes that ejected dark material about 20 m above the ocean and as high as 150 m. Ejecta from the vent formed a black-colored island about 100 m in diameter, according to observations conducted from the air by the Earthquake Research Institute of the University of Tokyo in cooperation with the Mainichi newspaper (figure 24). Occasionally, large boulders measuring more than several meters in size were also ejected. Observations from the Advanced Land Observing Satellite Daichi-2 and Sentinel-2 satellite images also confirmed the formation of this island (figure 23). Brown discolored water and floating pumice were present surrounding the island.

Figure 22. Map of Ioto showing the locations of recorded eruptions from 1889 through December 2023. The most recent eruption occurred during October through December 2023 and is highlighted in red just off the SE coast of the island and E of the 2001 eruption site. A single eruption highlighted in green was detected just off the NE coast of the island on 18 November 2023. From Ukawa et al. (2002), modified by JMA.

Figure 23. Satellite images showing the formation of the new island formation (white arrow) off the SE (Okinahama) coast of Ioto on 18 October 2023 (top left), 27 November 2023 (top right), 2 December 2023 (bottom left), and 12 December 2023 (bottom right). Discolored water was visible surrounding the new island. By December, much of the island had been eroded. Courtesy of Copernicus Browser.

Figure 24. Photo showing an eruption off the SE (Okinahama) coast of Ioto around 1230 on 30 October 2023. A column of water containing black ejecta is shown, which forms a new island. Occasionally, huge boulders more than several meters in size were ejected with the jet. Dark brown discolored water surrounded the new island. Photo has been color corrected and was taken from the S by the Earthquake Research Institute, University of Tokyo in cooperation of Mainichi newspaper. Courtesy of JMA.

The eruption continued during November. During an overflight on 3 November observers photographed the island and noted that material was ejected 169 m high, according to a news source. Explosions gradually became shorter, and, by the 3rd, they occurred every few seconds; dark and incandescent material were ejected about 800 m above the vent. On 4 November eruptions were accompanied by explosive sounds. Floating, brown-colored pumice was present in the water surrounding the island. There was a brief increase in the number of volcanic earthquakes during 8-14 November and 24-25 November. The eruption temporarily paused during 9-11 November and by 12 November eruptions resumed to the W of the island. On 10 November dark brown-to-dark yellow-green discolored water and a small amount of black floating material was observed (figure 25). A small eruption was reported on 18 November off the NE coast of the island, accompanied by white gas-and-steam plumes (figure 23). Another pause was recorded during 17-19 November, which then resumed on 20 November and continued erupting intermittently. According to a field survey conducted by the National Institute for Disaster Prevention Science and Technology on 19 November, a 30-m diameter crater was visible on the NE coast where landslides, hot water, and gray volcanic ash containing clay have occurred and been distributed previously. Erupted blocks about 10 cm in diameter were distributed about 90-120 m from the crater. JCG made observations during an overflight on 23 November and reported a phreatomagmatic eruption. Explosions at the main vent generated dark gas-and-ash plumes that rose to 200 m altitude and ejected large blocks that landed on the island and in the ocean (figure 26). Discolored water also surrounded the island. The size of the new island had grown to 450 m N-S x 200 m E-W by 23 November, according to JCG.

Figure 25. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 10 November showing discolored water and a small amount of black floating material were visible surrounding the island. Photo has been color corrected. Photographed by JCG courtesy of JMA.

Figure 26. Photo of the new land formed off the SE (Okinahama) coast of Ioto on 23 November showing a phreatomagmatic eruption that ejected intermittent pulses of ash and dark material that rose to 200 m altitude. Photo has been color corrected. Photographed by JCG courtesy of JMA.

The eruption continued through 11 December, followed by a brief pause in activity, which then resumed on 31 December, according to JMA. Intermittent explosions produced 100-m-high black plumes at intervals of several minutes to 30 minutes during 1-10 December. Overflights were conducted on 4 and 15 December and reported that the water surrounding the new island was discolored to dark brown-to-dark yellow-green (figure 27). No floating material was reported during this time. In comparison to the observations made on 23 November, the new land had extended N and part of it had eroded away. In addition, analysis by the Geospatial Information Authority of Japan using SAR data from Daichi-2 also confirmed that the area of the new island continued to decrease between 4 and 15 December. Ejected material combined with wave erosion transformed the island into a “J” shape, 500-m-long and with the curved part about 200 m offshore of Ioto. The island was covered with brown ash and blocks, and the surrounding water was discolored to greenish-brown and contained an area of floating pumice. JCG reported from an overflight on 4 December that volcanic ash-like material found around the S vent on the NE part of the island was newly deposited since 10 November (figure 28). By 15 December the N part of the “J” shaped island had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands (figure 27).

Figure 27. Photos of the new island formed off the SE (Okinahama) coast of Ioto on 4 December 2023 (left) and 15 December 2023 (right). No gas-and-ash emissions or lava flows were observed on the new land. Additionally, dark brown-to-dark yellow-green discolored water was observed surrounding the new land. During 4 and 15 December, the island had eroded to where the N part of the “J” shape had separated and migrated N, connecting to the Okinahama coast and the curved part of the “J” had eroded into two smaller islands. Courtesy of Copernicus Browser.

Figure 28. Photo of new volcanic ash-deposits (yellow dashed lines) near the S vent on the NE coast of Ioto taken by JCG on 4 December 2023. White gas-and-steam emissions were also visible (white arrow). Photo has been color corrected. Courtesy of JMA.

References. Ukawa, M., Fujita, E., Kobayashi, T., 2002, Recent volcanic activity of Iwo Jima and the 2001 eruption, Monthly Chikyu, Extra No. 39, 157-164.

Geologic Background. Ioto, in the Volcano Islands of Japan, lies within a 9-km-wide submarine caldera. The volcano is also known as Ogasawara-Iojima to distinguish it from several other "Sulfur Island" volcanoes in Japan. The triangular, low-elevation, 8-km-long island narrows toward its SW tip and has produced trachyandesitic and trachytic rocks that are more alkalic than those of other volcanoes in this arc. The island has undergone uplift for at least the past 700 years, accompanying resurgent doming of the caldera; a shoreline landed upon by Captain Cook's surveying crew in 1779 is now 40 m above sea level. The Motoyama plateau on the NE half of the island consists of submarine tuffs overlain by coral deposits and forms the island's high point. Many fumaroles are oriented along a NE-SW zone cutting through Motoyama. Numerous recorded phreatic eruptions, many from vents on the W and NW sides of the island, have accompanied the uplift.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); Japan Coast Guard (JCG) Volcano Database, Hydrographic and Oceanographic Department, 3-1-1, Kasumigaseki, Chiyoda-ku, Tokyo 100-8932, Japan (URL: https://www1.kaiho.mlit.go.jp/GIJUTSUKOKUSAI/kaiikiDB/kaiyo22-2.htm); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/); Asahi, 5-3-2, Tsukiji, Chuo Ward, Tokyo, 104-8011, Japan (URL: https://www.asahi.com/ajw/articles/15048458).

Puracé, located in Colombia, is a stratovolcano that contains a 500-m-wide summit crater. It is part of the Los Coconucos volcanic chain that is a NW-SE trending group of seven cones and craters. The most recent eruption occurred during March 2022 that was characterized by frequent seismicity and gas-and-steam emissions (BGVN 47:06). This report covers a brief eruption during November 2023 based on monthly reports from the Popayán Observatory, part of the Servicio Geologico Colombiano (SGC).

Activity during November 2022 through November 2023 primarily consisted of seismicity: VT-type events, LP-type events, HB-type events, and TR-type events (table 4). Maximum sulfur dioxide values were measured weekly and ranged from 259-5,854 tons per day (t/d) during November 2022 through April 2023. White gas-and-steam emissions were also occasionally reported.

SGC issued a report on 25 October that noted a significant increase in the number of earthquakes associated with rock fracturing. These earthquakes were located SE of the crater between Puracé and Piocollo at depths of 1-4 km. There were no reported variations in sulfur dioxide values, but SGC noted high carbon dioxide values, compared to those recorded in the first half of 2023.

SGC reported that at 1929 on 16 November the seismic network detected a signal that was possibly associated with a gas-and-ash emission, though it was not confirmed in webcam images due to limited visibility. On 17 November an observer confirmed ash deposits on the N flank. Webcam images showed an increase in degassing both inside the crater and from the NW flank, rising 700 m above the crater.

Table 4. Seismicity at Puracé during November 2022-November 2023. Volcano-tectonic (VT), long-period (LP), hybrid (HB), and tremor (TR) events are reported each month. Courtesy of SGC.

Geologic Background. Puracé is an active andesitic volcano with a 600-m-diameter summit crater at the NW end of the Los Coconucos Volcanic Chain. This volcanic complex includes nine composite and five monogenetic volcanoes, extending from the Puracé crater more than 6 km SE to the summit of Pan de Azúcar stratovolcano. The dacitic massif which the complex is built on extends about 13 km NW-SE and 10 km NE-SW. Frequent small to moderate explosive eruptions reported since 1816 CE have modified the morphology of the summit crater, with the largest eruptions in 1849, 1869, and 1885.

Information Contacts: Servicio Geologico Colombiano (SGC), Diagonal 53 No. 34-53 - Bogotá D.C., Colombia (URL: https://www.sgc.gov.co/volcanes).

Etna, located on the Italian island of Sicily, has had documented eruptions dating back to 1500 BCE. Activity typically originates from multiple cones at the summit, where several craters have formed and evolved. The currently active craters are Northeast Crater (NEC), Voragine (VOR), and Bocca Nuova (BN), and the Southeast Crater (SEC); VOR and BN were previously referred to as the “Central Crater”. The original Southeast crater formed in 1978, and a second eruptive site that opened on its SE flank in 2011 was named the New Southeast Crater (NSEC). Another eruptive site between the SEC and NSEC developed during early 2017 and was referred to as the "cono della sella" (saddle cone). The current eruption period began in November 2022 and has been characterized by intermittent Strombolian activity, lava flows, and ash plumes (BGVN 48:08). This report updates activity during July through October 2023, which includes primarily gas-and-steam emissions; during July and August Strombolian explosions, lava fountains, and lava flows were reported, based on weekly and special reports by the Osservatorio Etneo (OE), part of the Catania Branch of Italy's Istituo Nazionale di Geofisica e Vulcanologica (INGV) and satellite data.

Variable fumarolic degassing was reported at all summit craters (BN, VOR, NEC, and SEC) throughout the entire reporting period (table 15). The MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system based on the analysis of MODIS data showed frequent low-to-moderate power thermal anomalies during the reporting period (figure 399). During mid-August there was a pulse in activity that showed an increase in the power of the anomalies due to Strombolian activity, lava fountains, and lava flows. Infrared satellite imagery captured strong thermal anomalies at the central and southeast summit crater areas (figure 400). Accompanying thermal activity were occasional sulfur dioxide plumes that exceeded 2 Dobson Units (DUs) recorded by the TROPOMI instrument on the Sentinel-5P satellite (figure 401).

Table 15. Summary of activity at the four primary crater areas at the summit of Etna during July-October 2023. Information is from INGV weekly reports.

Figure 399. Frequent thermal activity at Etna varied in strength during July through October 2023, as shown on this MIROVA plot (Log Radiative Power). There was a spike in power during mid-August, which reflected an increase in Strombolian activity. Courtesy of MIROVA.

Figure 400. Infrared (bands B12, B11, B4) satellite images showing strong thermal anomalies at Etna’s central and Southeast crater areas on 21 July 2023 (top left), 27 August 2023 (top right), 19 September 2023 (bottom left), and 29 October 2023 (bottom right). Courtesy of Copernicus Browser.

Figure 401. Sulfur dioxide plumes that exceeded 2 Dobson Units (DUs) rose above Etna on 14 July 2023 (top left), 14 August 2023 (top right), 2 September 2023 (bottom left), and 7 October 2023 (bottom right). These plumes drifted NE, S, SE, and SW, respectively. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

Activity during July and August was relatively low and mainly consisted of degassing at the summit craters, particularly at SEC and BN. Cloudy weather prevented clear views of the summit during early July. During the night of 2 July some crater incandescence was visible at SEC. Explosive activity resumed at SEC during 9-10 July, which was characterized by sporadic and weak ash emissions that rapidly dispersed in the summit area (figure 402). INGV reported moderate Strombolian activity began at 2034 on 14 July and was confined to the inside of the crater and fed by a vent located in the E part of SEC. An ash emission was detected at 2037. A new vent opened on 15 July in the SE part of BN and began to produce continuous gas-and-steam emissions. During an inspection carried out on 28 July pulsating degassing, along with audible booms, were reported at two active vents in BN. Vigorous gas-and-steam emissions intermittently generated rings. On rare occasions, fine, reddish ash was emitted from BN1 and resuspended by the gas-and-steam emissions.

Figure 402. Webcam image taken by the Monta Cagliato camera showing an ash emission rising above Etna’s Southeast Crater (SEC) on 10 July 2023. Photo has been color corrected. Courtesy of INGV (Report 28/2023, ETNA, Bollettino Settimanale, 03/07/2023 - 09/07/2023).

Around 2000 on 13 August INGV reported a sudden increase in volcanic tremor amplitude. Significant infrasonic activity coincided with the tremor increase. Incandescent flashes were visible through the cloud cover in webcam images of SEC (figure 403). Strombolian activity at SEC began to gradually intensify starting at 2040 as seismicity continued to increase. The Aviation Color Code (ACC) was raised to Yellow (the second lowest-level on a four-color scale) at 2126 and then to Orange (the second highest-level on a four-color scale) at 2129 due to above-background activity. The activity rapidly transitioned from Strombolian activity to lava fountains around 2333 that rose 300-400 m above the crater (figure 403). Activity was initially focused on the E vent of the crater, but then the vent located above the S flank of the cone also became active. A lava flow from this vent traveled SW into the drainage created on 10 February 2022, overlapping with previous flows from 10 and 21 February 2022 and 21 May 2023, moving between Monte Barbagallo and Monte Frumento Supino (figure 404). The lava flow was 350 m long, oriented NNE-SSW, and descended to an elevation of 2.8 km. Flows covered an area of 300,000 m² and had an estimated volume of 900,000 m³. The ACC was raised to Red at 2241 based on strong explosive activity and ashfall in Rifugio Sapienza-Piano Vetore at 1.7 km elevation on the S flank. INGV reported that pyroclastic flows accompanied this activity.

Figure 403. Webcam images of the lava fountaining event at Etna during 13-14 August 2023 taken by the Milos (EMV) camera. Images show the start of the event with increasing incandescence (a-b), varying intensity in activity (c-e), lava fountaining and pyroclastic flows (f-g), and a strong ash plume (g). Courtesy of INGV (Report 33/2023, ETNA, Bollettino Settimanale, 08/08/2023 - 14/08/2023).

Figure 404. Map of the new lava flow (yellow) and vent (red) at SEC (CSE) of Etna on 13 August 2023. The background image is a shaded model of the terrain of the summit area obtained by processing Skysat images acquired during on 18 August. The full extent of the lava flow was unable to be determined due to the presence of ash clouds. The lava flow extended more than 350 m to the SSW and reached an elevation of 2.8 km and was located W of Mt. Frumento Supino. CSE = Southeast Crater; CNE = Northeast Crater; BN = Bocca Nuova; VOR = Voragine. Courtesy of INGV (Report 34/2023, ETNA, Bollettino Settimanale, 14/08/2023 - 20/08/2023).

Activity peaked between 0240 and 0330 on 14 August, when roughly 5-6 vents erupted lava fountains from the E to SW flank of SEC. The easternmost vents produced lava fountains that ejected material strongly to the E, which caused heavy fallout of incandescent pyroclastic material on the underlying flank, triggering small pyroclastic flows. This event was also accompanied by lightning both in the ash column and in the ash clouds that were generated by the pyroclastic flows. A fracture characterized by a series of collapse craters (pit craters) opened on the upper SW flank of SEC. An ash cloud rose a few kilometers above the crater and drifted S, causing ash and lapilli falls in Rifugio Sapienza and expanding toward Nicolosi, Mascalucia, Catania, and up to Syracuse. Ashfall resulted in operational problems at the Catania airport (50 km S), which lasted from 0238 until 2000. By 0420 the volcanic tremor amplitude values declined to background levels. After 0500 activity sharply decreased, although the ash cloud remained for several hours and drifted S. By late morning, activity had completely stopped. The ACC was lowered to Orange as volcanic ash was confined to the summit area. Sporadic, minor ash emissions continued throughout the day. At 1415 the ACC was lowered to Yellow and then to Green at 1417.

During the night of 14-15 August only occasional flashes were observed, which were more intense during avalanches of material inside the eruptive vents. Small explosions were detected at SEC at 2346 on 14 August and at 0900 on 26 August that each produced ash clouds which rapidly dispersed into the atmosphere (figure 405). According to a webcam image, an explosive event detected at 2344 at SEC generated a modest ash cloud that was rapidly dispersed by winds. The ACC was raised to Yellow at 2355 on 14 August due to increasing unrest and was lowered to Green at 0954 on 15 August.

Figure 405. Webcam image of an ash plume rising above Etna’s SEC at 0902 (local time) on 26 August taken by the Montagnola EMOV camera. Photo has been color corrected. Courtesy of INGV (Report 35/2023, ETNA, Bollettino Settimanale, 21/08/2023 - 27/08/2023).

Activity during September and October was relatively low and mainly characterized by variable degassing from BN and SEC. Intense, continuous, and pulsating degassing was accompanied by roaring sounds and flashes of incandescence at BN both from BN1 and the new pit crater that formed during late July (figure 406). The degassing from the new pit crater sometimes emitted vapor rings. Cloudy weather during 6-8 September prevented observations of the summit craters .

Figure 406. Webcam image (top) showing degassing from Etna’s Bocca Nuova (BN) crater accompanied by nighttime crater incandescence at 0300 (local time) on 2 September 2023 by the Piedimonte Etneo (EPVH) camera and a photo of incandescence at BN1 and the new pit crater (bottom) taken by an observatory scientist from the E rim of BN during a survey on 2 September 2023. Courtesy of INGV (Report 36/2023, ETNA, Bollettino Settimanale, 28/08/2023 - 03/09/2023).

Geologic Background. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: Sezione di Catania - Osservatorio Etneo, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/it/); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); NASA Global Sulfur Dioxide Monitoring Page, Atmospheric Chemistry and Dynamics Laboratory, NASA Goddard Space Flight Center (NASA/GSFC), 8800 Greenbelt Road, Goddard MD 20771, USA (URL: https://so2.gsfc.nasa.gov/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Suwanosejima is an 8-km-long island that consists of a stratovolcano and two active summit craters, located in the northern Ryukyu Islands, Japan. Volcanism over the past century has been characterized by Strombolian explosions, ash plumes, and ashfall. The current eruption began in October 2004 and has more recently consisted of frequent eruption plumes, explosions, and incandescent ejecta (BGVN 48:07). This report covers similar activity of ash plumes, explosions, and crater incandescence during July through October 2023 using monthly reports from the Japan Meteorological Agency (JMA) and satellite data.

Thermal activity during the reporting period was relatively low; only one low-power thermal anomaly was detected during mid-July and one during early August, based on a MIROVA (Middle InfraRed Observation of Volcanic Activity) Log Radiative Power graph of the MODIS thermal anomaly data. On two clear weather days, a thermal anomaly was visible in infrared satellite images (figure 81).

Figure 81. Infrared (bands B12, B11, B4) satellite imagery showing a thermal anomaly (bright yellow-orange) at the Otake crater of Suwanosejima on 23 September 2023 (left) and 18 October 2023 (right). Courtesy of Copernicus Browser.

Low-level activity was reported at the Otake crater during July and no explosions were detected. Eruption plumes rose as high as 1.8 km above the crater. On 13 July an ash plume rose 1.7 km above the crater rim, based on a webcam image. During the night of the 28th crater incandescence was visible in a webcam image. An eruptive event reported on 31 July produced an eruption plume that rose 2.1 km above the crater. Seismicity consisted of 11 volcanic earthquakes on the W flank, the number of which had decreased compared to June (28) and 68 volcanic earthquakes near the Otake crater, which had decreased from 722 in the previous month. According to observations conducted by the University of Tokyo Graduate School of Science, Kyoto University Disaster Prevention Research Institute, Toshima Village, and JMA, the amount of sulfur dioxide emissions released during the month was 400-800 tons per day (t/d).

Eruptive activity in the Otake crater continued during August and no explosions were reported. An eruptive event produced a plume that rose 1 km above the crater at 1447 on 12 August. Subsequent eruptive events were recorded at 0911 on 16 August, at 1303 on 20 August, and at 0317 on 21 August, which produced ash plumes that rose 1-1.1 km above the crater and drifted SE, SW, and W. On 22 August an ash plume was captured in a webcam image rising 1.4 km above the crater (figure 82). Multiple eruptive events were detected on 25 August at 0544, 0742, 0824, 1424, and 1704, which generated ash plumes that rose 1.1-1.2 km above the crater and drifted NE, W, and SW. On 28 August a small amount of ashfall was observed as far as 1.5 km from the crater. There were 17 volcanic earthquakes recorded on the W flank of the volcano and 79 recorded at the Otake crater during the month. The amount of sulfur dioxide emissions released during the month was 400-800 t/d.

Figure 82. Webcam image of an ash plume rising 1.4 km above Suwanosejima’s Otake crater rim on 22 August 2023. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, August 2023).

Activity continued at the Otake crater during September. Occasionally, nighttime crater incandescence was observed in webcam images and ashfall was reported. An eruptive event at 1949 on 4 September produced an ash plume that rose 1 km above the crater and drifted SW. On 9 September several eruption events were detected at 0221, 0301, and 0333, which produced ash plumes that rose 1.1-1.4 km above the crater rim and drifted W; continuous ash emissions during 0404-0740 rose to a maximum height of 2 km above the crater rim (figure 83). More eruptive events were reported at 1437 on 10 September, at 0319 on 11 September, and at 0511 and 1228 on 15 September, which generated ash plumes that rose 1-1.8 km above the crater. During 25, 27, and 30 September, ash plumes rose as high as 1.3 km above the crater rim. JMA reported that large blocks were ejected as far as 300 m from the center of the crater. There were 18 volcanic earthquakes detected beneath the W flank and 82 volcanic earthquakes detected near the Otake crater. The amount of sulfur dioxide released during the month ranged from 600 to 1,600 t/d.

Figure 83. Webcam image of an ash plume rising 2 km above Suwanosejima’s Otake crater rim on 9 September 2023. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, September 2023).

Activity during early-to-mid-October consisted of occasional explosions, a total number of 13, and ash plumes that rose as high as 1.9 km above the Otake crater rim on 29 October (figure 84). These explosions are the first to have occurred since June 2023. Continuous ash emissions were reported during 0510-0555 on 1 October. Explosions were recorded at 0304, 2141, and 2359 on 2 October, at 0112 on 3 October, and at 1326 on 6 October, which produced ash plumes that rose as high as 1 km above the crater rim and drifted SW and W. An explosion was noted at 0428 on 3 October, but emission details were unknown. A total of eight explosions were recorded by the seismic network at 1522 on 14 October, at 0337, 0433, 0555, 1008, and 1539 on 15 October, and at 0454 and 0517 on 16 October. Ash plumes from these explosions rose as high as 900 m above the crater and drifted SE. Eruptive events during 25-27 and 29-30 October generated plumes that rose as high as 1.9 km above the crater and drifted SE, S, and SW. Ash was deposited in Toshima village (3.5 km SSW). Eruptive activity occasionally ejected large volcanic blocks as far as 600 m from the crater. Nighttime crater incandescence was visible in webcams. Intermittent ashfall was reported as far as 1.5 km from the crater. There were 43 volcanic earthquakes detected on the W flank during the month, and 184 volcanic earthquakes detected near the Otake crater. The amount of sulfur dioxide emitted ranged between 400 and 900 t/d.

Figure 84. Webcam image of an ash plume rising 1.9 km above Suwanosejima’s Otake crater on 29 October 2023. Courtesy of JMA (Volcanic activity commentary for Suwanosejima, October 2023).

Geologic Background. The 8-km-long island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two active summit craters. The summit is truncated by a large breached crater extending to the sea on the E flank that was formed by edifice collapse. One of Japan's most frequently active volcanoes, it was in a state of intermittent Strombolian activity from Otake, the NE summit crater, between 1949 and 1996, after which periods of inactivity lengthened. The largest recorded eruption took place in 1813-14, when thick scoria deposits covered residential areas, and the SW crater produced two lava flows that reached the western coast. At the end of the eruption the summit of Otake collapsed, forming a large debris avalanche and creating an open collapse scarp extending to the eastern coast. The island remained uninhabited for about 70 years after the 1813-1814 eruption. Lava flows reached the eastern coast of the island in 1884. Only about 50 people live on the island.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Aira caldera, located in the northern half of Kagoshima Bay, Japan, contains the post-caldera Sakurajima volcano. Eruptions typically originate from the Minamidake crater, and since the 8th century, ash deposits have been recorded in the city of Kagoshima (10 km W), one of Kyushu’s largest cities. The Minamidake summit cone and crater has had persistent activity since 1955; the Showa crater on the E flank has also been intermittently active since 2006. The current eruption period began during March 2017 and has recently been characterized by intermittent explosions, eruption plumes, and ashfall (BGVN 48:07). This report updates activity during July through October 2023 and describes explosive events, ash plumes, nighttime crater incandescence, and ashfall, according to monthly activity reports from the Japan Meteorological Agency (JMA) and satellite data.

Thermal activity remained at low levels during this reporting period, according to the MIROVA (Middle InfraRed Observation of Volcanic Activity) system (figure 149). There was a slight increase in the number of anomalies during September through October. Occasional thermal anomalies were visible in infrared satellite images mainly at the Minamidake crater (Vent A is located to the left and Vent B is located to the right) (figure 150).

Table 30. Number of monthly explosive events, days of ashfall, area of ash covered, and sulfur dioxide emissions from Sakurajima’s Minamidake crater at Aira during July-October 2023. Note that smaller ash events are not listed. Ashfall days were measured at Kagoshima Local Meteorological Observatory and ashfall amounts represent material covering all the Kagoshima Prefecture. Data courtesy of JMA monthly reports.

Figure 149. Thermal activity at Sakurajima in the Aira caldera was relatively low during July through October 2023, based on this MIROVA graph (Log Radiative Power). There was an increase in the number of detected anomalies during September through October. Courtesy of MIROVA.

Figure 150. Infrared (bands B12, B11, B4) satellite images show a persistently strong thermal anomaly (bright yellow-orange) at the Minamidake crater at Aira’s Sakurajima volcano on 28 September 2023 (top left), 3 October 2023 (top right), 23 October 2023 (bottom left), and 28 October 2023 (bottom right). Vent A is located to the left and Vent B is to the right of Vent A; both vents are part of the Minamidake crater. Courtesy of Copernicus Browser.

JMA reported that during July, there were eight eruptions, three of which were explosion events in the Showa crater. Large blocks were ejected as far as 600 m from the Showa crater. Very small eruptions were occasionally reported at the Minamidake crater. Nighttime incandescence was observed in both the Showa and Minamidake crater. Explosions were reported on 16 July at 2314 and on 17 July at 1224 and at 1232 (figure 151). Resulting eruption plumes rose 700-2,500 m above the crater and drifted N. On 23 July the number of volcanic earthquakes on the SW flank of the volcano increased. A strong Mw 3.1 volcanic earthquake was detected at 1054 on 26 July. The number of earthquakes recorded throughout the month was 545, which markedly increased from 73 in June. No ashfall was observed at the Kagoshima Regional Meteorological Observatory during July. According to a field survey conducted during the month, the daily amount of sulfur dioxide emissions was 1,600-3,200 tons per day (t/d).

Figure 151. Webcam image showing a strong, gray ash plume that rose 2.5 km above the crater rim of Aira’s Showa crater at 1232 on 17 July 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, July 2023).

There were three eruptions reported at the Minamidake crater during August, each of which were explosive. The explosions occurred on 9 August at 0345, on 13 August at 2205, and on 31 August at 0640, which generated ash plumes that rose 800-2,000 m above the crater and drifted W. There were two eruptions detected at Showa crater; on 4 August at 2150 ejecta traveled 800 m from the Showa crater and associated eruption plumes rose 2.3 km above the crater. The explosion at 2205 on 13 August generated an ash plume that rose 2 km above the crater and was accompanied by large blocks that were ejected 600 m from the Minamidake crater (figure 152). Nighttime crater incandescence was visible in a high-sensitivity surveillance camera at both craters. Seismicity consisted of 163 volcanic earthquakes, 84 of which were detected on the SW flank. According to the Kagoshima Regional Meteorological Observatory there was a total of 7 g/m² of ashfall over the course of 10 days during the month. According to a field survey, the daily amount of sulfur dioxide emitted was 1,800-3,300 t/d.

Figure 152. Webcam image showing an eruption plume rising 2 km above the Minamidake crater at Aira at 2209 on 13 August 2023. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, August 2023).

During September, four eruptions were reported, three of which were explosion events. These events occurred at 1512 on 9 September, at 0018 on 11 September, and at 2211 on 13 September. Resulting ash plumes generally rose 800-1,100 m above the crater. An explosion produced an ash plume at 2211 on 13 September that rose as high as 1.7 km above the crater. Large volcanic blocks were ejected 600 m from the Minamidake crater. Smaller eruptions were occasionally observed at the Showa crater. Nighttime crater incandescence was visible at the Minamidake crater. Seismicity was characterized by 68 volcanic earthquakes, 28 of which were detected beneath the SW flank. According to the Kagoshima Regional Meteorological Observatory there was a total of 3 g/m² of ashfall over the course of seven days during the month. A field survey reported that the daily amount of sulfur dioxide emitted was 1,600-2,300 t/d.

Eruptive activity during October consisted of 69 eruptions, 33 of which were described as explosive. These explosions occurred during 4 and 11-21 October and generated ash plumes that rose 500-3,600 m above the crater and drifted S, E, SE, and N. On 19 October at 1648 an explosion generated an ash plume that rose 3.6 km above the crater (figure 153). No eruptions were reported in the Showa crater; white gas-and-steam emissions rose 100 m above the crater from a vent on the N flank. Nighttime incandescence was observed at the Minamidake crater. On 24 October an eruption was reported from 0346 through 0430, which included an ash plume that rose 3.4 km above the crater. Ejected blocks traveled 1.2 km from the Minamidake crater. Following this eruption, small amounts of ashfall were observed from Arimura (4.5 km SE) and a varying amount in Kurokami (4 km E) (figure 154). The number of recorded volcanic earthquakes during the month was 190, of which 14 were located beneath the SW flank. Approximately 61 g/m² of ashfall was reported over eight days of the month. According to a field survey, the daily amount of sulfur dioxide emitted was 2,200-4,200 t/d.

Figure 153. Webcam image showing an ash plume rising 3.6 km above the Minamidake crater at Aira at 1648 on 19 October 2023. Photo has been color corrected. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, October 2023).

Figure 154. Photo showing ashfall (light gray) in Kurokami-cho, Sakurajima on 24 October 2023 taken at 1148 following an eruption at Aira earlier that day. Courtesy of JMA monthly report (Sakurajima volcanic activity explanatory material, October 2023).

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Nishinoshima is a small island in the Ogasawara Arc, about 1,000 km S of Tokyo, Japan. It contains prominent submarine peaks to the S, W, and NE. Recorded eruptions date back to 1973, with the current eruption period beginning in October 2022. Eruption plumes and fumarolic activity characterize recent activity (BGVN 48:10). This report covers the end of the eruption for September through October 2023, based on information from monthly reports of the Japan Meteorological Agency (JMA) monthly reports, and satellite data.

No eruptive activity was reported during September 2023, although JMA noted that the surface temperature was slightly elevated compared to the surrounding area since early March 2023. The Japan Coast Guard (JCG) conducted an overflight on 20 September and reported white gas-and-steam plumes rising 3 km above the central crater of the pyroclastic cone, as well as multiple white gas-and-steam emissions emanating from the N, E, and S flanks of the crater to the coastline. In addition, dark reddish brown-to-green discolored water was distributed around almost the entire circumference of the island.

Similar low-level activity was reported during October. Multiple white gas-and-steam emissions rose from the N, E, and S flanks of the central crater of the pyroclastic cone and along the coastline; these emissions were more intense compared to the previous overflight observations. Dark reddish brown-to-green discolored water remained visible around the circumference of the island. On 4 October aerial observations by JCG showed a small eruption consisting of continuous gas-and-steam emissions emanating from the central crater, with gray emissions rising to 1.5 km altitude (figure 129). According to observations from the marine weather observation vessel Keifu Maru on 26 October, white gas-and-steam emissions persisted from the center of the pyroclastic cone, as well as from the NW, SW, and SE coasts of the island for about five minutes. Slightly discolored water was visible up to about 1 km.

Figure 129. Aerial photos of gray emissions rising from the central crater of Nishinoshima’s pyroclastic cone to an altitude of 1.5 km on 4 October 2023 taken at 1434 (left) and 1436 (right). Several white gas-and-steam emissions also rose from the N, E, and S flanks of the central crater. Both photos have been color corrected. Courtesy of JCG via JMA (monthly reports of activity at Nishinoshima, October, 2023).

Frequent low-to-moderate power thermal anomalies were recorded in the MIROVA graph (Middle InfraRed Observation of Volcanic Activity) during September (figure 130). Occasional anomalies were detected during October, and fewer during November through December. A thermal anomaly was visible in the crater using infrared satellite imagery on 6, 8, 11, 16, 18, 21, and 23 September and 8, 13, 21, 26, and 28 October (figure 131).

Figure 130. Low-to-moderate power thermal anomalies were detected at Nishinoshima during September through December 2023, showing a decrease in the frequency of anomalies after September, according to this MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 131. Infrared (bands B12, B11, B4) satellite images showing a strong thermal anomaly at the crater of Nishinoshima on 21 September 2023 (left) and 13 October 2023 (right). A strong gas-and-steam plume accompanied the thermal activity, extending NW. Courtesy of Copernicus Browser.

Geologic Background. The small island of Nishinoshima was enlarged when several new islands coalesced during an eruption in 1973-74. Multiple eruptions that began in 2013 completely covered the previous exposed surface and continued to enlarge the island. The island is the summit of a massive submarine volcano that has prominent peaks to the S, W, and NE. The summit of the southern cone rises to within 214 m of the ocean surface 9 km SSE.

Information Contacts: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

Kīlauea is on the island of Hawai’i and overlaps the E flank of the Mauna Loa volcano. Its East Rift Zone (ERZ) has been intermittently active for at least 2,000 years. An extended eruption period began in January 1983 and was characterized by open lava lakes and lava flows from the summit caldera and the East Rift Zone. During May 2018 magma migrated into the Lower East Rift Zone (LERZ) and opened 24 fissures along a 6-km-long NE-trending fracture zone that produced lava flows traveling in multiple directions. As lava emerged from the fissures, the lava lake at Halema'uma'u drained and explosions sent ash plumes to several kilometers altitude (BGVN 43:10).

The current eruption period started during September 2021 and has been characterized by low-level lava effusions in the active Halema’uma’u lava lake (BGVN 48:01). This report covers three notable eruption periods during February, June, and September 2023 consisting of lava fountaining, lava flows, and spatter during January through September 2023 using information from daily reports, volcanic activity notices, and abundant photo, map, and video data from the US Geological Survey's (USGS) Hawaiian Volcano Observatory (HVO).

Activity during January 2023. Small earthquake swarms were recorded on 2 January 2023; increased seismicity and changes in the pattern of deformation were noted on the morning of 5 January. At around 1500 both the rate of deformation and seismicity drastically increased, which suggested magma movement toward the surface. HVO raised the Volcano Alert Level (VAL) to Watch (the second highest level on a four-level scale) and the Aviation Color Code (ACC) to Orange (the second highest color on a four-color scale) at 1520.

Multiple lava fountains and lava effusions from vents in the central eastern portion of the Halema’uma’u crater began on 5 January around 0434; activity was confined to the eastern half of the crater and within the basin of the western half of the crater, which was the focus of the eruption in 2021-2022 (figure 525). Incandescence was visible in webcam images at 1634 on 5 January, prompting HVO to raise the VAL to Warning (the highest level on a four-level scale) and the ACC to Red (the highest color on a four-color scale). Lava fountains initially rose as high as 50 m above the vent at the onset of the eruption (figure 526) but then declined to a more consistent 5-6 m height in the proceeding days. By 1930 that same day, lava had covered most of the crater floor (an area of about 1,200,000 m²) and the lava lake had a depth of 10 m. A higher-elevation island that formed during the initial phase of the December 2020 eruption remained exposed, appearing darker in images, along with a ring of older lava around the lava lake that was active prior to December 2022. Overnight during 5-6 January the lava fountains continued to rise 5 m high, and the lava effusion rate had slowed.

Figure 525. A reference map of Kīlauea showing activity on 6 January 2023, based on measurements taken from the crater rim at approximately 0900. Multiple eruptive vents (orange color) are on the E floor of Halema’uma’u crater effusing into a lava lake (red color). Lava from these vents flowed laterally across the crater floorcovering an area of 880,000 m2. The full extent of new lava from this eruption (red and pink colors) is approximately 1,120,000 m2. An elevated part of the lake (yellow color) that is higher in elevation compared to the rest of the crater floor was not covered in lava flows. Courtesy of USGS, HVO.

Figure 526. Image of the initial lava fountain at the onset of Kīlauea’s eruption on 5 January 2023 from a newly opened vent in the Halema’uma’u crater at 0449. This lava fountain rose as high as 50 m and ejected lava across the crater floor. Courtesy of USGS, HVO.

On 6 January at 0815 HVO lowered the VAL to Watch and the ACC to Orange due to the declining effusion rates. Sulfur dioxide emission rates ranged from 3,000-12,500 tonnes per day (t/d), the highest value of which was recorded on 6 January. Lava continued to erupt from the vents during 6-8 January, although the footprint of the active area had shrunk; a similar progression has been commonly observed during the early stages of recent eruptions at Halema’uma’u. On 9 January HVO reported one dominant lava fountain rising 6-7 m high in the E half of the crater. Lava flows built up the margins of the lake, causing the lake to be perched. On 10 January the eastern lava lake had an area of approximately 120,000 m² that increased to 250,000 m² by 17 January. During 13-31 January several small overflows occurred along the margins of the E lake. A smaller area of lava was active within the basin in the W half of the crater that had been the focus of activity during 2021-2022. On 19 January just after 0200 a small ooze-out was observed on the crater’s W edge.

Activity during February 2023. Activity continued in the E part of Halema’uma’u crater, as well as in a smaller basin in the W part of the 2021-2022 lava lake (figure 527). The E lava lake contained a single lava fountain and frequent overflows. HVO reported that during the morning of 1 February the large E lava lake began to cool and crust over in the center of the lake; two smaller areas of lava were observed on the N and S sides by the afternoon. The dominant lava fountain located in the S part of the lava lake paused for roughly 45 minutes at 2315 and resumed by midnight, rising 1-2 m. At 0100 on 2 February lava from the S part was effusing across the entire E lava lake area, covering the crusted over portion in the center of the lake and continuing across the majority of the previously measured 250,000 m² by 0400. A small lava pond near the E lake produced an overflow around 0716 on 2 February. On 3 February some lava crust began to form against the N and E levees, which defined the 250,000 m² eastern lava lake. The small S lava fountain remained active, rising 1-6 m high during 3-9 February; around 0400 on 5 February occasional bursts doubled the height of the lava fountain.

Figure 527. An aerial visual and thermal image taken of Kīlauea’s Halema’uma’u crater on 2 February 2023. The largest lava lake is in the E part of the crater, although lava has also filled areas that were previously active in the W part of the crater. The colors of the map indicate temperature, with blues indicative of cooler temperatures and reds indicative of warmer temperatures. Courtesy of USGS, HVO.

A large breakout occurred overnight during 2100 on 4 February to 0900 on 5 February on the N part of the crater floor, equal to or slightly larger in size than the E lava lake. A second, smaller lava fountain appeared in the same area of the E lava lake between 0300 and 0700 on 5 February and was temporarily active. This large breakout continued until 7 February. A small, brief breakout was reported in the S of the E lava lake around midnight on 7 February. In the W lake, as well as the smaller lava pond in the central portion of the crater floor, contained several overflows during 7-10 February and intermittent fountaining. Activity at the S small lava pond and the small S lava fountain within the E lake declined during 9-10 February. The lava pond in the central portion of the crater floor had nearly continuous, expansive flows during 10-13 February; channels from the small central lava pond seemed to flow into the larger E lake. During 13-18 February a small lava fountain was observed in the small lava pond in the central portion of the crater floor. Continuous overflows persisted during this time.

Activity in the eastern and central lakes began to decline in the late afternoon of 17 February. By 18 February HVO reported that the lava effusions had significantly declined, and that the eastern and central lakes were no longer erupting. The W lake in the basin remained active but at a greatly reduced level that continued to decline. HVO reported that this decrease in activity is attributed to notable deflationary tilt that began early on the morning of 17 February and lasted until early 19 February. By 19 February the W lake was mostly crusted over although some weak lava flows remained, which continued through 28 February. The sulfur dioxide emission rates ranged 250-2,800 t/d, the highest value of which was recorded on 6 February.

Activity during March 2023. The summit eruption at Halema’uma’u crater continued at greatly reduced levels compared to the previous two months. The E and central vents stopped effusing lava, and the W lava lake remained active with weak lava flows; the lake was mostly crusted over, although slowly circulating lava intermittently overturned the crust. By 6 March the lava lake in the W basin had stopped because the entire surface was crusted over. The only apparent surface eruptive activity during 5-6 March was minor ooze-outs of lava onto the crater floor, which had stopped by 7 March. Several hornitos on the crater floor still glowed through 12 March according to overnight webcam images, but they did not erupt any lava. A small ooze-out of lava was observed just after 1830 in the W lava lake on 8 March, which diminished overnight. The sulfur dioxide emission rate ranged from 155-321 t/d on 21 March. The VAL was lowered to Advisory, and the ACC was lowered to Yellow (the second lowest on a four-color scale) on 23 March due to a pause in the eruption since 7 March.

Activity during April-May 2023. The eruption at Halema’uma’u crater was paused; no lava effusions were visible on the crater floor. Sulfur dioxide emission rates ranged from 75-185 t/d, the highest of which was measured on 22 April. During May and June summit seismicity was elevated compared to seismicity that preceded the activity during January.

Activity during June 2023. Earthquake activity and changes in the patterns of ground deformation beneath the summit began during the evening of 6 June. The data indicated magma movement toward the surface, prompting HVO to raise the VAL to Watch and the ACC to Orange. At about 0444 on 7 June incandescence in Halema’uma’u crater was visible in webcam images, indicating that a new eruption had begun. HVO raised the VAL to Warning and the ACC to Red (the highest color on a four-color scale). Lava flowed from fissures that had opened on the crater floor. Multiple minor lava fountains were active in the central E portion of the Halema’uma’u crater, and one vent opened on the W wall of the caldera (figure 528). The eruptive vent on the SW wall of the crater continued to effuse into the lava lake in the far SW part of the crater (figure 529). The largest lava fountain consistently rose 15 m high; during the early phase of the eruption, fountain bursts rose as high as 60 m. Lava flows inundated much of the crater floor and added about 6 m depth of new lava within a few hours, covering approximately 10,000 m². By 0800 on 7 June lava filled the crater floor to a depth of about 10 m. During 0800-0900 the sulfur dioxide emission rate was about 65,000 t/d. Residents of Pahala (30 km downwind of the summit) reported minor deposits of fine, gritty ash and Pele’s hair. A small spatter cone had formed at the vent on the SW wall by midday, and lava from the cone was flowing into the active lava lake. Fountain heights had decreased from the onset of the eruption and were 4-9 m high by 1600, with occasional higher bursts. Inflation switched to deflation and summit earthquake activity greatly diminished shortly after the eruption onset.

Figure 528. Photo of renewed activity at Kīlauea’s Halema’uma’u crater that began at 0444 on 7 June 2023. Lava flows cover the crater floor and there are several active source vents exhibiting lava fountaining. Courtesy of USGS, HVO.

Figure 529. Photo of a lava fountain on the SW wall of Kīlauea’s Halema’uma’u crater on 7 June 2023. By midday a small cone structure had been built up. The fissure was intermittently obscured by gas-and-steam plumes. Courtesy of USGS, HVO.

At 0837 on 8 June HVO lowered the VAL to Watch and the ACC to Orange because the initial high effusion rates had declined, and no infrastructure was threatened. The surface of the lava lake had dropped by about 2 m, likely due to gas loss by the morning of 8 June. The drop left a wall of cooled lava around the margins of the crater floor. Lava fountain heights decreased during 8-9 June but continued to rise to 10 m high. Active lava and vents covered much of the W half of Halema’uma’u crater in a broad, horseshoe-shape around a central, uplifted area (figure 530). The preliminary average effusion rate for the first 24 hours of the eruption was about 150 cubic meters per second, though the estimate did not account for vesiculated lava and variations in crater floor topography. The effusion rate during the very earliest phases of the eruption appeared significantly higher than the previous three summit eruptions based on the rapid coverage of the entire crater floor. An active lava lake, also referred to as the “western lava lake” was centered within the uplifted area and was fed by a vent in the NE corner. Two small active lava lakes were located just SE from the W lava lake and in the E portion of the crater floor.

Figure 530. A compilation of thermal images taken of Kīlauea’s Halema’uma’u crater on 7 June 2023 (top left), 8 June 2023 (top right), 12 June 2023 (bottom left), and 16 June 2023 (bottom right). The initial high effusion rates that consisted of numerous lava fountains and lava flows that covered the entire crater floor began to decline and stabilize. A smaller area of active lava was detected in the SW part of the crater by 12 June. The colors of the thermal map represent temperature, with blue colors indicative of cooler temperatures and red colors indicative of warmer temperatures. Courtesy of USGS, HVO.

During 8-9 June the lava in the central lava lake had a thickness of approximately 1.5 m, based on measurements from a laser rangefinder. During 9-12 June the height of the lava fountains decreased to 9 m high. HVO reported that the previously active lava lake in the E part of the crater appeared stagnant during 10-11 June. The surface of the W lake rose approximately 1 m overnight during 11-12 June, likely due to the construction of a levee around it. Only a few small fountains were active during 12-13 June; the extent of the active lava had retreated so that all activity was concentrated in the SW and central parts of Halema’uma’u crater. Intermittent spattering from the vent on the SW wall was visible in overnight webcam images during 13-18 June. On the morning of 14 June a weak lava effusion originated from near the western eruptive vent, but by 15 June there were no signs of continued activity. HVO reported that other eruptive vents in the SW lava lake had stopped during this time, following several days of waning activity; lava filled the lake by about 0.5 m. Lava circulation continued in the central lake and no active lava was reported in the northern or eastern parts of the crater. Around 0800 on 15 June the top of the SW wall spatter cone collapsed, which was followed by renewed and constant spattering from the top vent and a change in activity from the base vent; several new lava flows effused from the top of the cone, as well as from the pre-existing tube-fed flow from its base. Accumulation of lava on the floor resulted in a drop of the central basin relative to the crater floor, allowing several overflows from the SW lava lake to cascade into the basin during the night of 15 June into the morning of 16 June.

Renewed lava fountaining was reported at the eruptive vent on the SW side of the crater during 16-19 June, which effused lava into the far SW part of the crater. This activity was described as vigorous during midday on 16 June; a group of observatory geologists estimated that the lava was consistently ejected at least 10 m high, with some spatter ejected even higher and farther. Deposits from the fountain further heightened and widened the spatter cone built around the original eruptive vent in the lower section of the crater wall. Multiple lava flows from the base of the cone were fed into the SW lava lake and onto the southwestern-most block from the 2018 collapse within Halema’uma’u on 17 June (figure 531); by 18 June they focused into a single flow feeding into the SW lava lake. On the morning of 19 June a second lava flow from the base of the eruptive cone advanced into the SW lava lake.

Figure 531. Nighttime photo of the upwelling area at the base of the spatter cone at Kīlauea’s Halema’uma’u crater on 17 June 2023. This upwelling feeds a lava flow that spreads out to the E of the spatter cone. Courtesy of M. Cappos, USGS.

Around 1600 on 19 June there was a rapid decline in lava fountaining and effusion at the eruptive vent on the SW side of the crater; vent activity had been vigorous up to that point (figure 532). Circulation in the lava lake also slowed, and the lava lake surface dropped by several meters. Overnight webcam images showed some previously eruptive lava still flowing onto the crater floor, which continued until those flows began to cool. By 21 June no lava was erupting in Halema’uma’u crater. Overnight webcam images during 29-30 June showed some incandescence from previously erupted lava flows as they continued to cool. Seismicity in the crater declined to low levels. Sulfur dioxide emission rates ranged 160-21,000 t/d throughout the month, the highest measurement of which was recorded on 8 June. On 30 June the VAL was lowered to Advisory (the second level on a four-level scale) and the ACC was lowered to Yellow. Gradual inflation was detected at summit tiltmeters during 19-30 June.

Figure 532. Photos showing vigorous lava fountaining and lava flows at Kīlauea’s Halema’uma’u crater at the SW wall eruptive vent on 18 June 2023 at 1330 (left). The eruption stopped abruptly around 1600 on 19 June 2023 and no more lava effusions were visible, as seen from the SW wall eruptive vent at 1830 on 19 June 2023 (right). Courtesy of M. Patrick, USGS.

Activity during July-August 2023. During July, the eruption paused; no lava was erupting in Halema’uma’u crater. Nighttime webcam images showed some incandescence from previously erupted lava as it continued to cool on the crater floor. During the week of 14 August HVO reported that the rate in seismicity increased, with 467 earthquakes of Mw 3.2 and smaller occurring. Sulfur dioxide emission rates remained low, ranging from 75-86 t/d, the highest of which was recorded on 10 and 15 August. On 15 August beginning at 0730 and lasting for several hours, a swarm of approximately 50 earthquakes were detected at a depth of 2-3 km below the surface and about 2 km long directly S of Halema’uma’u crater. HVO reported that this was likely due to magma movement in the S caldera region. During 0130-0500 and 1700-2100 on 21 August two small earthquake swarms of approximately 20 and 25 earthquakes, respectively, occurred at the same location and at similar depths. Another swarm of 50 earthquakes were recorded during 0430-0830 on 23 August. Elevated seismicity continued in the S area through the end of the month.

Activity during September 2023. Elevated seismicity persisted in the S summit with occasional small, brief seismic swarms. Sulfur dioxide measurements were relatively low and were 70 t/d on 8 September. About 150 earthquakes occurred during 9-10 September, and tiltmeter and Global Positioning System (GPS) data showed inflation in the S portion of the crater.

At 0252 on 10 September HVO raised the VAL to Watch and the ACC to Orange due to increased earthquake activity and changes in ground deformation that indicated magma moving toward the surface. At 1515 the summit eruption resumed in the E part of the caldera based on field reports and webcam images. Fissures opened on the crater floor and produced multiple minor lava fountains and flows (figure 533). The VAL and ACC were raised to Warning and Red, respectively. Gas-and-steam plumes rose from the fissures and drifted downwind. A line of eruptive vents stretched approximately 1.4 km from the E part of the crater into the E wall of the down dropped block by 1900. The lava fountains at the onset of the eruption had an estimated 50 m height, which later rose 20-25 m high. Lava erupted from fissures on the down dropped block and expanded W toward Halema’uma’u crater. Data from a laser rangefinder recorded about 2.5 m thick of new lava added to the W part of the crater. Sulfur dioxide emissions were elevated in the eruptive area during 1600-1500 on 10 September, measuring at least 100,000 t/d.

Figure 533. Photo of resumed lava fountain activity at Kīlauea’s Halema’uma’u crater on 10 September 2023. The main lava fountain rises approximately 50 m high and is on the E crater margin. Courtesy of USGS, HVO.

At 0810 on 11 September HVO lowered the VAL and ACC back to Watch and Orange due to the style of eruption and the fissure location had stabilized. The initial extremely high effusion rates had declined (but remained at high levels) and no infrastructure was threatened. An eruption plume, mainly comprised of sulfur dioxide and particulates, rose as high as 3 km altitude. Several lava fountains were active on the W side of the down dropped block during 11-15 September, while the easternmost vents on the down dropped block and the westernmost vents in the crater became inactive on 11 September (figure 534). The remaining vents spanned approximately 750 m and trended roughly E-W. The fed channelized lava effusions flowed N and W into Halema’uma’u. The E rim of the crater was buried by new lava flows; pahoehoe lava flows covered most of the crater floor except areas of higher elevation in the SW part of the crater. The W part of the crater filled about 5 m since the start of the eruption, according to data from a laser rangefinder during 11-12 September. Lava fountaining continued, rising as high as 15 m by the morning of 12 September. During the morning of 13 September active lava flows were moving on the N and E parts of the crater. The area N of the eruptive vents that had active lava on its surface became perched and was about 3 m higher than the surrounding ground surface. By the morning of 14 September active lava was flowing on the W part of the down dropped block and the NE parts of the crater. The distances of the active flows progressively decreased. Spatter had accumulated on the S (downwind) side of the vents, forming ramparts about 20 m high.

Figure 534. Photo of a strong lava fountain in the E part of Kīlauea’s Halema’uma’u crater taken on the morning of 11 September 2023. The lava fountains rise as high as 10-15 m. Courtesy of J. Schmith, USGS.

Vigorous spattering was restricted to the westernmost large spatter cone with fountains rising 10-15 m high. Minor spattering occurred within the cone to the E of the main cone, but HVO noted that the fountains remained mostly below the rim of the cone. Lava continued to effuse from these cones and likely from several others as well, traveled N and W, confined to the W part of the down-dropped block and the NE parts of Halema’uma’u. Numerous ooze-outs of lava were visible over other parts of the crater floor at night. Laser range-finder measurements taken of the W part of the crater during 14-15 September showed that lava filled the crater by 10 m since the start of the eruption. Sulfur dioxide emissions remained elevated after the onset of the eruption, ranging 20,000-190,000 t/d during the eruption activity, the highest of which occurred on 10 September.

Field crews observed the eruptive activity on 15 September; they reported a notable decrease or stop in activity at several vents. Webcam images showed little to no fountaining since 0700 on 16 September, though intermittent spattering continued from the westernmost large cone throughout the night of 15-16 September. Thermal images showed that lava continued to flow onto the crater floor. On 16 September HVO reported that the eruption stopped around 1200 and that there was no observable activity anywhere overnight or on the morning of 17 September. HVO field crews reported that active lava was no longer flowing onto Halema’uma’u crater floor and was restricted to a ponded area N of the vents on the down dropped block. They reported that spattering stopped around 1115 on 16 September. Nighttime webcam images showed some incandescence on the crater floor as lava continued to cool. Field observations supported by geophysical data showed that eruptive tremor in the summit region decreased over 15-16 September and returned to pre-eruption levels by 1700 on 16 September. Sulfur dioxide emissions were measured at a rate of 800 t/d on 16 September while the eruption was waning, and 200 t/d on 17 September, which were markedly lower compared to measurements taken the previous week of 20,000-190,000 t/d.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: Hawaiian Volcano Observatory (HVO), U.S. Geological Survey, PO Box 51, Hawai'i National Park, HI 96718, USA (URL: http://hvo.wr.usgs.gov/).

Tinakula is a remote 3.5 km-wide island in the Solomon Islands, located 640 km ESE of the capital, Honiara. The current eruption period began in December 2018 and has more recently been characterized by intermittent lava flows and thermal activity (BGVN 48:06). This report covers similar activity during June through November 2023 using satellite data.

During clear weather days (20 July, 23 September, 23 October, and 12 November), infrared satellite imagery showed lava flows that mainly affected the W side of the island and were sometimes accompanied by gas-and-steam emissions (figure 54). The flow appeared more intense during July and September compared to October and November. According to the MODVOLC thermal alerts, there were a total of eight anomalies detected on 19 and 21 July, 28 and 30 October, and 16 November. Infrared MODIS satellite data processed by MIROVA (Middle InfraRed Observation of Volcanic Activity) detected a small cluster of thermal activity occurring during late July, followed by two anomalies during August, two during September, five during October, and five during November (figure 55).

Figure 54. Infrared (bands B12, B11, B4) satellite images showed lava flows mainly affecting the W flank of Tinakula on 20 July 2023 (top left), 23 September 2023 (top right), 23 October 2023 (bottom left), and 12 November 2023 (bottom right). Some gas-and-steam emissions accompanied this activity. Courtesy of Copernicus Browser.

Figure 55. Low-power thermal anomalies were sometimes detected at Tinakula during July through November 2023, as shown on this MIROVA plot (Log Radiative Power). A small cluster of thermal anomalies were detected during late July. Then, only two anomalies were detected during August, two during September, five during October, and five during November. Courtesy of MIROVA.

Geologic Background. The small 3.5-km-wide island of Tinakula is the exposed summit of a massive stratovolcano at the NW end of the Santa Cruz islands. It has a breached summit crater that extends from the summit to below sea level. Landslides enlarged this scarp in 1965, creating an embayment on the NW coast. The Mendana cone is located on the SE side. The dominantly andesitic volcano has frequently been observed in eruption since the era of Spanish exploration began in 1595. In about 1840, an explosive eruption apparently produced pyroclastic flows that swept all sides of the island, killing its inhabitants. Recorded eruptions have frequently originated from a cone constructed within the large breached crater. These have left the upper flanks and the steep apron of lava flows and volcaniclastic debris within the breach unvegetated.

Information Contacts: MIROVA (Middle InfraRed Observation of Volcanic Activity), a collaborative project between the Universities of Turin and Florence (Italy) supported by the Centre for Volcanic Risk of the Italian Civil Protection Department (URL: http://www.mirovaweb.it/); Hawai'i Institute of Geophysics and Planetology (HIGP) - MODVOLC Thermal Alerts System, School of Ocean and Earth Science and Technology (SOEST), Univ. of Hawai'i, 2525 Correa Road, Honolulu, HI 96822, USA (URL: http://modis.higp.hawaii.edu/); Copernicus Browser, Copernicus Data Space Ecosystem, European Space Agency (URL: https://dataspace.copernicus.eu/browser/).

An "underwater explosion" and "pumice swirl" ~30 km wide were reported at 19.10°S, 175.41°E (200 km SW of Nadi, Fiji) on 16 October at 1058 from Air Pacific flight 914 (Nadi to Sydney, Australia). At 1450, the crew of a second Air Pacific flight (enroute from Auckland, New Zealand) noted pumice visible in the sea 130 km from Nadi (on the W coast of Fiji's largest island, Viti Levu) [see also 15:11-12].

Although no historical volcanism has been reported near the observation site, the area is near a spreading center described by Gill and Whelan (1989). Another possible source of the pumice is Monowai Seamount (25.92°S, 177.15°W), 1,100 km to the ESE, where submarine activity was observed from the HMNZS Tui on 13 August. On 30 May-18 June and 5-7 September, the Polynesian Seismic Net recorded T-phase activity, centered in the Monowai area, that had characteristics typical of shallow submarine eruptions.

Reference. Gill, J., and Whelan, P., 1989, Early rifting of an oceanic island arc (Fiji) Produced shoshonitic to tholeiitic basalts: JGR, v. 94, no. B4, p. 4561-4578.

Geologic Background. Reports of floating pumice from an unknown source, hydroacoustic signals, or possible eruption plumes seen in satellite imagery.

Information Contacts: J. Latter, DSIR Geophysics, Wellington.

A seven-member team of USGS volcanologists visited the CNMI 24 September-6 October at the request of the Office of Civil Defense. The following is from a report by Richard Moore.

"On Agrigan, the team established a new EDM network within the summit caldera, and hope to remeasure it in 1991. At that time, geologic investigations terminated by tropical storm Hattie on 2 October 1990 would be continued.

"A revolving drum seismograph operated continuously 28 September-1 October at a village near the coast, and a portable seismograph operated intermittently 28-29 September at several sites on the caldera floor, recorded no sustained microearthquake activity or volcanic tremor on Agrigan.

"The team discovered a boiling hot spring, associated terrace deposits, and solfataras at the 1917 eruption vent (Agrigan's most recent) on the floor of the 1.5-km-diameter caldera. Steam was being emitted from several areas at the base of the caldera wall. Temperatures of the boiling hot spring and 25 solfataras measured by thermocouple were all 98°C. Water from the hot spring had a pH of 2.0. Chemical analysis of the water is in progress. Several measurements (using Kitagawa and Draeger tubes) of the abundances of various gases emitted by the solfataras are summarized in table 1.

Table 1. Range in compositions of gas samples collected at Agrigan, September-October 1990. Courtesy of Richard Moore.

"The team found no evidence of new fuming on Agrigan (suggested by reports in August and cause of the island's evacuation; 15:7). Hot spring terraces composed of siliceous sinter covered an area of ~20 x 7 m² below the boiling hot spring. The terraces are now mostly dry, with current deposition of silica limited to a few square meters adjacent to the hot spring, suggesting that activity was more vigorous sometime in the past. However, fluctuations in the volume of flow from the spring may occur as a result of seasonal variations in rainfall."

Geologic Background. The highest of the Marianas arc volcanoes, Agrigan contains a 500-m-deep, flat-floored caldera. The elliptical island is 8 km long; its summit is the top of a massive 4000-m-high submarine volcano. Deep radial valleys dissect the flanks of the thickly vegetated stratovolcano. The elongated caldera is 1 x 2 km wide and is breached to the NW, from where a prominent lava flow extends to the coast and forms a lava delta. The caldera floor is surfaced by fresh-looking lava flows and also contains two cones that may have formed during the only historical eruption in 1917. This eruption deposited large blocks and 3 m of ash and lapilli on a village on the SE coast, prompting its evacuation.

Information Contacts: R. Moore, USGS; R. Koyanagi and M. Sako, HVO.

Minami-dake cone exploded once in October, on the 4th, following 37 days of quiescence. No additional explosions had occurred as of 14 November. The October explosion was the 113th of 1990 and caused no damage. The maximum ash plume height, 3,500 m above the crater, occurred during a quiet emission on the 2nd. A monthly total of 130 g/m² of ash was deposited 10 km W of the crater . . . .

Geologic Background. The Aira caldera in the northern half of Kagoshima Bay contains the post-caldera Sakurajima volcano, one of Japan's most active. Eruption of the voluminous Ito pyroclastic flow accompanied formation of the 17 x 23 km caldera about 22,000 years ago. The smaller Wakamiko caldera was formed during the early Holocene in the NE corner of the caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim and built an island that was joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4,850 years ago, after which eruptions took place at Minamidake. Frequent eruptions since the 8th century have deposited ash on the city of Kagoshima, located across Kagoshima Bay only 8 km from the summit. The largest recorded eruption took place during 1471-76.

Information Contacts: JMA.

The October total of recorded earthquakes was 202, an increase from 144 in September. Steam emission remained unchanged, reaching 600 m height during October. Seismicity has been at high levels for the last 3 years, since the eruption of January-February 1988.

Geologic Background. Akan is a 13 x 24 km caldera located immediately SW of Kussharo caldera in eastern Hokkaido. The elongated, irregular outline of the caldera rim reflects its incremental formation during major explosive eruptions from the early to mid-Pleistocene. There are four post-caldera stratovolcanoes, three at the SW end of the caldera and the other on the NE side. Conical Oakandake was frequently active during the Holocene. The 1-km-wide Nakamachineshiri crater of Meakandake was formed during a major pumice-and-scoria eruption about 13,500 years ago. The Meakandake group, composed of nine overlapping cones E of Lake Akan, has produced mild phreatic eruptions since the beginning of the 19th century. The main cone of Meakandake proper has a triple crater at its summit. Although recorded eruptions at Meakandake have consisted of minor phreatic explosions, four major magmatic eruptions with pyroclastic flows have also occurred during the Holocene.

Information Contacts: JMA.

A seven-member team of USGS volcanologists visited the CNMI 24 September-6 October at the request of the Office of Civil Defense. The team installed [a seismic station] on Anatahan . . .; data are telemetered to Saipan and recorded at Civil Defense headquarters. Quoted material below is from a report by Richard Moore.

"No significant earthquakes have occurred on Anatahan since installation of the seismic and telemetry system on 29 September. Reoccupation of the EDM network established in April showed small changes in line lengths, in accord with the lack of local seismicity. The lake in the eastern crater . . . was full again on 1 October 1990. The water was discolored, but not boiling."

Felt seismicity 30 March-1 April and turbulence in the crater lake of Anatahan prompted the evacuation of Anatahan Island. The island has remained uninhabited since 4 April.

Geologic Background. The elongate, 9-km-long island of Anatahan in the central Mariana Islands consists of a large stratovolcano with a 2.3 x 5 km compound summit caldera. The larger western portion of the caldera is 2.3 x 3 km wide, and its western rim forms the island's high point. Ponded lava flows overlain by pyroclastic deposits fill the floor of the western caldera, whose SW side is cut by a fresh-looking smaller crater. The 2-km-wide eastern portion of the caldera contained a steep-walled inner crater whose floor prior to the 2003 eruption was only 68 m above sea level. A submarine cone, named NE Anatahan, rises to within 460 m of the sea surface on the NE flank, and numerous other submarine vents are found on the NE-to-SE flanks. Sparseness of vegetation on the most recent lava flows had indicated that they were of Holocene age, but the first historical eruption did not occur until May 2003, when a large explosive eruption took place forming a new crater inside the eastern caldera.

Information Contacts: R. Moore, USGS; R. Koyanagi and M. Sako, HVO.

Strombolian activity and lava production continued, and occasional small nuées ardentes were observed during October. An average of 18 explosions (with three or four strong events) were recorded daily (figure 31) . . . . Explosions ejected blocks and bombs that were deposited to ~1 km from the crater, and produced ash columns 1 km high. White and occasionally bluish gas plumes were carried NW, W, and SW; acid rain continued to cause damage. At the end of October, two blocky lava flows were observed extending down the NW and SW flanks.

Figure 31. Number of explosions/day (top) and hours/day of tremor (bottom) recorded at Arenal by the Univ Nacional, October 1990.

Tremor was nearly continuous, averaging 23 hours/day (figure 31), and increased in intensity towards the end of the month as the number of explosions decreased. Deformation measurements indicated continued deflation of the volcano since 1986, with occasional pulses of inflation during explosive stages (figure 32). The area around the active summit crater (C) continued to grow by accumulation of pyroclastic materials. The rate of this accumulation, which totaled 9 m from early 1987 through May 1988, has decreased (figure 33).

Figure 32. Electronic tilt measurements at Arenal, January-October 1990. Courtesy of the Univ Nacional.

Figure 33. Growth of the active crater at Arenal, April 1987-August 1990. Courtesy of the Univ Nacional.

Geologic Background. Conical Volcán Arenal is the youngest stratovolcano in Costa Rica and one of its most active. The 1670-m-high andesitic volcano towers above the eastern shores of Lake Arenal, which has been enlarged by a hydroelectric project. Arenal lies along a volcanic chain that has migrated to the NW from the late-Pleistocene Los Perdidos lava domes through the Pleistocene-to-Holocene Chato volcano, which contains a 500-m-wide, lake-filled summit crater. The earliest known eruptions of Arenal took place about 7000 years ago, and it was active concurrently with Cerro Chato until the activity of Chato ended about 3500 years ago. Growth of Arenal has been characterized by periodic major explosive eruptions at several-hundred-year intervals and periods of lava effusion that armor the cone. An eruptive period that began with a major explosive eruption in 1968 ended in December 2010; continuous explosive activity accompanied by slow lava effusion and the occasional emission of pyroclastic flows characterized the eruption from vents at the summit and on the upper western flank.

Information Contacts: J. Barquero, V. Barboza, E. Fernández, and R. van der Laat, OVSICORI; G. Soto and R. Barquero, ICE.

High seismicity . . . continued through October, but declined slightly from previous months. A monthly total of 105 earthquakes and 19 tremor episodes were recorded, declining from 206 and 24 respectively in September. Seismicity was continuing to decline as of 14 November.

Geologic Background. Asamayama, Honshu's most active volcano, overlooks the resort town of Karuizawa, 140 km NW of Tokyo. The volcano is located at the junction of the Izu-Marianas and NE Japan volcanic arcs. The modern Maekake cone forms the summit and is situated east of the remnant of an older andesitic volcano, Kurofuyama, which was destroyed by a late-Pleistocene landslide about 20,000 years before present (BP). Growth of a dacitic shield volcano was accompanied by pumiceous pyroclastic flows, the largest of which occurred about 14,000-11,000 BP, and by growth of the Ko-Asamayama lava dome on the east flank. Maekake, capped by the Kamayama pyroclastic cone that forms the present summit, is probably only a few thousand years old and has observed activity dating back at least to the 11th century CE. Maekake has had several major Plinian eruptions, the last two of which occurred in 1108 (Asamayama's largest Holocene eruption) and 1783 CE.

Information Contacts: JMA.

No ash was erupted during October . . . . Crater 1 . . . continued to emit white steam that rose to 900 m above the crater. Weak ash emission was observed on 13 November, and glow from vents on the crater bottom was seen during fieldwork that night. The amplitude and number of volcanic tremor episodes increased in late October, reaching levels similar to September's and continuing at those levels through early November.

Geologic Background. The 24-km-wide Asosan caldera was formed during four major explosive eruptions from 300,000 to 90,000 years ago. These produced voluminous pyroclastic flows that covered much of Kyushu. The last of these, the Aso-4 eruption, produced more than 600 km3 of airfall tephra and pyroclastic-flow deposits. A group of 17 central cones was constructed in the middle of the caldera, one of which, Nakadake, is one of Japan's most active volcanoes. It was the location of Japan's first documented historical eruption in 553 CE. The Nakadake complex has remained active throughout the Holocene. Several other cones have been active during the Holocene, including the Kometsuka scoria cone as recently as about 210 CE. Historical eruptions have largely consisted of basaltic to basaltic andesite ash emission with periodic strombolian and phreatomagmatic activity. The summit crater of Nakadake is accessible by toll road and cable car, and is one of Kyushu's most popular tourist destinations.

Information Contacts: JMA.

The following is a report from Páll Einarsson. "An intense earthquake swarm started on the N part of the Reykjanes Ridge on 30 October (figure 3). The first event recorded by seismographs in Iceland occurred at 1021 and had a magnitude of 3.5. Smaller events occurred at 1052 and 1152. The epicenters cannot be located accurately, but appear to be near 63°N, or ~180 km SW of Reykjavík. At 1229, activity in this area increased dramatically, and for the next 19 hours hundreds of earthquakes were recorded. The largest events approached M 5 and at least 14 were of M 4 or larger. For large parts of this time, the seismographs showed continuous motion due to the dense sequence of small and large earthquakes. However, motion resembling volcanic tremor could not be identified.

Figure 3. Sketch map showing the approximate location of the earthquake swarms near 63°N and 63.7°N on the Reykjanes Ridge. After Perry and others (1980).

"The intense activity came to a rather abrupt halt at about 0800 on 31 October, but activity at a lower level continued, gradually diminishing. A temporary increase occurred 5-6 November (figure 4).

Figure 4. Number of earthquakes from the Reykjanes Ridge swarm area recorded daily at the Háhryggur seismic station in SW Iceland. Detection threshold of the station for this area is near magnitude 2.5. Courtesy of P. Einarsson.

"A second swarm started 3 November closer to Iceland, near 63.7°N. It began at 1426 with an event of M 3.8. Thirty events were recorded in the area that day, and five events the following day. This swarm was small and short-lived, and probably unrelated to the first one.

"The question of whether or not the swarm at 63°N is related to intrusive or extrusive activity at the sea floor cannot be answered from the available seismic data. Earthquake swarms are common on the Reykjanes Ridge and its landward continuation on the Reykjanes Peninsula. None of the recent swarms on the peninsula have been accompanied by eruptive activity, and they do not resemble the seismic swarms that accompany magmatic intrusions in the Krafla area along the rift zone in NE Iceland. Intrusion tremor, commonly observed at Krafla, has not been recorded during the swarms on the Reykjanes Peninsula despite a relatively dense seismograph network there.

"The current swarm at 63°N is unusual in both intensity and duration. The large distance to the nearest seismograph (roughly 150 km) means that intrusion and extrusion tremor could have occurred without being observed. Some characteristics of the swarm - for example the slow beginning, the high density of events at its culmination, and the abrupt end - in some respects resemble those of some of the Krafla eruptive events. If an analogy is drawn, one could speculate that the intense part of the swarm accompanied an intrusion of magma and that a dyke propagated for 19 hours. If an eruption occurred, it most likely began at about 0800 on 31 October when the seismic activity suddenly dropped to a lower level. Eruptive activity may have ended on 5 November, when there was a temporary increase in earthquakes."

A U.S. Navy P3 aircraft overflew the swarm area on 2 November between 1000 and 1400. Five sonobuoys were deployed; the central sonobuoy (at 63°15'40"N, 24°11'52") detected 50 Hz noise at 97 dB; sound intensity at four others (~ 9 km N, S, E, and W) was about 85 dB.

The following is from Jón Olafsson. "In response to the earthquake swarm on the Reykjanes Ridge, an international team assembled in Reykjavík on 2 November, sailing at midnight on the RV Bjarni Saemundsson of Iceland's Marine Research Institute.

"Investigations were concentrated on the area of the ridge crest between 62.9°N and 63.3°N, where the water depth ranged from 100 to 500 m. The ship is equipped with echosounders, sonar, and a CTD (Conductivity-Temperature-Depth) + light transmissometer with a rosette for water column sampling. On board were sonobuoys (provided by the U.S. Navy), equipment for analysis of dissolved silica, and a bottom dredge. Signs of possible eruptive activity were sought by deployment of sonobuoys, and water sampling on sections along and to the sides of the ridge crest. No signs could be detected of explosive activity of the type that created Surtsey in 1963, which would have given rise to extensive silica anomalies. However, the water above a segment of the ridge centered at 63.1°N showed some anomalous properties, particularly decreased light transmissivity and water column stability. A hydrothermal region was discovered near the summit of a seamount in this region, but has most likely been there beforehand, judging from previous information from fishermen. On the afternoon of 5 November, two nearby earthquake shocks were felt on the ship. Reports of earthquakes also came from deep-sea trawlers in this region, confirming that the research effort was in the region of seismic activity. Twelve dredge hauls brought up some fresh basalts but none were newly erupted.

"The ship returned to Reykjavík on 6 November with water samples for analysis of helium isotopes, manganese, methane, and hydrogen. Processing of these samples and the instrument records will be conducted in the UK, Iceland, and USA."

Locations of four of the largest earthquakes in the swarm were determined at the U.S. National Earthquake Information Center on 4 November (table 2). Arrival time values were obtained from the NEIC database, and from two seismic stations in Iceland (~150 and 250 km from the epicentral area), reported by Páll Einarsson. The following is from Eric Bergman.

Table 2. Relocations of four large earthquakes from the Reykjanes Ridge swarm, 30-31 October 1990. Courtesy of Eric Bergman.

"The swarm events were relocated as part of a multiple-event relocation analysis for earthquakes on the Reykjanes Ridge between 62.5°N and 63.5°N. In all, 30 well-recorded earthquakes were relocated, using the hypocentroidal decomposition technique. Locations were estimated using the 1968 Herrin tables for P-wave travel times, except for the two Icelandic stations. Because the Herrin tables assume a thick continental crust, the theoretical travel times are longer than the true travel times for these phases, which propagate predominantly as refracted waves along the oceanic Moho with a velocity of around 8 km/s. Theoretical travel times for the two Icelandic stations were calculated by dividing the epicentral distance by 8.0 km/s. This admittedly crude estimate is a substantial improvement over the standard tables and is in good agreement with other data. No station corrections were used in the relocation. All focal depths were fixed at 10 km, consistent with many studies of the depth distribution of mid-ocean ridge seismicity. Further work is needed to refine this type of analysis, and it should be recognized that the locations reported here are to some extent biased by these assumptions. The results of the analysis will also change as more arrival data accumulate."

Reference. Perry, R.K., Fleming, H.S., Cherkis, N.Z., Feden, R.H., and Vogt, P.R., 1980, Bathymetry of the Norwegian-Greenland and western Barents Seas: U.S. Naval Research Laboratory-Acoustics Division, map and chart series MC-21

Geologic Background. The Eldey volcanic system is located on the northernmost part of the Reykjanes Ridge and is submarine with the exception of Eldey Island and the skerries (small rocky islands) Eldeyjardrangur, Geirfugladrangur, and Geirfuglasker. Maximum water depth within the system is about 250 m. Characteristic activity consists of explosive submarine basaltic eruptions. Six small eruptions have been located within this system during the last 1,100 years, the last occurring in 1926 CE. Bulletin reports that are included here cover a larger area of the Reykjanes Ridge south of Iceland without a clear source or enough evidence for a separate volcano entry.

Information Contacts: P. Einarsson, Univ of Iceland; J. Olafsson, Marine Research Institute; E. Bergman, NEIC; P. Vogt, Naval Research Laboratory; T. Stroh, Univ of Washington. Scientific team on the RV Bjarni Saemundsson: Jón Olafsson, Icelandic Marine Research Institute (leader); Johnson R. Cann, Univ of Leeds (deputy leader); Kjartan Thors, S. Kristmansson, and Jón Benjaminsson, Icelandic Marine Research Institute; David Francis, Univ of Leeds; Cherry Walker, Univ of Durham; and Marie de Angelis, State Univ of New York, Stony Brook. Sponsoring Institutions: Icelandic Marine Research Institute; Natural Environmental Research Council, UK; and RIDGE Office, National Science Foundation, USA.

The following, from IIV, covers April-September 1990.

Summit crater activity. Eruptive activity was at Bocca Nuova and La Voragine, while only degassing was observed at the SE and NE subterminal craters. At the beginning of July, the mild degassing that had characterized the central vents during previous months evolved to Strombolian activity, sporadically ejecting juvenile products that reached the rim of Bocca Nuova. An intense eruptive episode began at Bocca Nuova on 7 August at about 1130, lasting for ~ 40 minutes. Strong Strombolian activity alternated with lava fountaining, producing a thick deposit (~10 cm maximum) of vesiculated scoria and Pele's Hair that accumulated on the N and NW sides of the crater rim. Wind carried lighter tephra 10 km NE, where it reached the villages of Vena and Presa (figure 38). Weak Strombolian activity followed, stopping early the next day. During the same period, La Voragine was limited to moderate Strombolian activity that stopped on 8 August. Increased tremor amplitude was recorded during the night of 7-8 August (see below), then tremor declined to low levels.

Figure 38. Sketch map of Etna, showing tephra dispersal during the 7 August Strombolian activity and lava fountaining.

Collapse of part of the wall between Bocca Nuova and La Voragine 9-10 August produced a landslide deposit that covered pre-existing vents on Bocca Nuova's floor. This deposit was soon penetrated by explosive activity, which formed two new vents characterized by weak Strombolian activity.

Throughout this period, activity at the SE subterminal crater remained limited to degassing. However, a considerable enlargement of the vent was observed in June, accompanied by strong incandescence of the inner walls. The temperature of the fumarolic gas, measured 8 August, reached 615°C. By the end of August, a larger degassing vent (~ 10 m across) had formed on the crater floor where fumarolic activity had previously been most intense. This vent produced only strong gas emission, without explosive episodes. Activity at the summit craters was limited to degassing of variable intensity in September.

Fault seismicity. Seismicity alternated between phases of relative quiet (April-June, September) and moderate to intense activity (July-August).

Moderate activity April-June was broken by four seismic sequences that occurred 25 April, 17-18 May, and 1-2 and 30 June (figure 39b). Seismic energy release (figure 39a) was also moderate (maximum M 3.0 on 17 May) and a total of 101 shocks of M >= 1 were recorded. The April-May seismicity mainly affected the W sector of the volcano, with seismic activity moving to the E (Valle del Bove) and NE flanks in June (figure 40). Average focal depths were ~15 km, except for the 1 June sequence, which had a focal zone at a depth of <=10 km (figure 41).

Figure 39. Top a): cumulative seismic energy release at Etna's S flank ESP station (figure 38) in the square-root of ergs (solid line) and radial component of ground tilt at nearby borehole station SPC (dashed line with squares). Bottom b): number of earthquakes (M³1) recorded at ESP station; April-September 1990.

Figure 40. Epicenters of earthquakes (M >= 2.5) at Etna, April-September 1990.

Figure 41. Foci of earthquakes shown in figure 40, projected onto a N-S cross-section passing through Etna's summit.

During the next two months, the most significant seismic episodes took place on 3 and 8 July, and 27 August. These sequences plus a general increase in background activity caused a significant change in the slope of the cumulative strain release curve. Energy associated with single events remained moderate, never exceeding M 3.1. The total of 148 events recorded in July decreased to 97 in August. The upper NE flank (10-25 km depth) and the Valle del Bove (6-12 km depth) were the areas most affected.

Another seismic sequence (78 events of M >= 1) occurred on the NW flank on 3 September; the average calculated focal depth was about 24 ± 4 km. Seismic activity then returned to moderate levels for the rest of the month.

Volcanic tremor. During April, May, and the first part of June, volcanic tremor amplitude recorded at a reference station (ESP) on the S flank fluctuated from low to moderate values (7-20 mV/_Hz). Beginning in the second half of June, an amplitude increase was observed (20-30 mV/_Hz) that lasted until 7 August. During the night of 7-8 August, a sudden further increase in tremor amplitude coincided with the violent Strombolian activity from Bocca Nuova (see above). After this episode, tremor amplitude returned to low levels (5-8 mV/_Hz), remaining at similar values until the end of September.

Ground deformation. EDM measurements were performed on two geodimeter networks, on the S and SW flanks. The southern network was measured in June, about a year after the last measurement in May 1989. The area covered by the network includes part of the main fracture system that affected the SE flank during the September-October 1989 eruption (14:8-10). Comparisons between May 1989 and June 1990 data showed significant distance variations, mostly for lines in the higher altitude sector of the network. The resulting deformation pattern was characterized by a significant areal contraction. The deformation ellipse was strongly polarized with the minimum extension axis (contraction) trending approximately N29°E. The southwestern EDM network was reoccupied in July, showing only minor slope distance variations from the previous measurements in June 1989. A weak areal contraction was observed. The calculated deformation ellipse had a minimum extension axis (contraction) striking approximately N7°E.

Tilt data were collected at a biaxial borehole station (SPC) on the S flank, close to the ESP seismic station. Recording was interrupted early April-early June by vandalism. The radial component indicated continuous inflation of the volcanic edifice from the beginning of July until early September, closely paralleling the seismic strain release (figure 39a). During the same period, the tangential component remained nearly flat, showing fluctuations within the confidence limit of about ± 2 µrads.

Geologic Background. Mount Etna, towering above Catania on the island of Sicily, has one of the world's longest documented records of volcanism, dating back to 1500 BCE. Historical lava flows of basaltic composition cover much of the surface of this massive volcano, whose edifice is the highest and most voluminous in Italy. The Mongibello stratovolcano, truncated by several small calderas, was constructed during the late Pleistocene and Holocene over an older shield volcano. The most prominent morphological feature of Etna is the Valle del Bove, a 5 x 10 km caldera open to the east. Two styles of eruptive activity typically occur, sometimes simultaneously. Persistent explosive eruptions, sometimes with minor lava emissions, take place from one or more summit craters. Flank vents, typically with higher effusion rates, are less frequently active and originate from fissures that open progressively downward from near the summit (usually accompanied by Strombolian eruptions at the upper end). Cinder cones are commonly constructed over the vents of lower-flank lava flows. Lava flows extend to the foot of the volcano on all sides and have reached the sea over a broad area on the SE flank.

Information Contacts: IIV.

"Photographs taken by Civil Defense personnel in early August 1990 from a fixed-wing airplane showed vigorous fuming."

Geologic Background. The small 2-km-wide island of Farallon de Pajaros (also known as Uracas) is the northernmost and most active volcano of the Mariana Islands. Its relatively frequent eruptions dating back to the mid-19th century have caused the andesitic volcano to be referred to as the "Lighthouse of the western Pacific." The symmetrical, sparsely vegetated summit is the central cone within a small caldera cutting an older edifice, remnants of which are seen on the SE and southern sides near the coast. Flank fissures have fed lava flows that form platforms along the coast. Eruptions have been recorded from both summit and flank vents.

Information Contacts: R. Moore, USGS; R. Koyanagi and M. Sako, HVO.

Seismicity decreased slightly during October. One high-frequency earthquake (M 2.8) was felt in Pasto (10 km E) on 5 October. Earthquakes were centered in distinct zones: under, NE, and E of the crater. Low-frequency earthquakes remained at low levels of occurrence and energy. Spasmodic tremor was variable, and was associated with ash emissions on 17 and 18 October.

Deformation measurements showed little change, although dry tiltmeters continued to show low levels of deformation with an inflationary trend. Electronic tilt 2 km E of the crater (Peladitos station) showed changes of 1-8 Nrad.

The SO₂ flux, measured by COSPEC, decreased slightly from 2,378 t/d on 1 October, to 1,994 t/d at the end of the month.

Geologic Background. Galeras, a stratovolcano with a large breached caldera located immediately west of the city of Pasto, is one of Colombia's most frequently active volcanoes. The dominantly andesitic complex has been active for more than 1 million years, and two major caldera collapse eruptions took place during the late Pleistocene. Long-term extensive hydrothermal alteration has contributed to large-scale edifice collapse on at least three occasions, producing debris avalanches that swept to the west and left a large open caldera inside which the modern cone has been constructed. Major explosive eruptions since the mid-Holocene have produced widespread tephra deposits and pyroclastic flows that swept all but the southern flanks. A central cone slightly lower than the caldera rim has been the site of numerous small-to-moderate eruptions since the time of the Spanish conquistadors.

Information Contacts: INGEOMINAS-OVP.

"Weak fumarolic emissions were noted from the SE side of the W summit crater during an overflight in early September. No unusual activity was observed."

Geologic Background. This little-known volcano is one of several major calderas on the island of New Britain. The 10 x 12 km Hargy caldera, whose floor is 150 m above sea level, contains an inner caldera with a steep west-facing wall. A caldera lake on the SE side drains through a narrow gap in the northern caldera wall. The latest caldera-forming eruption of Hargy volcano took place about 11,000 years ago. The dacitic Galloseulo lava cone rises above and partially overtops the western rim of the caldera. A double crater occupies a larger 700-m-wide crater. Numerous small eruptions have taken place at Galloseulo over the past 7000 years, the last occurring about 1000 years ago.

Information Contacts: C. McKee and I. Itikarai, RVO.

Activity decreased, following ash emissions on 4 October . . . . Seismicity and steam emission declined rapidly following the 4 October activity, and no subsequent ash emissions had occurred as of 14 November. No tremor episodes were recorded during October.

Geologic Background. Izu-Oshima volcano in Sagami Bay, east of the Izu Peninsula, is the northernmost of the Izu Islands. The broad, low stratovolcano forms an 11 x 13 km island constructed over the remnants of three older dissected stratovolcanoes. It is capped by a 4-km-wide caldera with a central cone, Miharayama, that has been the site of numerous recorded eruptions datining back to the 7th century CE. More than 40 cones are located within the caldera and along two parallel rift zones trending NNW-SSE. Although it is a dominantly basaltic volcano, strong explosive activity has occurred at intervals of 100-150 years throughout the past few thousand years. A major eruption in 1986 produced spectacular lava fountains up to 1,600 m high and a 16-km-high eruption column; more than 12,000 people were evacuated from the island.

Information Contacts: JMA.

Lava . . . moved down the S flank and continued to enter the ocean during October (figure 73). At the beginning of the month, lava from a persistent tube system along the E side of the flow field formed a coastal front 750 m wide (in the Kaimu area; figure 72). The E margin of the flow advanced along the coast in front of the Kalapana Shores subdivision, which had been evacuated by the beginning of October. Lava breakouts destroyed two homes in the subdivision on 7 October. By mid-October, lava had nearly reached the W edge of the 1750 flow, > 500 m E of the former Kaimu Bay, and ocean entries were active along a front 1 km wide. A lava breakout from the flow's main ("Woodchip") tube at ~40 m elevation destroyed a home in the upper Kalapana Gardens subdivision on 15 October. A low to moderate number of intermediate-depth long-period earthquakes were evident from the beginning of the month. These peaked 6-7 October when almost 100 were counted, then subsided about 10 October.

Figure 73. Lava produced by Kīlauea's East rift zone eruption, 1983-90. Arrows indicate paths of recent flows, with the "Woodchip" flow on the E side of the lava field, the "Royal Gardens" flow on the W side. Crosses at the coast mark new lava entries into the sea in the Wahaula area.

Flows that emerged from the inflated area at the base of Kupaianaha's shield in early October moved down the W side of the flow field, the most active as channelized aa that reached relatively level terrain near the coast on 12 October. By the 15th, the distal end of the flow was pahoehoe and had advanced to below 30 m elevation. Less-active lobes of the same flow were observed upslope in Royal Gardens subdivision, but did not destroy any of the subdivision's remaining homes. Lava reached the ocean on 20 October along the W side of the flow field (near Kupapau Point) and was moving through newly formed tubes by the 22nd. A large lava breakout on the E side of the flow field destroyed another home in Kalapana Gardens on 19 October, while activity declined at the coast to only two small entries.

On 22 October a flurry of long-period events occurred between 0500 and 0800, averaging ~30/hour. Long-period seismicity increased again at about 2100, accompanied by summit tremor. The number of intermediate-depth long-period earthquakes, which had resumed in mid-October, peaked at about 380 on the 23rd. That day, lava movement viewed through a skylight at ~350 m elevation was slower than it had been in the past several weeks, and a decline in activity at all coastal entries was evident by 24 October. However, a new lobe from the E side of the flow field reached the ocean on the 24th (at Hakuma Point), an entry that remained intermittently active through early November, and lava that had moved through Royal Gardens in mid-October entered the sea 29 October on the W side of the flow field (just E of Wahaula). By 31 October, lava was flowing into the ocean at several points in the Wahaula area along a front 700 m wide, and the flow feeding this entry was moving through tubes upslope. . . . Intermediate-depth long-period seismicity declined from its peak on 23 October to a few tens of events/day at the end of the month, and dropped further in early November.

A lava pond was seen on 31 October in the base of Pu`u `O`o Crater, where a pair of lava ponds had been active in August and September. Lava at Kupaianaha remained deep in the vent and covered by a frozen crust.

Geologic Background. Kilauea overlaps the E flank of the massive Mauna Loa shield volcano in the island of Hawaii. Eruptions are prominent in Polynesian legends; written documentation since 1820 records frequent summit and flank lava flow eruptions interspersed with periods of long-term lava lake activity at Halemaumau crater in the summit caldera until 1924. The 3 x 5 km caldera was formed in several stages about 1,500 years ago and during the 18th century; eruptions have also originated from the lengthy East and Southwest rift zones, which extend to the ocean in both directions. About 90% of the surface of the basaltic shield volcano is formed of lava flows less than about 1,100 years old; 70% of the surface is younger than 600 years. The long-term eruption from the East rift zone between 1983 and 2018 produced lava flows covering more than 100 km2, destroyed hundreds of houses, and added new coastline.

Information Contacts: T. Moulds and P. Okubo, HVO.

Seismicity has remained at high levels... (figure 4). During October, 213 earthquakes (up from 184 in September) and 29 tremor episodes... were recorded. Tremor amplitudes were similar to previous months. Earthquakes were centered 1 km E of... Yugama Crater. Seismicity remained similar as of 14 November. No changes in surface activity were observed.

Figure 4. Monthly number of earthquakes at Kusatsu-Shirane, January 1978-October 1990. Arrows at top of figure mark eruptions. Courtesy of JMA.

Geologic Background. The Kusatsu-Shiranesan complex, located immediately north of Asama volcano, consists of a series of overlapping pyroclastic cones and three crater lakes. The andesitic-to-dacitic volcano was formed in three eruptive stages beginning in the early to mid-Pleistocene. The Pleistocene Oshi pyroclastic flow produced extensive welded tuffs and non-welded pumice that covers much of the E, S, and SW flanks. The latest eruptive stage began about 14,000 years ago. Historical eruptions have consisted of phreatic explosions from the acidic crater lakes or their margins. Fumaroles and hot springs that dot the flanks have strongly acidified many rivers draining from the volcano. The crater was the site of active sulfur mining for many years during the 19th and 20th centuries.

Information Contacts: JMA.

"Activity returned to a low level in October . . . . Emissions from Crater 3 consisted mainly of occasional weak to moderate, white and grey, ash and vapour clouds. Deep, low, explosion and rumbling noises were heard on 6 and 7 October, respectively. Weak steady glow was observed on the 6th and the 9th. Activity at Crater 3 was somewhat subdued during the last week of October. Crater 2 released weak and occasionally moderate white and at times blue vapour throughout the month. Deep weak rumbling noises were heard between 16 and 28 October and steady weak glow was seen throughout the month."

Geologic Background. Langila, one of the most active volcanoes of New Britain, consists of a group of four small overlapping composite basaltic-andesitic cones on the lower E flank of the extinct Talawe volcano in the Cape Gloucester area of NW New Britain. A rectangular, 2.5-km-long crater is breached widely to the SE; Langila was constructed NE of the breached crater of Talawe. An extensive lava field reaches the coast on the N and NE sides of Langila. Frequent mild-to-moderate explosive eruptions, sometimes accompanied by lava flows, have been recorded since the 19th century from three active craters at the summit. The youngest and smallest crater (no. 3 crater) was formed in 1960 and has a diameter of 150 m.

Information Contacts: C. McKee and I. Itikarai, RVO.

A group of scientists visited . . . 7-8 August, and were the first to reach the crater floor since June-July 1988.

"Considerable activity in the N crater was observed between March and August, concentrated around the centers T5/T9 and at the E end of the T4/T7 ridge (figure 18). No vent opened S of the saddle between the two craters (M1M2), but lava continued to flow S and the area of lava occupying the floor of the S depression increased slightly. Emission of steam and sulfur fumes continued, particularly N and E of the crater walls and E rim. No eruption of lava on the crater walls or rim had occurred since the formation of features C1, D, and the cluster of cones at A3/A5 (all pre-1988; 13:01). However, the top of T5/T9... reached the level of the E crater rim.

"At 0830 on 7 August, when the party... reached the E crater rim, shimmering heat was observed rising from the top of T5/T9, and there was noise like ocean surf from a small vent on the E end of T4/T7. There was an occasional spatter of fine fragments as lava splashed out of the top of T14.

Figure 18. Active crater at Ol Doinyo Lengai, 7 August 1990, looking NE (top) and SE (bottom). The stippled area represents fresh lava. Tracings of photos courtesy of C. Nyamweru.

"Two large cones, T14 and T14A, are located on the E edge of ridge T4/T7. On the N slope of T14A, younger, dark gray material was visible overlying the heavily weathered brown material that formed the surface of the ridge in May. When first seen at about 0830, T14 was pale gray to white, with a few small vertical cracks on its upper slopes. During the morning, the noise of moving lava continued, with some episodes of silence. By 1200, parts of the cone's top cracked and bulged when lava bubbles burst within it. Between 1240 and 1307, part of the upper slope of the cone collapsed and there was a relatively violent eruption from a SW-facing vent near the top of the cone. Liquid lava was ejected to 10 m above the top of the cone, and also spilled over the edge of the vent, 10 m above the surrounding crater floor.

"Vigorous activity continued for much of the afternoon; occasionally there were 7-10 bursts (sprays) of lava in a 20-second period. At times the lava was thrown up from the vent, and at others it surged over the edge. Periodically, three separate tongues of lava were visible, following each other down the slope of the cone. The flows did not extend any distance away from the base of the cone, and the volumes of lava erupted were very small. After about 1500, the rate of activity gradually slowed, but it continued until at least 1900, when several large clots of lava were thrown as much as 40 m W of T14 (onto the slopes of T14A). Observation ceased at about 2000 and resumed at 0730 on 8 August. Little overnight change was apparent. On the morning of the 8th, moving lava was audible deep below T14, shimmering heat rose from the open vent of T14A, and steam came from the W end of T4/T7 (the oldest part of this feature).

"The tallest cone, T5/T9, extended up 30 m to a single peak, without a large open vent. It had not changed since the 9 July overflight. The slopes were mostly pale grey to white, with slight darkening by fumes at the very top, from which shimmering heat was rising. An open vent over 2 m across (H6) was still visible on the N slope of T5/T9, but there was no sign of activity.

"A low dome or 'blister,' T15, was located a few meters from H6 and... was the source of shimmering heat and noise of moving lava. A flow (F18), that had escaped N and W from this vent had reached the W wall of the crater (probably within 1 or 2 days of the 7 August visit). This flow was smooth, mostly dark brown, and still slightly warm on the 7th; cracking sounds could still be heard from below its surface.

"Cone T10 was almost entirely covered by lava from T5/T9;

"No sign of new effusion was visible at cones T8 or T11. The upper slopes of T8 were stained by considerable amounts of sulfur, and partial collapse of a small section of its lower W slope had occurred. Steam and sulfur fumes were being emitted from T11. In the center of the cone, a hole 2 m across (base not visible) contained bright yellow-orange stalactites, some >50 cm long. The overhanging N slope of the cone had not changed much since late 1988.

"Strong fumaroles were found on the W wall (around D and A5), on the N wall (near C1), and on the E wall, where extensive sulfur staining was present. Small steam sources were also found on the walls of the S depression. In general, emission of steam was very strong . . . .

"The saddle between the two craters, M1/M2, had possibly widened with increased flow of lava from N to S. No vents have opened in the S depression. Patches of burned vegetation have resulted on the S slopes, probably set afire by the heat from lava when it flowed against the surrounding slope, as observed in November 1988."

Geologic Background. The symmetrical Ol Doinyo Lengai is the only volcano known to have erupted carbonatite tephras and lavas in historical time. The prominent stratovolcano, known to the Maasai as "The Mountain of God," rises abruptly above the broad plain south of Lake Natron in the Gregory Rift Valley. The cone-building stage ended about 15,000 years ago and was followed by periodic ejection of natrocarbonatitic and nephelinite tephra during the Holocene. Historical eruptions have consisted of smaller tephra ejections and emission of numerous natrocarbonatitic lava flows on the floor of the summit crater and occasionally down the upper flanks. The depth and morphology of the northern crater have changed dramatically during the course of historical eruptions, ranging from steep crater walls about 200 m deep in the mid-20th century to shallow platforms mostly filling the crater. Long-term lava effusion in the summit crater beginning in 1983 had by the turn of the century mostly filled the northern crater; by late 1998 lava had begun overflowing the crater rim.

Information Contacts: C. Nyamweru, Kenyatta Univ.

"Activity declined further in October. Both craters intermittently released very weak emissions of thin white vapour. No noises or glow were observed from either crater. The decline in daily earthquake totals . . . continued in October and by the end of the month averaged only ~150 (compared to ~1,200 during former inter-eruptive periods). The amplitude of these events also decreased to a very low level. No significant changes were observed in tilt measurements."

Geologic Background. The 10-km-wide island of Manam, lying 13 km off the northern coast of mainland Papua New Guinea, is one of the country's most active volcanoes. Four large radial valleys extend from the unvegetated summit of the conical basaltic-andesitic stratovolcano to its lower flanks. These valleys channel lava flows and pyroclastic avalanches that have sometimes reached the coast. Five small satellitic centers are located near the island's shoreline on the northern, southern, and western sides. Two summit craters are present; both are active, although most observed eruptions have originated from the southern crater, concentrating eruptive products during much of the past century into the SE valley. Frequent eruptions, typically of mild-to-moderate scale, have been recorded since 1616. Occasional larger eruptions have produced pyroclastic flows and lava flows that reached flat-lying coastal areas and entered the sea, sometimes impacting populated areas.

Information Contacts: C. McKee and I. Itikarai, RVO.

A swarm of earthquakes began at 0624 on 5 October 1990, in the sea ~20 km SW of Mikura-jima Island (figure 3). A second burst occurred 13 October, following a gradual decline from the 5th, and a third burst occurred on 21 October (figure 4). A total of about 30 shocks were felt by residents on Mikura-jima and nine were felt on Miyake-jima Island (at Miyake-jima Weather Station), ~40 km NNE of the epicentral area. The largest event was M 4.3 and occurred on 27 October. Depths of most of the located events ranged from 20 to 30 km, although depth control was poor.

Figure 3. Epicentral distribution of earthquakes (M >= 3.0) off Mikura-jima, October 1990. Courtesy of JMA.

Figure 4. Daily number of located earthquakes off Mikura-jima, October 1990. Courtesy of JMA.

The swarm occurred in the vicinity of Mikura Seamount, a cone-shaped feature with a summit ~300 m below sea level and 1,400 m above the surrounding sea floor. No surface phenomena were reported in the area in October, nor have any been reported in historical time. The last swarm near this site took place in December 1982, and was more vigorous, including one M 6.4 event.

Information Contacts: JMA.

A seven-member team of USGS volcanologists visited the CNMI 24 September-6 October at the request of the Office of Civil Defense. The team installed [a seismic station on] Pagan; data are telemetered to Saipan and recorded at Civil Defense headquarters. Quoted material below is from a report by Richard Moore.

"Remeasurements of the distances between two permanent glass reflectors installed in 1983 on the SW flank showed no significant changes in line lengths since 1984. Seismic data telemetered to Saipan showed no significant earthquake activity on Pagan after installation of the station in early October. Most of the abandoned village has been destroyed by alluvial debris derived from 1981 and younger rocks. Eruptions less vigorous than that of 15 May 1981 occurred intermittently from late May 1981 until October 1988. The USGS team observed a prominent SO₂-bearing plume emitted from Pagan 28 September-3 October."

Geologic Background. Pagan Island, the largest and one of the most active of the Mariana Islands volcanoes, consists of two stratovolcanoes connected by a narrow isthmus. Both North and South Pagan stratovolcanoes were constructed within calderas, 7 and 4 km in diameter, respectively. North Pagan at the NE end of the island rises above the flat floor of the northern caldera, which may have formed less than 1,000 years ago. South Pagan is a stratovolcano with an elongated summit containing four distinct craters. Almost all of the recorded eruptions, which date back to the 17th century, have originated from North Pagan. The largest eruption during historical time took place in 1981 and prompted the evacuation of the sparsely populated island.

Information Contacts: R. Moore, USGS; R. Koyanagi and M. Sako, HVO.

Vigorous fumarolic activity continued during October. The crater lake level, which had been dropping during August and September, rose 2 m due to heavy rainfall, but began to fall again at the end of the month. Fumarolic activity within the lake remained similar to previous months; in the SE part of the lake, a small pool of sulfur remained visible. Temperatures of up to 80°C were recorded in the crater lake, 17°C in the peripheral cold water springs, and 91°C from the 1953-55 dome fumaroles. On 13 September, rain water had a pH of 2.7 at the dome and 3.2 at the crater rim.

Daily averages of 250 low-frequency earthquakes (<2.5 Hz) were recorded, similar to levels in May. A maximum of 373 events was recorded 28 October (figure 34). High-frequency earthquakes were rare. Only three were locatable (M 2.2-2.6), the largest felt by local residents. Short tremor episodes (<6 hour durations), less frequent than in August and September, were also occasionally recorded.

Figure 34. Number of earthquakes/day recorded at Poás by the Univ Nacional, October 1990.

Periodic S radial inflation was measured in 1990 through early November (figure 35); the general trend was one of minor inflation relative to the 1989 baseline. Inflation in April-May coincided with eruptions of sediment from the bottom of the lake. Deformation measurements by EDM (1 km S of the crater) registered several different inflationary peaks that geologists believed were influenced by local disturbances such as subsoil water or changes in temperature.

Figure 35. Dry tilt (top) and electronic tilt (bottom) measurements at Poás, January-October 1990. Courtesy of the Univ Nacional.

Geologic Background. The broad vegetated edifice of Poás, one of the most active volcanoes of Costa Rica, contains three craters along a N-S line. The frequently visited multi-hued summit crater lakes of the basaltic-to-dacitic volcano are easily accessible by vehicle from the nearby capital city of San José. A N-S-trending fissure cutting the complex stratovolcano extends to the lower N flank, where it has produced the Congo stratovolcano and several lake-filled maars. The southernmost of the two summit crater lakes, Botos, last erupted about 7,500 years ago. The more prominent geothermally heated northern lake, Laguna Caliente, is one of the world's most acidic natural lakes, with a pH of near zero. It has been the site of frequent phreatic and phreatomagmatic eruptions since an eruption was reported in 1828. Eruptions often include geyser-like ejections of crater-lake water.

Information Contacts: J. Barquero, V. Barboza, E. Fernández, and R. van der Laat, OVSICORI; G. Soto and R. Barquero, ICE.

"Seismicity remained at a low level in October. The total number of events recorded was 101 . . . . All events were of M_L <=1. Only two were locatable, on the N and W sides of the caldera seismic zone. No significant changes were observed from levelling, EDM, tilt, tide gauge, and gravity measurements."

Geologic Background. The low-lying Rabaul caldera on the tip of the Gazelle Peninsula at the NE end of New Britain forms a broad sheltered harbor utilized by what was the island's largest city prior to a major eruption in 1994. The outer flanks of the asymmetrical shield volcano are formed by thick pyroclastic-flow deposits. The 8 x 14 km caldera is widely breached on the east, where its floor is flooded by Blanche Bay and was formed about 1,400 years ago. An earlier caldera-forming eruption about 7,100 years ago is thought to have originated from Tavui caldera, offshore to the north. Three small stratovolcanoes lie outside the N and NE caldera rims. Post-caldera eruptions built basaltic-to-dacitic pyroclastic cones on the caldera floor near the NE and W caldera walls. Several of these, including Vulcan cone, which was formed during a large eruption in 1878, have produced major explosive activity during historical time. A powerful explosive eruption in 1994 occurred simultaneously from Vulcan and Tavurvur volcanoes and forced the temporary abandonment of Rabaul city.

Information Contacts: C. McKee and I. Itikarai, RVO.

The following report covers the period 1 August-12 November. "The last explosive event at Redoubt, on 21 April, generated an ash plume to 7.5-9 km and a small N flank pyroclastic flow. A portion of the existing lava dome was destroyed during this event. Dome growth continued until approximately mid-June, punctuated only by occasional small rockfalls off the dome's N face. The estimated volume of the current lava dome is 10-15 x 10⁶ m³. Field crews have observed consistent fumarolic activity from the dome's S side, summit, and E and W margins. An early September search for high-temperature fumaroles on the dome's accessible N face was unsuccessful. COSPEC measurements documented a steady decline in SO₂ emission from an average of 1,000-2,400 t/d in early August to 160-590 t/d in October and early November.

"Seismicity beneath Redoubt diminished over the summer and into the fall, but remained elevated relative to pre-eruption levels. In early November, low-level tremor was observed for the first time since April, associated with increased steaming on the lava dome and occasional minor steam and ash emissions.

October steam and ash emissions. "Beginning in late August, AVO seismologists noted intermittent bursts of seismicity containing multiple phases and extended codas on flank stations. These bursts occurred at rates of one to several/day, but no relationship between them and eruptive activity was established until 29 October, when AVO received a pilot report of ash on the snow-covered flanks of Redoubt. The ash deposit was thin (< 0.5 mm) and did not extend beyond the base of the volcano (a lateral distance of about 5 km). The deposit must have been emplaced since 27 October, the date of last overflight prior to the pilot report. On 30 October at 1637, a small seismic event lasting 7-8 minutes was recorded on flank stations. Within 30 minutes, personnel at the Drift River oil terminal and an AVO crew in a helicopter reported a small, diffuse ash cloud drifting E of the summit. The cumulative tephra deposit from the late October events is very fine-grained, consisting primarily of plagioclase and a minor amount of altered and unaltered mafic crystals.

"The steam and ash emissions have thus far produced plumes that rise at most 300-600 m above the summit, and only minor amounts of ash have been deposited outside the summit crater. They are reminiscent of the 'gas and ash emissions' documented at Mt. St. Helens between 1981 and 1986, and are interpreted to reflect increasing access of meltwater to hot interior dome rocks. Their onset in late summer approximately coincided with the beginning of snowfall high on the volcano and may reflect some seasonal control related to the increasing availability of snowmelt. Alternatively, the quiescent dome may have cooled and fractured sufficiently to allow ingress of greater amounts of water to its hot interior.

Early November seismo-phreatic crisis. "On 5 November, AVO seismologists noted low-amplitude tremor on flank stations. During the next week, tremor fluctuated in intensity several times. More intense periods appeared to follow the 'emissions' described above. No concurrent change was observed in the occurrence of long-period events or volcano-tectonic earthquakes.

"Observations of the dome from fixed wing aircraft 6-8 November revealed no sign of avalanching or large-scale changes in dome morphology. Fumarolic activity appeared heightened compared to the previous week and new steaming was observed on the dome's N flank.

"An 8 November COSPEC flight measured SO₂ emission of 580 t/d, consistent with results of the past few weeks. Continuing phreatic activity, in the form of steam emissions that occasionally contain ash, is expected."

Further Reference. Brantley, S., ed., 1990, The eruption of Redoubt volcano, Alaska, December 14, 1989-August 31, 1990: USGS Circular 1061, 33 p.

Geologic Background. Redoubt is a glacier-covered stratovolcano with a breached summit crater in Lake Clark National Park about 170 km SW of Anchorage. Next to Mount Spurr, Redoubt has been the most active Holocene volcano in the upper Cook Inlet. The volcano was constructed beginning about 890,000 years ago over Mesozoic granitic rocks of the Alaska-Aleutian Range batholith. Collapse of the summit 13,000-10,500 years ago produced a major debris avalanche that reached Cook Inlet. Holocene activity has included the emplacement of a large debris avalanche and clay-rich lahars that dammed Lake Crescent on the south side and reached Cook Inlet about 3,500 years ago. Eruptions during the past few centuries have affected only the Drift River drainage on the north. Historical eruptions have originated from a vent at the north end of the 1.8-km-wide breached summit crater. The 1989-90 eruption had severe economic impact on the Cook Inlet region and affected air traffic far beyond the volcano.

Information Contacts: AVO Staff.

Fieldwork on 11 and 26 September, and 9 October, monitored changes in deformation and in Crater Lake. The lake appeared gray, with upwelling occurring over the N vents during all three visits, over the central vents on 11 September (when visibility was poor), and on rare occasions on 9 October. Yellow slicks were observed over the central vents on 26 September and 9 October. Undercutting of snow along the shore of the lake (around 1 m above lake level) and the wide channel cut through snow at the outlet, suggested that a surge (potentially related to minor phreatic activity) had occurred sometime before new snow (which may have fallen on 8 September) re-covered part of the exposed area.

Water temperatures increased to a maximum of 35°C by the end of September ... then decreased to 31°C by 9 October (figure 9). Lake Mg/Cl ratios were 0.052 on 26 September and 0.053 on 9 October, slightly higher than 22 August (0.051).

Seismicity has remained at very low levels since late August. A few small earthquakes (M <= 2.5) were recorded (instrument failure prevented monitoring 28 September-3 October). Tremor amplitude was low, except on 14-15 September when moderate amplitudes were recorded (figure 10).

Figure 10. Qualitative ampliude of tremor at Ruapehu, July-October 1990.

Deformation measurements showed that the decrease in crater width that began in early September continued through early October, although distances had not yet returned to pre-inflation, early July values (figures 9 and 11).

Figure 11. Apparent deformation (in millimeters) at Ruapehu, 22 August-9 October (left) and 20 July-9 October 1990 (right).

Geologic Background. Ruapehu, one of New Zealand's most active volcanoes, is a complex stratovolcano constructed during at least four cone-building episodes dating back to about 200,000 years ago. The dominantly andesitic 110 km3 volcanic massif is elongated in a NNE-SSW direction and surrounded by another 100 km3 ring plain of volcaniclastic debris, including the NW-flank Murimoto debris-avalanche deposit. A series of subplinian eruptions took place between about 22,600 and 10,000 years ago, but pyroclastic flows have been infrequent. The broad summait area and flank contain at least six vents active during the Holocene. Frequent mild-to-moderate explosive eruptions have been recorded from the Te Wai a-Moe (Crater Lake) vent, and tephra characteristics suggest that the crater lake may have formed as recently as 3,000 years ago. Lahars resulting from phreatic eruptions at the summit crater lake are a hazard to a ski area on the upper flanks and lower river valleys.

Information Contacts: B. Scott and I. Nairn, DSIR Rotorua; P. Otway and S. Sherburn, DSIR Wairakei; J. Allen and R. O'Brien, Dept of Conservation, Whakapapa.

Many small ash emissions occurred during October, although seismicity remained at low levels. Two small swarms of high-frequency earthquakes were recorded on 14 and 22 October. Tremor episodes (2 cm² maximum reduced displacement) were prominent and were occasionally associated with small ash emissions. Although EDM measurements showed important changes, dry-tilt did not show ground deformation. Similarly, EDM indicated 6.6 µrad of deformation at one station during September, while dry-tilt did not show any significant changes. The average SO₂ flux for the month, measured by COSPEC, was 1,630 t/d, compared to 2,448 t/d in September.

Geologic Background. Nevado del Ruiz is a broad, glacier-covered volcano in central Colombia that covers more than 200 km2. Three major edifices, composed of andesitic and dacitic lavas and andesitic pyroclastics, have been constructed since the beginning of the Pleistocene. The modern cone consists of a broad cluster of lava domes built within the caldera of an older edifice. The 1-km-wide, 240-m-deep Arenas crater occupies the summit. The prominent La Olleta pyroclastic cone located on the SW flank may also have been active in historical time. Steep headwalls of massive landslides cut the flanks. Melting of its summit icecap during historical eruptions, which date back to the 16th century, has resulted in devastating lahars, including one in 1985 that was South America's deadliest eruption.

Information Contacts: C. Carvajal, INGEOMINAS, Manizales.

At 0207 on 5 November, the start of a brief explosive episode and ash emission was signalled by 2 minutes of low-amplitude seismicity, followed by an increase to high-amplitude seismicity and the failure of several sensors on the summit dome. Pilots reported the plume at altitudes of ~7.5-9 km traveling SE at 90-110 km/hr; ash was reported as far as Fossil, Oregon (~200 km SE). Strong seismicity lasted for 6 minutes, then decreased to normal levels over the following 2-3 hours.

Geologists visiting the crater that day found that the explosive activity took place at a vent on the N side of the lava dome. Two seismic stations and a steel tower were destroyed, but others continued to function. Hot dome blocks and finer-grained material blanketed the snow on the crater floor, NW and N of the dome; blocks up to 2 m in diameter were scattered on the lower part of the W crater wall (NW of the dome). Rock avalanches and hot debris from the explosions moved down the N side of the dome and across the crater floor, abrading and melting snow and ice. The resultant small debris flow traveled out of the crater into the North Fork of the Toutle River, where it formed a small mudflow that extended 16-19 km.

Fine tephra was collected from the extreme limit of deposition, but had not yet been analyzed at press time. Small quantities of fresh-appearing glass had been found in tephra emitted on 6 January (SEAN 14:12).

No precursory events to the 5 November activity have been identified, although two distinctive "cigar-shaped" events (closely spaced, small, shallow earthquakes with concurrent tremor) lasting several hours were recorded 25 and 26 October. Similar signals were associated with the 6 January ash emission, and were recorded 24 September, when no ash was emitted. These signals have been identified at Old Faithful Geyser (Yellowstone Caldera, USA), and Ruiz (Colombia), where they are thought to represent hydrothermal venting or near-surface movement of fluids.

The 5 November ash emission was very similar to the previous explosive events on [6] December and 6 January (SEAN 14:11 and 14:12). An event on 25 April produced similar explosion-type seismic signals, but bad weather prevented observations and no ash or eruption plume was reported (BGVN 15:04). Each of the events was short-lived (up to 18 hours) and produced little ash. Although the January episode also caused rock and snow avalanches, the November activity was the first to produce a mudflow in the last two years.

Geologic Background. Prior to 1980, Mount St. Helens was a conical volcano sometimes known as the Fujisan of America. During the 1980 eruption the upper 400 m of the summit was removed by slope failure, leaving a 2 x 3.5 km breached crater now partially filled by a lava dome. There have been nine major eruptive periods beginning about 40-50,000 years ago, and it has been the most active volcano in the Cascade Range during the Holocene. Prior to 2,200 years ago, tephra, lava domes, and pyroclastic flows were erupted, forming the older edifice, but few lava flows extended beyond the base of the volcano. The modern edifice consists of basaltic as well as andesitic and dacitic products from summit and flank vents. Eruptions in the 19th century originated from the Goat Rocks area on the N flank, and were witnessed by early settlers.

Information Contacts: W. Scott and S. Brantley, CVO; SAB.

At the end of August, explosive activity in Crater 1 became nearly continuous and tremor amplitude increased. The monthly average tremor amplitude was twice as high in September as in August. The daily number of events that saturated seismometers oscillated around a mean of 30 until 20 September before rapidly decreasing (figure 9). Saturating events and tremor amplitude reached a minimum during the first week in October, then remained at levels similar to those preceding the strong activity in the second half of July.

Figure 9. Number of seismometer-saturating events/day (solid line); and average tremor amplitude (dashed line) at Stromboli, mid-June to November 1990. Courtesy of M. Riuscetti.

Volcano guides reported the following activity. 1-6 September: Ejection of hot lapilli was continuous from vents 1 and 2 in crater C1 (figure 10). Violent explosions with ash emission (150-200 m high) occurred from C3. 7-12 September: Activity was similar from C1 and C3. Block ejection and gas emission took place from C2. 13-20 September: Ejection of hot lapilli and noisy gas emission occurred from C1, while continuous minor explosions ejected small blocks from C2. Tephra was filling C3, where 4 new vents were forming on 15 September. 21 September-4 October: Most activity was concentrated in C1 and C3, with frequent explosions ejecting hot lapilli to as high as 200 m.

Figure 10. Sketch from "Pizzo sopra la Fossa" looking NW at the summit of Stromboli, October 1990. Courtesy of M. Riuscetti.

Geologists visited the summit area 4-10 October. The activity was more vigorous than had been seen during 25 years of study at Stromboli. Strong explosions at 15-20-minute intervals fed powerful brown ash emissions that reached about 300 m height (from vent 4 of C3). Nearly continuous bomb ejection from vent 1 of C1 was evident at night and gases were red. Stronger explosions were synchronous from many of the vents in the 3 craters. Lava spilled out every few tenths of a second from one small cone (1) in C3. One vent (3) in C1 ejected gas nearly every second. Fumarolic activity was very intense, especially from the W rim of C3. At least 3 new vents had formed (3 in C1 and 2 & 3 in C3) with continuous whistling and rare explosions.

11-16 October: Activity continued, but with an apparent slight decline. 17-28 October: Observations from the summit area were not available, but seismicity and reports from a village at the foot of the volcano suggested decreasing activity.

Geologic Background. Spectacular incandescent nighttime explosions at Stromboli have long attracted visitors to the "Lighthouse of the Mediterranean" in the NE Aeolian Islands. This volcano has lent its name to the frequent mild explosive activity that has characterized its eruptions throughout much of historical time. The small island is the emergent summit of a volcano that grew in two main eruptive cycles, the last of which formed the western portion of the island. The Neostromboli eruptive period took place between about 13,000 and 5,000 years ago. The active summit vents are located at the head of the Sciara del Fuoco, a prominent scarp that formed about 5,000 years ago due to a series of slope failures which extends to below sea level. The modern volcano has been constructed within this scarp, which funnels pyroclastic ejecta and lava flows to the NW. Essentially continuous mild Strombolian explosions, sometimes accompanied by lava flows, have been recorded for more than a millennium.

Information Contacts: M. Riuscetti, Univ di Udine.

"Activity remained at a low level in October. Emissions from the summit crater consisted mainly of white vapour released at moderate rates. Seismic activity remained at a very low level."

Geologic Background. The symmetrical basaltic-to-andesitic Ulawun stratovolcano is the highest volcano of the Bismarck arc, and one of Papua New Guinea's most frequently active. The volcano, also known as the Father, rises above the N coast of the island of New Britain across a low saddle NE of Bamus volcano, the South Son. The upper 1,000 m is unvegetated. A prominent E-W escarpment on the south may be the result of large-scale slumping. Satellitic cones occupy the NW and E flanks. A steep-walled valley cuts the NW side, and a flank lava-flow complex lies to the south of this valley. Historical eruptions date back to the beginning of the 18th century. Twentieth-century eruptions were mildly explosive until 1967, but after 1970 several larger eruptions produced lava flows and basaltic pyroclastic flows, greatly modifying the summit crater.

Information Contacts: C. McKee and I. Itikarai, RVO.

Seismicity increased during October, with a monthly total of 549 recorded events (up from 251 in September); 15 shocks were felt 3.9 km SW of the volcano (at UWS). Earthquake swarms occurred on the 17th, 23rd, and 31st, and during the night of 13-14 November, when four shocks were felt at the weather station. Epicenters were located roughly in two groups, one in the central part of the Shimabara peninsula, the other in the sea about 15 km W of the summit (figure 6). The seismicity in the central part of the peninsula was the first there since July. Previous swarms were generally centered at sea.

Figure 6. Epicentral distribution of earthquakes around Unzen, October 1990. Courtesy of JMA.

Tremor resumed on 10 October after 20 days of absence, and as many as 10 episodes/day were recorded through the end of the month. The monthly total of 81 tremor episodes was an increase from 42 in September. Tremor amplitudes were similar to previous months.

Weak, continuous tremor started on seismographs near the volcano at 0322 on 17 November. By dawn (around 0600), an eruption had already begun and residents saw a white plume rising from the volcano; the exact start time of the eruption was unknown. An air and ground survey by JMA and Kyushu Univ revealed that two steam plumes were being continuously erupted from new E flank vents; one was [650] m E of the summit (Fugen-dake), the other about 100 m S of the first vent. The steam plumes were about 300 m high and occasionally contained ash. Weak ashfall was noted downwind. No explosion sounds were heard, and no clear shocks were recorded by seismographs. The amplitude of continuous tremor gradually declined during the day, fading away at around 1900. Steam was still being erupted from one of the vents the next day, and was continuing, but declining, through 19 November. Seismicity was unchanged after the eruption. No damage was reported.

Geologic Background. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: JMA.

During a brief 17 October visit to the rim of 1978/90 Crater complex, white steam emissions were relatively voluminous. A lake had been re-established in the SE portion (R.F. Crater). The vent observed 3 October under the SW wall was apparently still present, although viewing conditions were poor. The margins of TV1 Crater appeared unchanged, and it was emitting white steam at low pressure. Strong, audible fumaroles were present in the area NW of the 20-m-high non-extrusive rock spine (first seen on 30 August and located 15 m W of TV1 Crater), where transparent vapors emerged from the crater floor, condensing to white steam above. East of 1978/90 Crater, Donald Duck vent was emitting wispy white vapor and its floor and vent area were clearly visible.

There was no evidence for further eruptive activity since 3 October from within 1978/90 Crater, TV1 Crater, or Donald Duck Crater.

Geologic Background. The uninhabited Whakaari/White Island is the 2 x 2.4 km emergent summit of a 16 x 18 km submarine volcano in the Bay of Plenty about 50 km offshore of North Island. The island consists of two overlapping andesitic-to-dacitic stratovolcanoes. The SE side of the crater is open at sea level, with the recent activity centered about 1 km from the shore close to the rear crater wall. Volckner Rocks, sea stacks that are remnants of a lava dome, lie 5 km NW. Descriptions of volcanism since 1826 have included intermittent moderate phreatic, phreatomagmatic, and Strombolian eruptions; activity there also forms a prominent part of Maori legends. The formation of many new vents during the 19th and 20th centuries caused rapid changes in crater floor topography. Collapse of the crater wall in 1914 produced a debris avalanche that buried buildings and workers at a sulfur-mining project. Explosive activity in December 2019 took place while tourists were present, resulting in many fatalities. The official government name Whakaari/White Island is a combination of the full Maori name of Te Puia o Whakaari ("The Dramatic Volcano") and White Island (referencing the constant steam plume) given by Captain James Cook in 1769.

Information Contacts: B. Scott, DSIR, Rotorua; S. Sherburn, DSIR, Wairakei.

"Emissions from the summit crater were very weak during an overflight in early September. Mild fumarolic emissions were noted from the NW flank of Pago."

Geologic Background. The active Pago cone has grown within the Witori caldera (5.5 x 7.5 km) on the northern coast of central New Britain contains the active Pago cone. The gently sloping outer caldera flanks consist primarily of dacitic pyroclastic-flow and airfall deposits produced during a series of five major explosive eruptions from about 5,600 to 1,200 years ago, many of which may have been associated with caldera formation. Pago cone may have formed less than 350 years ago; it has grown to a height above the caldera rim, and a series of ten dacitic lava flows from it covers much of the caldera floor. The youngest of these was erupted during 2002-2003 from vents extending from the summit nearly to the NW caldera wall. The Buru caldera cuts the SW flank.

Information Contacts: C. McKee and I. Itikarai, RVO.