Pacaya, located in Guatemala, is a highly active volcano that has previously produced continuous Strombolian explosions, multiple lava flows, and the formation of a small cone within the crater due to the constant deposition of ejected material (BGVN 45:02). This reporting period updates information from February through July 2020 consisting of similar activity that dominantly originates from the Mackenney crater. Information primarily comes from reports by the Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH) in Guatemala and various satellite data.

Strombolian explosions were recorded consistently throughout this reporting period. During February 2020, explosions ejected incandescent material 100 m above the Mackenney crater. At night and during the early morning the explosions were accompanied by incandescence from lava flows. Multiple lava flows were active during most of February, traveling primarily down the SW and NW flanks and reaching 500 m on 25 February. On 5 February the lava flow on the SW flank divided into three flows measuring 200, 150, and 100 m. White and occasionally blue gas-and-steam emissions rose up to 2.7 km altitude on 11 and 14 February and drifted in multiple directions. On 16 February Matthew Watson utilized UAVs (Unmanned Aerial Vehicle) to take detailed, close up photos of Pacaya and report that there were five active vents at the summit exhibiting lava flows from the summit, gas-and-steam emissions, and small Strombolian explosions (figure 122).

Figure 122. Drone image of active summit vents at Pacaya on 16 February 2020 with incandescence and white gas-and-steam emissions. Courtesy of Matthew Watson, University of Bristol, posted on 17 February 2020.

Activity remained consistent during March with Strombolian explosions ejecting material 100 m above the crater accompanied by occasional incandescence and white and occasionally blue gas-and-steam emissions drifting in multiple directions. Multiple lava flows were detected on the NW and W flanks reaching as far as 400 m on 9-10 March.

In April, frequent Strombolian explosions were accompanied by active lava flows moving dominantly down the SW flank and white gas-and-steam emissions. These repeated explosions ejected material up to 100 m above the crater and then deposited it within the Mackenney crater, forming a small cone. On 27 April seismicity increased at 2140 due to a lava flow moving SW as far as 400 m (figure 123); there were also six strong explosions and a fissure opened on the NW flank in front of the Los Llanos Village, allowing gas-and-steam to rise.

Figure 123. Infrared image of Pacaya on 28 April 2020, showing a lava flow approximately 500 m long and moving down the S flank on the day after seismicity increased and six strong explosions were detected. Courtesy of ISIVUMEH (Reporte Volcán de Pacaya July 2020).

During May, Strombolian explosions continued to eject incandescent material up to 100 m above the Mackenney crater, accompanied by active lava flows on 1-2, 17-18, 22, 25-26, and 29-30 May down the SE, SW, NW, and NE flanks up to 700 m on 30 May. White gas-and-steam emissions continued to be observed up to 100 m above the crater drifting in multiple directions. Between the end of May and mid-June, the plateau between the Mackenney cone and the Cerro Chiquito had become inundated with lava flows (figure 124).

Figure 124. Aerial views of the lava flows at Pacaya to the NW during a) 18 September 2019 and b) 16 June 2020 showing the lava flow advancement toward the Cerro Chiquito. Courtesy of INSIVUMEH (Reporte Volcán de Pacaya July 2020).

Lava flows extended 700 m on 8 June down multiple flanks. On 9 June, a lava flow traveled N and NW 500 m and originating from a vent on the N flank about 100 m below the Mackenney crater. Active lava flows continued to originate from this vent through at least 19 June while white gas-and-steam emissions were observed rising 300 m above the crater. At night and during the early mornings of 24 and 29 June Strombolian explosions were observed ejecting incandescent material up to 200 m above the crater (figure 125). These explosions continued to destroy and then rebuild the small cone within the Mackenney crater with fresh ejecta. Active lava flows on the SW flank were mostly 100-600 m long but had advanced to 2 km by 30 June.

On 10 July a 1.2 km lava flow divided in two which moved on the NE and N flanks. On 11 July, another 800 m lava flow divided in two, on the N and NE flanks (figure 126). On 14 and 19 July, INSIVUMEH registered constant seismic tremors and stated they were associated with the lava flows. No active lava flows were observed on 18-19 July, though some may have continued to advance on the SW, NW, N, and NE flanks. On 20 July, lava emerged from a vent at the NW base of the Mackenney cone near Cerro Chino, extending SE. Strombolian explosions ejected incandescent material up to 200 m above the crater on 22 July, accompanied by active incandescent lava flows on the SW, N, NW, NE, and W flanks. Three lava flows on the NW flank were observed on 22-24 July originating from the base of the Mackenney cone. Explosive activity during 22 July vibrated the windows and roofs of the houses in the villages of San Francisco de Sales, El Patrocinio, El Rodeo, and others located 4 km from the volcano. The lava flow activity had decreased by 25 July, but remnants of the lava flow on the NW flank persisted with weak incandescence observed at night, which was no longer observed by 26 July. Strombolian explosions continued to be detected through the rest of the month, accompanied by frequent white gas-and-steam emissions that extended up to 2 km from the volcano; no active lava flows were observed.

Figure 125. Photos of Pacaya on 11 July 2020 showing Strombolian explosions and lava flows moving down the N and NE flanks. Courtesy of William Chigna, CONRED, posted on 12 July 2020.

Figure 126. Infrared image of Pacaya on 20 July 2020 showing a hot lava flow accompanied by gas-and-steam emissions. Courtesy of INSIVUMEH (BEPAC 47 Julio 2020-22).

During February through July 2020, multiple lava flows and thermal anomalies within the Mackenney crater were detected in Sentinel-2 thermal satellite imagery (figure 127). These lava flows were observed moving down multiple flanks and were occasionally accompanied by white gas-and-steam emissions. Thermal anomalies were also recorded by the MIROVA (Middle InfraRed Observation of Volcanic Activity) system during 10 August through July 2020 within 5 km of the crater summit (figure 128). There were a few breaks in thermal activity from early to mid-March, late April, early May, and early June; however, each of these gaps were followed by a pulse of strong and frequent thermal anomalies. According to the MODVOLC algorithm, 77 thermal alerts were recorded within the summit crater during February through July 2020.

Figure 127. Sentinel-2 thermal satellite images of Pacaya showing thermal activity (bright yellow-orange) primarily as lava flows originating from the summit crater during February to July 2020 frequently accompanied by white gas-and-steam emissions. All images with "Atmospheric penetration" (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Figure 128. The MIROVA thermal activity graph (Log Radiative Power) at Pacaya during 10 August to July 2020 shows strong, frequent thermal anomalies through late July with brief gaps in activity during early to mid-March, late April, early May, and early June. Courtesy of MIROVA.

Geologic Background. Eruptions from Pacaya, one of Guatemala's most active volcanoes, are frequently visible from Guatemala City, the nation's capital. This complex basaltic volcano was constructed just outside the southern topographic rim of the 14 x 16 km Pleistocene Amatitlán caldera. A cluster of dacitic lava domes occupies the southern caldera floor. The post-caldera Pacaya massif includes the ancestral Pacaya Viejo and Cerro Grande stratovolcanoes and the currently active Mackenney stratovolcano. Collapse of Pacaya Viejo between 600 and 1500 years ago produced a debris-avalanche deposit that extends 25 km onto the Pacific coastal plain and left an arcuate somma rim inside which the modern Pacaya volcano (Mackenney cone) grew. A subsidiary crater, Cerro Chino, was constructed on the NW somma rim and was last active in the 19th century. During the past several decades, activity has consisted of frequent strombolian eruptions with intermittent lava flow extrusion that has partially filled in the caldera moat and armored the flanks of Mackenney cone, punctuated by occasional larger explosive eruptions that partially destroy the summit of the growing young stratovolcano.

Information Contacts: Instituto Nacional de Sismologia, Vulcanologia, Meteorologia e Hydrologia (INSIVUMEH), Unit of Volcanology, Geologic Department of Investigation and Services, 7a Av. 14-57, Zona 13, Guatemala City, Guatemala (URL: http://www.insivumeh.gob.gt/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Matthew Watson, School of Earth Sciences at the University of Bristol (Twitter: @Matthew__Watson, https://twitter.com/Matthew__Watson); William Chigna, CONRED (URL: https://twitter.com/william_chigna).

Sangeang Api is a 13-km-wide island located off the NE coast of Sumbawa Island, part of Indonesia's Lesser Sunda Islands. Documentation of historical eruptions date back to 1512. The most recent eruptive episode began in July 2017 and included frequent Strombolian explosions, ash plumes, and block avalanches. The previous report (BGVN 45:02) described activity consisting of a new lava flow originating from the active Doro Api summit crater, short-lived explosions, and ash-and-gas emissions. This report updates information during February through July 2020 using information from the Darwin Volcanic Ash Advisory Center (VAAC) reports, Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, or CVGHM) reports, and various satellite data.

Volcanism during this reporting period was relatively low compared to the previous reports (BGVN 44:05 and BGVN 45:02). A Darwin VAAC notice reported an ash plume rose 2.1 km altitude and drifted E on 10 May 2020. Another ash plume rose to a maximum of 3 km altitude drifting NE on 10 June, as seen in HIMAWARI-8 satellite imagery.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data detected a total of 12 low power thermal anomalies within 5 km from the summit during February through May 2020 (figure 42). No thermal anomalies were recorded during June and July according to the MIROVA graph. Though the MODVOLC algorithm did not detect any thermal signatures between February to July, many small thermal hotspots within the summit crater could be seen in Sentinel-2 thermal satellite imagery (figure 43).

Figure 42. Thermal anomalies at Sangeang Api from 10 August 2019 through July 2020 recorded by the MIROVA system (Log Radiative Power) were infrequent and low power during February through May 2020. No thermal anomalies were detected during June and July. Courtesy of MIROVA.

Figure 43. Sentinel-2 thermal satellite imagery using “Atmospheric penetration” (bands 12, 11, 8A) rendering showed small thermal hotspots (orange-yellow) at the summit of Sangeang Api during February through June 2020. Courtesy of Sentinel Hub Playground.

Geologic Background. Sangeang Api volcano, one of the most active in the Lesser Sunda Islands, forms a small 13-km-wide island off the NE coast of Sumbawa Island. Two large trachybasaltic-to-tranchyandesitic volcanic cones, Doro Api and Doro Mantoi, were constructed in the center and on the eastern rim, respectively, of an older, largely obscured caldera. Flank vents occur on the south side of Doro Mantoi and near the northern coast. Intermittent historical eruptions have been recorded since 1512, most of them during in the 20th century.

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/); 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Stromboli is a stratovolcano located in the northeastern-most part of the Aeolian Islands composed of two active summit vents: the Northern (N) Crater and the Central-South (CS) Crater that are situated at the head of the Sciara del Fuoco, a large scarp that runs from the summit down the NW side of the volcano. The ongoing eruption began in 1934 and has been characterized by regular Strombolian explosions in both summit craters, ash plumes, and occasional lava flows (BGVN 45:08). This report updates activity from January to April 2020 with information primarily from daily and weekly reports by Italy's Istituto Nazionale di Geofisica e Vulcanologia (INGV) and various satellite data.

Activity was consistent during this reporting period. Explosion rates ranged from 1-20 per hour and were of variable intensity, producing material that rose from less than 80 to over 250 m above the vents (table 8). Strombolian explosions were often accompanied by gas-and-steam emissions, spattering, and lava flows which has resulted in fallout deposited on the Sciara del Fuoco and incandescent blocks rolling toward the coast up to a few hundred meters down the slopes of the volcano. According to INGV, the average SO2 emissions measured 300-650 tons/day.

Table 8. Summary of activity at Stromboli during January-April 2020. Low-intensity activity indicates ejecta rising less than 80 m, medium-intensity is ejecta rising less than 150 m, and high-intensity is ejecta rising over 200 m above the vent. Data courtesy of INGV.

During January 2020, explosive activity mainly originated from three vents in the N crater and at least three vents in the CS crater. Ejecta from numerous Strombolian explosions covered the slopes on the upper Sciara del Fuoco, some of which rolled hundreds of meters down toward the coast. Explosion rates varied from 2-12 per hour in the N crater and 9-14 per hour in the CS crater; ejected material rose 80-200 m above the craters. According to INGV, a small cone growing in the S1 crater produced some explosions that ejected coarse material mixed with fine ash. On 18 and 19 January a lava flow was observed, both of which originated in the N crater. In addition, two explosions were detected in the N crater that was associated with two landslide events.

Explosive activity in February primarily originated from 2-3 eruptive vents in the N crater and at least three vents in the CS crater (figure 177). The Strombolian explosions ejected material 80-250 m above the craters, some of which fell onto the upper part of the Sciara. Explosion rates varied from 3-12 per hour in the N crater and 2-14 per hour in the CS crater (figure 178). On 3 February a short-lived lava flow was reported in the N crater.

Figure 177. A drone image showed spattering accompanied by gas-and-steam emissions at Stromboli rising above the N crater on 15 February 2020. Courtesy of INGV (Rep. No. 08/2020, Stromboli, Bollettino Settimanale, 10/02/2020 - 16/02/2020, data emissione 18/02/2020).

Figure 178. a) Strombolian explosions during the week of 17-23 February 2020 in the N1 crater of Stromboli were seen from Pizzo Sopra La Fossa. b) Spattering at Stromboli accompanied by white gas-and-steam emissions was detected in the N1 and S2 craters during the week of 17-23 February 2020. c) Spattering at Stromboli accompanied by a dense ash plume was seen in the N1 and S2 craters during the week of 17-23 February 2020. All photos by F. Ciancitto, courtesy of INGV (Rep. No. 09/2020, Stromboli, Bollettino Settimanale, 17/02/2020 - 23/02/2020, data emissione 25/02/2020).

Ongoing explosive activity continued into March, originating from three eruptive vents in the N crater and at least three vents in the CS crater. Ejected lapilli and bombs rose 80-250 m above the craters resulting in fallout covering the slopes in the upper Sciara del Fuoco with blocks rolling down the slopes toward the coast and explosions varied from 4-13 per hour in the N crater and 1-16 per hour in the CS crater. Discontinuous spattering was observed during 9-19 March. On 19 March, intense spattering was observed in the N crater, which produced a lava flow that stretched along the upper part of the Sciara for a few hundred meters. Another lava flow was detected in the N crater on 28 March for about 4 hours into 29 March, which resulted in incandescent blocks breaking off the front of the flow and rolling down the slope of the volcano. On 30 March a lava flow originated from the N crater and remained active until the next day on 31 March. Landslides accompanied by incandescent blocks rolling down the Sciara del Fuoco were also observed.

Strombolian activity accompanied by gas-and-steam emissions continued into April, primarily produced in 3-4 eruptive vents in the N crater and 2-3 vents in the CS crater. Ejected material from these explosions rose 80-250 m above the craters, resulting in fallout products covering the slopes on the Sciara and blocks rolling down the slopes. Explosions varied from 4-15 per hour in the N crater and 1-10 per hour in the CS crater. On 1 April a thermal anomaly was detected in satellite imagery accompanied by gas-and-steam and ash emissions downstream of the Sciara del Fuoco. A lava flow was observed on 15 April in the N crater accompanied by gas-and-steam and ash emissions; at the front of the flow incandescent blocks detached and rolled down the Sciara (figure 179). This flow continued until 16 April, ending by 0956; a thermal anomaly persisted downslope from the lava flow. Spatter was ejected tens of meters from the vent. Another lava flow was detected on 19 April in the N crater, followed by detached blocks from the front of the flow rolling down the slopes. Spattering continued during 20-21 April.

Figure 179. A webcam image of an ash plume accompanied by blocks ejected from Stromboli on 15 April 2020 rolling down the Sciara del Fuoco. Courtesy of INGV via Facebook posted on 15 April 2020.

Moderate thermal activity occurred frequently during 16 October to April 2020 as recorded in the MIROVA Log Radiative Power graph using MODIS infrared satellite information (figure 180). The MODVOLC thermal alerts recorded a total of 14 thermal signatures over the course of nine different days between late February and mid-April. Many of these thermal signatures were captured as hotspots in Sentinel-2 thermal satellite imagery in both summit craters (figure 181).

Figure 180. Low to moderate thermal activity at Stromboli occurred frequently during 16 October-April 2020 as shown in the MIROVA graph (Log Radiative Power). Courtesy of MIROVA.

Figure 181. Thermal anomalies (bright yellow-orange) at Stromboli were observed in thermal satellite imagery from both of the summit vents throughout January-April 2020. Images with Atmospheric Penetration rendering (bands 12, 11, 8A); courtesy of Sentinel Hub Playground.

Geologic Background. Spectacular incandescent nighttime explosions at this volcano have long attracted visitors to the "Lighthouse of the Mediterranean." Stromboli, the NE-most of the Aeolian Islands, 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 horseshoe-shaped scarp formed about 5,000 years ago due to a series of slope failures that extend 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: Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Catania, Piazza Roma 2, 95123 Catania, Italy (URL: http://www.ct.ingv.it/en/, Facebook: https://www.facebook.com/ingvvulcani/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Columbia’s broad, glacier-capped Nevado del Ruiz has an eruption history documented back 8,600 years, and historical observations since 1570. It’s profound notoriety stems from an eruption on 13 November 1985 that produced an ash plume and pyroclastic flows onto the glacier, triggering large lahars that washed down 11 valleys, inundating most severely the towns of Armero (46 km W) and Chinchiná (34 km E) where approximately 25,000 residents were killed. It remains the second deadliest volcanic eruption of the 20th century after Mt. Pelee killed 28,000 in 1902. Ruiz remained quiet for 20 years after the September 1985-July 1991 eruption until a new explosive event occurred in February 2012; a series of explosive events lasted into 2013. Renewed activity beginning in November 2014 included ash and gas-and-steam plumes, ashfall, and the appearance of a lava dome inside the Arenas crater in August 2015 which has regularly displayed thermal anomalies through 2019. This report covers ongoing activity from January-June 2020 using information primarily from reports by the Servicio Geologico Colombiano (SGC) and the Observatorio Vulcanológico y Sismológico de Manizales, the Washington Volcanic Ash Advisory Center (VAAC) notices, and various sources of satellite data.

Gas and ash emissions continued at Nevado del Ruiz throughout January-June 2020; they generally rose to 5.8-6.1 km altitude with the highest reported plume at 7 km altitude during early March. SGC confirmed the presence of the growing lava dome inside Arenas crater during an overflight in January; infrared satellite imagery indicated a continued heat source from the dome through April. SGC interpreted repeated episodes of ‘drumbeat seismicity’ as an indication of continued dome growth throughout the period. Small- to moderate-density sulfur dioxide emissions were measured daily with satellite instruments. The MIROVA graph of thermal activity indicated a heat source consistent with a growing dome from January through April (figure 102).

Figure 102. The MIROVA graph of thermal activity at Nevado del Ruiz from 2 July 2019 through June 2020 indicated persistent thermal anomalies from mid-November 2019-April 2020. Courtesy of MIROVA.

Activity during January-March 2020. During January 2020 some of the frequent tremor seismic events were associated with gas and ash emissions, and several episodes of “drumbeat” seismicity were recorded; they have been related by SGC to the growth of the lava dome on the floor of the Arenas crater. An overflight on 10 January, with the support of the Columbian Air Force, confirmed the presence of the dome which was first proposed in August 2015 (BGVN 42:06) (figure 103). The Arenas crater had dimensions of 900 x 980 m elongate to the SW-NE and was about 300 m deep (figure 104). The dome inside the crater was estimated to be 173 m in diameter and 60 m high with an approximate volume of 1,500,000 m³ (figures 105 and 106). In addition to the dome, the scientists also noted ash deposits on the summit ice cap (figure 107). The Washington VAAC reported an ash plume on 19 January that rose to 5.5 km altitude and drifted SW, dissipating quickly. On 30 January they reported an ash plume visible in satellite imagery extending 15 km NW from the summit at 5.8 km altitude. A single MODVOLC alert was issued on 15 January and data from the VIIRS satellite instrument reported thermal anomalies inside the summit crater on 14 days of the month. Sulfur dioxide plumes with DU values greater than 2 were recorded by the TROPOMI satellite instrument daily during the month.

Figure 103. SGC confirmed the presence of a lava dome inside the Arenas crater at Nevado del Ruiz on 10 January 2020. The dome is shown in brown, and zones of fumarolic activity are labelled around the dome. Courtesy of SGC (El Nuevo Domo de Lava del Volcán Nevado del Ruiz y la Geomorfología Actual del Cráter Arenas 2020).

Figure 104. A view of the Arenas crater at the summit of Nevado del Ruiz on 10 January 2020 (left) is compared with a view from 2010 (right). They were both taken during overflights supported by the Colombian Air Force (FAC). Ash deposits on the ice fields are visible in both images. Fumarolic activity rises from the inner walls of the crater in January 2020. Courtesy of SGC (El Nuevo Domo de Lava del Volcán Nevado del Ruiz y la Geomorfología Actual del Cráter Arenas 2020).

Figure 105. The dome inside the Arenas crater at Nevado del Ruiz appeared dark against the crater rim and ash-covered ice field on 10 January 2020. Features observed include (A) the edge of the Arenas crater, (B) a secondary crater 150 m in diameter located to the west, (C) interior cornices, (D) the lava dome, (E) a depression in the center of the dome caused by possible subsidence and cooling of the lava, (F) a source of gas and ash emission with a diameter of approximately 15 m (secondary crater), and (G, H, and I) several sources of gas emission located around the crater. Courtesy of SGC (El Nuevo Domo de Lava del Volcán Nevado del Ruiz y la Geomorfología Actual del Cráter Arenas 2020).

Figure 106. Images of the summit of Nevado del Ruiz captured by the PlanetScope satellite system on 14 March 2018 (A) and 10 January 2020 (B) show the lava dome at the bottom of Arenas crater. Courtesy Planet Lab Inc. and SGC (El Nuevo Domo de Lava del Volcán Nevado del Ruiz y la Geomorfología Actual del Cráter Arenas 2020).

Figure 107. Ash covered the snow and ice field around the Arenas crater at the summit of Nevado del Ruiz on 10 January 2020. The lava dome is the dark area on the right. Courtesy of SGC (posted on Twitter @sgcol).

The Washington VAAC reported multiple ash plumes during February 2020. On 4 February an ash plume was observed in satellite imagery drifting 35 km W from the summit at 5.8 km altitude. The following day a plume rose to 6.1 km altitude and extended 37 km W from the summit before dissipating by the end of the day (figure 108). On 6 February an ash cloud was observed in satellite imagery centered 45 km W of the summit at 5.8 km altitude. Although it had dissipated by midday, a hotspot remained in shortwave imagery until the evening. Late in the day another plume rose to 6.7 km altitude and drifted W. Diffuse ash was seen in satellite imagery on 13 February fanning towards the W at 5.8 km altitude. On 18 February at 1720 UTC the Bogota Meteorological Weather Office (MWO) reported an ash emission drifting NW at 5.8 km altitude; a second plume was reported a few hours later at the same altitude. Intermittent emissions continued the next day at 5.8-6.1 km altitude that reached as far as 50 km NW before dissipating. A plume on 21 February rose to 6.7 km altitude and drifted W (figure 109). Occasional emissions on 25 February at the same altitude reached 25 km SW of the summit before dissipating. A discrete ash emission around 1550 UTC on 26 February rose to 6.1 km altitude and drifted W. Two similar plumes were reported the next day. On 28 and 29 February plumes rose to 5.8 km altitude and drifted W.

Figure 108. Emissions rose from the Arenas crater at Nevado del Ruiz on 5 February 2020. The Washington VAAC reported an ash plume that day that rose to 6.1 km altitude and drifted 37 km W before dissipating. Courtesy of Camilo Cupitre.

Figure 109. Emissions rose from the Arenas crater at Nevado del Ruiz around 0600 on 21 February 2020. The Washington VAAC reported ash emissions that day that rose to 6.7 km altitude and drifted W. Courtesy of Manuel MR.

SGC reported several episodes of drumbeat type seismicity on 2, 8, 9, and 27 February which they attributed to effusion related to the growing lava dome in the summit crater. Sentinel-2 satellite imagery showed ring-shaped thermal anomalies characteristic of dome growth within Arenas crater several times during January and February (figure 110). The VIIRS satellite instrument recorded thermal anomalies on twelve days during February.

Figure 110. Persistent thermal anomalies from Sentinel-2 satellite imagery during January and February 2020 suggested that the lava dome inside Nevado del Ruiz’s Arenas crater was still actively growing. Atmospheric penetration rendering (bands 12, 11, 8A) courtesy of Sentinel Hub Playground.

On 4, 14, and 19 March 2020 thermal anomalies were visible in Sentinel-2 satellite data from within the Arenas crater. Thermal anomalies were recorded by the VIIRS satellite instrument on eight days during the month. Several episodes of drumbeat seismicity were recorded during the first half of the month and on 30-31 March. Distinct SO₂ plumes with DU values greater than 2 were recorded by the TROPOMI satellite instrument daily throughout February and March (figure 111). The Washington VAAC reported an ash emission on 1 March that rose to 5.8 km altitude and drifted NW; it was centered 15 km from the summit when detected in satellite imagery. The next day a plume was seen in satellite imagery moving SW at 7.0 km altitude, extending nearly 40 km from the summit. Additional ash emissions were reported on 4, 14, 15, 21, 28, 29, and 31 March; the plumes rose to 5.8-6.7 km altitude and drifted generally W, some reaching 45 km from the summit before dissipating.

Figure 111. Distinct SO2 plumes with Dobson values (DU) greater than 2 were recorded by the TROPOMI satellite instrument daily during February and March 2020. Ecuador’s Sangay produced smaller but distinct plumes most of the time as well. Dates are shown at the top of each image. Courtesy of NASA’s Sulfur Dioxide Monitoring Page.

Activity during April-June 2020. The Washington VAAC reported an ash emission that rose to 6.7 km altitude and drifted W on 1 April 2020. On 2 April, emission plumes were visible from the community of Tena in the Cundinamarca municipality which is located 100 km ESE (figure 112). The unusually clear skies were attributed to the reduction in air pollution in nearby Bogota resulting from the COVID-19 Pandemic quarantine. On 4 April the Bogota MWO reported an emission drifting SW at 5.8 km altitude. An ash plume on 8 April rose to 6.7 km altitude and drifted W. On 25 April the last reported ash plume from the Washington VAAC for the period rose to 6.1 km altitude and was observed in satellite imagery moving W at 30 km from the summit; after that, only steam and gas emissions were observed.

Figure 112. On the evening of 2 April 2020, emission plumes from Nevado del Ruiz were visible from Santa Bárbara village in Tena, Cundinamarca municipality which is located 100 km ESE. The unusually clear skies were attributed to the reduction in air pollution in the nearby city of Bogota resulting from the COVID-19 Pandemic quarantine. Photo by Williama Garcia, courtesy of Semana Sostenible (3 April 2020).

Distinct SO₂ plumes with DU values greater than 2 were recorded by the TROPOMI satellite instrument daily throughout the month. On 13 April, a Sentinel-2 thermal image showed a hot spot inside the Arenas crater largely obscured by steam and clouds. Cloudy images through May and June prevented observation of additional thermal anomalies in satellite imagery, but the VIIRS thermal data indicated anomalies on 3, 4, and 26 April. SGC reported low-energy episodes of drumbeat seismicity on 4, 9, 10, 12, 15, 16, 20, and 23 April which they interpreted as related to growth of the lava dome inside the Arenas crater. The seismic events were located 1.5-2.0 km below the floor of the crater.

Small emissions of ash and gas were reported by SGC during May 2020 and the first half of June, with the primary drift direction being NW. Gas and steam plumes rose 560-1,400 m above the summit during May and June (figure 113). Drumbeat seismicity was reported a few times each month. Sulfur dioxide emissions continued daily; increased SO₂ activity was recorded during 10-13 June (figure 114).

Figure 113. Gas and steam plumes rose 560-1,400 m above the summit of Nevado del Ruiz during May and June 2020, including in the early morning of 11 June. Courtesy of Carlos-Enrique Ruiz.

Figure 114. Increased SO2 activity during 10-13 June 2020 at Nevado del Ruiz was recorded by the TROPOMI instrument on the Sentinel-5P satellite. Sangay also emitted SO2 on those days. Courtesy of NASA Global Sulfur Dioxide Monitoring Page.

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: El Servicio Geológico Colombiano (SGC), Diagonal 53 No. 34-53 - Bogotá D.C., Colombia (URL: https://www.sgc.gov.co/volcanes, https://twitter.com/sgcol); 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/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS OSPO, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: www.ospo.noaa.gov/Products/atmosphere/vaac, archive at: http://www.ssd.noaa.gov/VAAC/archive.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/); NASA Worldview (URL: https://worldview.earthdata.nasa.gov/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Camilo Cupitre (URL: https://twitter.com/Ccupitre/status/1225207439701704709); Manuel MR (URL: https://twitter.com/ElPlanetaManuel/status/1230837262088384512); Semana Sostenible (URL: https://sostenibilidad.semana.com/actualidad/articulo/fumarola-del-nevado-del-ruiz-fue-captada-desde-tena-cundinamarca/49597); Carlos-Enrique Ruiz (URL: https://twitter.com/Aleph43/status/1271800027841794049).

Japan's 24-km-wide Asosan caldera on the island of Kyushu has been active throughout the Holocene. Nakadake has been the most active of 17 central cones for 2,000 years; all historical activity is from Nakadake Crater 1. The largest ash plume in 20 years occurred on 8 October 2016. Asosan remained quiet until renewed activity from Crater 1 began in mid-April 2019; explosions with ash plumes continued through the first half of 2020 and are covered in this report. The Japan Meteorological Agency (JMA) provides monthly reports of activity; the Tokyo Volcanic Ash Advisory Center (VAAC) issues aviation alerts reporting on possible ash plumes, and Sentinel-2 satellite images provide data on ash emissions and thermal activity.

The Tokyo VAAC issued multiple daily reports of ash plumes from Nakadake Crater 1 from 1 January-14 June 2020. They were commonly at 1.8-2.1 km altitude, and often drifted E or S. JMA reported that ashfall continued downwind from the ash plumes until mid-June; seismic activity was relatively high during January and February and decreased steadily after that time. The measured SO₂ emissions ranged from 1,000-4,900 tons per day through mid-June and dropped to 500 tons per day during the second half of June. Intermittent thermal activity was recorded at the crater through mid-May.

Explosive activity during January-June 2020. Ash plumes rose up to 1.1 km above the crater rim at Nakadake Crater 1 during January 2020 (figure 70). Ashfall was confirmed downwind of an explosion on 7 January. During February, ash plumes rose up to 1.7 km above the crater, and ashfall was again reported downwind. The crater camera provided by the Aso Volcano Museum occasionally observed incandescence at the floor of the crater during both months. Incandescence was also occasionally observed with the Kusasenri webcam (3 km W) and was seen on 20 February from a webcam in Minamiaso village (8 km SW).

Figure 70. Ash plumes rose up to 1.1 km above the Nakadake Crater 1 at Asosan during January 2020 (left) and up to 1.7 km above the crater during February 2020 (right) as seen in these images from the Kusasenri webcam. Ashfall was reported downwind multiple times. Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, January and February 2020).

During March 2020, ash plumes rose as high as 1.3 km. Ashfall was reported on 9 March in Ichinomiyamachi, Aso City (figure 71). In field surveys conducted on the 18th and 25th, there was no visible water inside the crater, and high-temperature grayish-white plumes were observed. The temperature at the base of the plume was measured at 300°C (figure 72).

Figure 71. Ashfall from Asosan appeared on 9 March 2020 in Ichinomiyamachi, Aso City around 10 km N. Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, March 2020).

Figure 72. During a field survey of Nakadake Crater 1 at Asosan on 25 March 2020, JMA staff observed a gray ash plume rising from the crater floor (left). The maximum temperature of the ash plume was measured at about 300°C with an infrared thermal imaging device (right). Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, March 2020).

Occasional incandescence was observed at the bottom of the crater during April and May 2020; ash plumes rose 1.1 km above the crater on most days in April and were slightly higher, rising to 1.8 km during May, although activity was more intermittent (figure 73). A brief increase in SO₂ activity was reported by JMA during field surveys on 7 and 8 May; satellite data captured small plumes of SO₂ on 1 and 6 May (figure 74). A brief increase in tremor amplitude was reported by JMA on 16 May.

Figure 73. Although activity at Asosan’s Nakadake Crater 1 was more intermittent during April and May 2020 than earlier in the year, ash plumes were still reported most days and incandescence was seen at the bottom of the crater multiple times until 15 May. Left image taken 11 May 2020 from the Kusachiri webcam; right image taken from the crater webcam on 10 May provided by the Aso Volcano Museum. Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, May 2020).

Figure 74. The TROPOMI instrument on the Sentinel-5P satellite detected small but distinct SO2 plumes from Asosan on 1 and 6 May 2020. Additional small plumes are visible from Aira caldera’s Sakurajima volcano. Courtesy of NASA’s Global Sulfur Dioxide Monitoring Page.

The last report of ash emissions at Nakadake Crater 1 from the Tokyo VAAC was on 14 June 2020. JMA also reported that no eruption was observed after mid-June. On 8 June they reported an ash plume that rose 1.4 km above the crater. During a field survey on 16 June, only steam was observed at the crater; the plume rose about 100 m (figure 75). In addition, a small plume of steam rose from a fumarole on the S crater wall.

Figure 75. A steam plume rose about 100 m from the floor of Nakadake Crater 1 on 16 June 2020. A small steam plume was also observed by the S crater wall. Courtesy of JMA (Volcanic activity commentary material for Mt. Aso, June 2020).

Thermal activity during January-June 2020. Sentinel-2 satellite data indicated thermal anomalies present at Nakadake Crater 1 on 2 January, 6 and 21 February, 16 April, and 11 May (figure 76). In addition, thermal anomalies from agricultural fires appeared in satellite images on 11 February, 7 and 17 March (figure 77). The fires were around 5 km from the crater, thus they appear on the MIROVA thermal anomaly graph in black, but are likely unrelated to volcanic activity (figure 78). No thermal anomalies were recorded in satellite data from the Nakadake Crater 1 after 11 May, and none appeared in the MIROVA data as well.

Figure 76. Thermal anomalies appeared at Asosan’s Nakadake Crater 1 on 2 January, 6 and 21 February, 16 April and 11 May 2020. On 2 January a small ash plume drifted SSE from the crater (left). On 6 February a dense ash plume drifted S from the crater (center). Only a small steam plume was visible above the crater on 21 February (right). Images use Atmospheric penetration rendering (bands 12, 11, 8A), courtesy of Sentinel Hub Playground.

Figure 77. Thermal anomalies from agricultural fires located about 5 km from the crater appeared in satellite images on 11 February, and 7 and 17 March 2020. Although a dense ash plume drifted SSE from the crater on 11 February (left), no thermal anomalies appear at the crater on these dates. Images use Atmospheric penetration rendering (bands 12, 11, 8A), courtesy of Sentinel Hub Playground.

Figure 78. The MIROVA project plot of Log Radiative Power at Asosan from 29 June 2019 through June 2020 shows only a few small thermal alerts within 5 km of the summit crater during January-June 2020, and a spike in activity during February and March located around 5 km away. These data correlate well with the Sentinel-2 satellite data that show intermittent thermal anomalies at the summit throughout January-May and agricultural fires located several kilometers from the crater during February and March. Courtesy of MIROVA.

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: Japan Meteorological Agency (JMA), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/jma/indexe.html); 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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Sakurajima rises from Kagoshima Bay, which fills the Aira Caldera near the southern tip of Japan's Kyushu Island. Frequent explosive and occasional effusive activity has been ongoing for centuries. The Minamidake summit cone has been the location of persistent activity since 1955; the adjacent Showa crater on its E flank has also been intermittently active since 2006. Numerous explosions and ash-bearing emissions have been occurring each month at either Minamidake or Showa crater since the latest eruptive episode began in late March 2017. This report covers ongoing activity at Minamidake from January through June 2020; the Japan Meteorological Agency (JMA) provides regular reports on activity, and the Tokyo VAAC (Volcanic Ash Advisory Center) issues tens of reports each month about the frequent ash plumes.

Activity continued during January-June 2020 at Minamidake crater with tens of explosions each month. The Tokyo VAAC issued multiple daily reports of ash emissions during January and February. Less activity occurred during the first half of March but picked up again with multiple daily reports from mid-March through mid-April. Emissions were more intermittent but continued through early June, when activity decreased significantly. JMA reported explosions with ash plumes rising 2.5-4.2 km above the summit, and ejecta traveling generally up to 1,700 m from the crater, although a big explosion in early June send a large block of tephra 3 km from the crater (table 23). Thermal anomalies were visible in satellite imagery on a few days most months and were persistent in the MIROVA thermal anomaly data from November 2019 through early June 2020 (figure 94). Incandescence was often visible at night in the webcams through early June; the Showa crater remained quiet throughout the period.

Table 23. Monthly summary of eruptive events recorded at Sakurajima's Minamidake crater within the Aira Caldera, January through June 2020. The number of events that were explosive in nature are in parentheses. No events were recorded at the Showa crater during this time. Ashfall is measured at the Kagoshima Local Meteorological Observatory (KLMO), 10 km W of Showa crater. Data courtesy of JMA (January to June 2020 monthly reports).

Figure 94. Persistent thermal anomalies were recorded in the MIROVA thermal energy data for the period from 2 July 2019 through June 2020. Thermal activity increased in October 2019 and remained steady through May 2020, decreasing abruptly at the beginning of June. Courtesy of MIROVA.

Explosions continued at Minamidake crater during January 2020 with 65 ash plumes reported. The highest ash plume rose 2.5 km above the crater on 30 January, and incandescent ejecta reached up to 1,700 m from the Minamidake crater on 22 and 29 January (figure 95). Slight inflation of the volcano since September 2019 continued to be measured with inclinometers and extensometers on Sakurajima Island. Field surveys conducted on 15, 20, and 31 January measured the amount of sulfur dioxide gas released as very high at 3,400-4,700 tons per day, as compared with 1,000-3,000 tons in December 2019.

Figure 95. An explosion at the Minamidake summit crater of Aira’s Sakurajima volcano on 29 January 2020 produced an ash plume that rose 2.5 km above the crater rim and drifted SE (left). On 22 January incandescent ejecta reached 1,700 m from the summit during explosive events. Courtesy of JMA (Sakurajima Volcanic Activity Commentary, January 2020).

About the same number of explosions produced ash plumes during February 2020 (67) as in January (65) (figure 96). On 10 February a large block was ejected 1,800 m from the crater, the first to reach that far since 5 February 2016. The tallest plume, on 26 February rose 2.6 km above the crater. Sentinel-2 satellite imagery indicated two distinct thermal anomalies within the Minamidake crater on both 1 and 6 February (figure 97). Activity diminished during March 2020 with only 10 explosions out of 26 eruptive events. On 21 March a large bomb reached 1,700 m from the crater. The tallest ash plume rose 3 km above the crater on 17 March. Scientists noted during an overflight on 16 March that a small steam plume was rising from the inner wall on the south side of the Showa crater; a larger steam plume rose to 300 m above the Minamidake crater and drifted S (figure 98). Sulfur dioxide emissions were similar in February (1,900 to 3,100 tons) and March (1,300 to 3,400 tons per day).

Figure 96. An ash plume rose from the Minamidake crater at the summit of Aira’s Sakurajima volcano on 6 February 2020 at 1752 local time, as seen looking S from the Kitadake crater. Courtesy of JMA (Sakurajima Volcanic Activity Commentary, February 2020).

Figure 97. Sentinel-2 satellite imagery revealed two distinct thermal anomalies within the Minamidake crater at Aira’s Sakurajima volcano on 1 and 6 February 2020. Images use Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub playground.

Figure 98. During an overflight of Aira’s Sakurajima volcano on 16 March 2020, JMA captured this view to the SW of the Kitadake crater on the right, the steam-covered Minamidake crater in the center, and the smaller Showa crater on the left adjacent to Minamidake. Courtesy of JMA and the Maritime Self-Defense Force 1st Air Group P-1 (Sakurajima Volcanic Activity Commentary, March 2020).

During April 2020, ejecta again reached as far as 1,700 m from the crater; 14 explosions were identified from the 51 reported eruptive events, an increase from March. The tallest plume, on 4 April, rose 3.8 km above the crater (figure 99). The same number of eruptive events occurred during May 2020, but 24 were explosive in nature. A large plume on 9 May rose to 4.2 km above the rim of Minamidake crater, the tallest of the period (figure 100). On 20 May, incandescent ejecta reached 1,300 m from the summit. Sulfur dioxide emissions during April (1,700-2,100 tons per day) and May (1,200-2,700 tons per day) were slightly lower than previous months.

Figure 99. A large ash plume at Aira’s Sakurajima volcano rose from Minamidake crater at 1621 on 4 April 2020. The plume rose to 3.8 km above the crater and drifted SE. Courtesy of JMA (Sakurajima Volcanic Activity Commentary, April 2020).

Figure 100. Activity continued at Aira’s Sakurajima volcano during May 2020. A large plume rose to 4.2 km above the summit and drifted N in the early morning of 9 May (left). The Kaigata webcam located at the Osumi River National Highway Office captured abundant incandescent ejecta reaching 1,300 m from the crater during the evening of 20 May. Courtesy of JMA (Sakurajima Volcanic Activity Commentary, May 2020).

A major explosion on 4 June 2020 produced 137 Pa of air vibration at the Seto 2 observation point on Sakurajima Island. It was the first time that air vibrations exceeding 100 Pa have been observed at the Seto 2 station since the 21 May 2015 explosion at the Showa crater. The ash plume associated with the explosion rose 1.5 km above the crater rim. During an 8 June field survey conducted in Higashisakurajima-cho, Kagoshima City, a large impact crater believed to be associated with this explosion was located near the coast 3 km SSW from Minamidake. The crater formed by the ejected block was about 6 m in diameter and 2 m deep (figure 101); fragments found nearby were 10-20 cm in diameter (figure 102). A nearby roof was also damaged by the blocks. Smaller bombs were found in Kurokami-cho, Kagoshima City, around 4- 5 km E of Minamidake on 5 June; the largest fragment was 5 cm in diameter. Multiple ash plumes rose to 3 km or more above the summit during the first ten days of June; explosions on 4 and 5 June reached 3.7 km above the crater (figure 103). Larger than normal inflation and deflation before and after the explosions was recorded during early June in the inclinometers and extensometers located on the island. Incandescence at the summit was observed at night through the first half of June. The Tokyo VAAC issued multiple daily ash advisories during 1-10 June after which activity declined abruptly. Two brief explosions on 23 June and one on 28 June were the only two additional ash explosions reported in June.

Figure 101. A large crater measuring 6 m wide and 2 m deep was discovered 3 km from the Minamidake crater in Higashisakurajima, part of Kagoshima City, on 8 June 2020. It was believed to be from the impact of a large block ejected during the 4 June explosion at Aira’s Sakurajima volcano. Photo courtesy of Kagoshima City and JMA (Sakurajima Volcanic Activity Commentary, June 2020).

Figure 102. Fragments 10-30 cm in diameter from a large bomb that traveled 3 km from Minamidake crater on Sakurajima were found a few days after the 4 June 2020 explosion at Aira. Courtesy of JMA, photo courtesy of Kagoshima City (Sakurajima Volcanic Activity Commentary, June 2020).

Figure 103. An ash plume rose 3.7 km above the Minamidake crater at Aira’s Sakurajima volcano on 5 June 2020 and was recorded in Sentinel-2 satellite imagery. Image uses Atmospheric penetration rendering (bands 12, 11, 8A). Courtesy of Sentinel Hub playground.

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 Aira caldera, along with several post-caldera cones. The construction of Sakurajima began about 13,000 years ago on the southern rim of Aira caldera and built an island that was finally joined to the Osumi Peninsula during the major explosive and effusive eruption of 1914. Activity at the Kitadake summit cone ended about 4850 years ago, after which eruptions took place at Minamidake. Frequent historical eruptions, recorded since the 8th century, have deposited ash on Kagoshima, one of Kyushu's largest cities, located across Kagoshima Bay only 8 km from the summit. The largest historical 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/); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Nevados de Chillán is a complex of late-Pleistocene to Holocene stratovolcanoes in the Chilean Central Andes. An eruption started with a phreatic explosion and ash emission on 8 January 2016 from a new crater (Nicanor) on the E flank of the Nuevo crater, itself on the NW flank of the large Volcán Viejo stratovolcano. Strombolian explosions and ash emissions continued throughout 2016 and 2017; a lava dome within the Nicanor crater was confirmed in early January 2018. Explosions and pyroclastic flows continued during 2018 and 2019, with several lava flows appearing in late 2019. This report covers continuing activity from January-June 2020 when ongoing explosive events produced ash plumes, pyroclastic flows, and the growth of new dome inside the crater. Information for this report is provided primarily by Chile's Servicio Nacional de Geología y Minería (SERNAGEOMIN)-Observatorio Volcanológico de Los Andes del Sur (OVDAS), and by the Buenos Aires Volcanic Ash Advisory Center (VAAC).

Explosions with ash plumes rising up to three kilometers above the summit area were intermittent from late January through early June 2020. Some of the larger explosions produced pyroclastic flows that traveled down multiple flanks. Thermal anomalies within the Nicanor crater were recorded in satellite data several times each month from February through June. A reduction in overall activity led SERNAGEOMIN to lower the Alert Level from Orange to Yellow (on a 4-level, Green-Yellow-Orange-Red scale) during the first week of March, although tens of explosions with ash plumes were still recorded during March and April. Explosive activity diminished in early June and SERNAGEOMIN reported the growth of a new dome inside the Nicanor crater. By the end of June, a new flow had extended about 100 m down the N flank. Thermal activity recorded by the MIROVA project showed a drop in thermal energy in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in thermal and explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June (figure 52).

Figure 52. MIROVA thermal anomaly data for Nevados de Chillan from 8 September 2019 through June 2020 showed a drop in thermal activity in mid-December 2019 after the lava flows of September-November stopped advancing. A decrease in activity in January and February 2020 was followed by an increase in explosive activity in March and April. Renewed thermal activity from the growth of a new dome inside the Nicanor crater was recorded beginning in mid-June. Courtesy of MIROVA.

Weak gas emissions were reported daily during January 2020 until a series of explosions began on the 21st. The first explosion rose 100 m above the active crater; the following day, the highest explosion rose 1.6 km above the crater. The Buenos Aires VAAC reported pulse emissions visible in satellite imagery on 21 and 24 January that rose to 3.9-4.3 km altitude and drifted SE and NE, respectively. Intermittent explosions continued through 26 January. Incandescent ejecta was observed during the night of 28-29 January. The VAAC reported an isolated emission on 29 January that rose to 5.2 km altitude and drifted E. A larger explosion on 30 January produced an ash plume that SERNAGEOMIN reported at 3.4 km above the crater (figure 53). It produced pyroclastic flows that traveled down ravines on the NNE and SE flanks. The Washington VAAC reported on behalf of the Buenos Aires VAAC that an emission was observed in satellite imagery on 30 January that rose to 4.9 km altitude and was moving rapidly E, reaching 15 km from the summit at midday. The altitude of the ash plume was revised two hours later to 7.3 km, drifting NNE and rapidly dissipating. Satellite images identified two areas of thermal anomalies within the Nicanor crater that day. One was the same emission center (CE4) identified in November 2019, and the second was a new emission center (CE5) located 60 m NW.

Figure 53. A significant explosion and ash plume from the Nicanor crater at Nevados de Chillan on 30 January 2020 produced an ash plume reported at 7.3 km altitude. The left image was taken within one minute of the initial explosion. Images posted by Twitter accounts #EmergenciasÑuble (left) and T13 (right); original photographers unknown.

When the weather permitted, low-altitude mostly white degassing was seen during February 2020, often with traces of fine-grained particulate material. Incandescence at the crater was observed overnight during 4-5 February. The Buenos Aires VAAC reported an emission on 14 February visible in the webcam. The next day, an emission was visible in satellite imagery at 3.9 km altitude that drifted E. Episodes of pulsating white and gray plumes were first observed by SERNAGEOMIN beginning on 18 February and continued through 25 February (figure 54). The Buenos Aires VAAC reported pulses of ash emissions moving SE on 18 February at 4.3 km altitude. Ash drifted E the next day at 3.9 km altitude and a faint plume was briefly observed on 20 February drifting N at 3.7 km altitude before dissipating. Sporadic pulses of ash moved SE from the volcano on 22 February at 4.3 km altitude, briefly observed in satellite imagery before dissipating. Thermal anomalies were visible from the Nicanor crater in Sentinel-2 satellite imagery on 23 and 28 February.

Figure 54. An ash emission at Nevados de Chillan on 18 February 2020 was captured in Sentinel-2 satellite imagery drifting SE (left). Thermal anomalies within the Nicanor crater were measured on 23 (right) and 28 February. Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Only low-altitude degassing of mostly steam was reported for the first half of March 2020. When SERNAGEOMIN lowered the Alert Level from Orange to Yellow on 5 March, they reduced the affected area from 5 km NE and 3 km SW of the crater to a radius of 2 km around the active crater. Thermal anomalies were recorded at the Nicanor crater in Sentinel-2 imagery on 4, 9, 11, 16, and 19 March (figure 55). A new series of explosions began on 19 March; 44 events were recorded during the second half of the month (figure 56). Webcams captured multiple explosions with dense ash plumes; on 25 and 30 March the plumes rose more than 2 km above the crater. Fine-grained ashfall occurred in Las Trancas (10 km SW) on 25 March. Pyroclastic flows on 25 and 30 March traveled 300 m NE, SE, and SW from the crater. Incandescence was observed at night multiple times after 20 March. The Buenos Aires VAAC reported several discrete pulses of ash that rose to 4.3 km altitude and drifted SE on 20 and 21 March, SW on 25 March, and SE on 29 and 30 March. Another ash emission rose to 5.5 km altitude later on 30 March and drifted SE.

Figure 55. Sentinel-2 Satellite imagery of Nevados de Chillan during March 2020 showed thermal anomalies on five different dates at the Nicanor crater, including on 9, 11, and 16 March. A second thermal anomaly of unknown origin was also visible on 11 March about 2 km SW of the crater (center). Images use Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

Figure 56. Forty-four explosive events were recorded at Nevados de Chillan during the second half of March 2020 including on 19 March. Courtesy of SERNAGEOMIN webcams and chillanonlinenoticia.

In their semi-monthly reports for April 2020, SERNAGEOMIN reported 94 explosive events during the first half of the month and 49 during the second half; many produced dense ash plumes. The Buenos Aires VAAC reported frequent intermittent ash emissions during 1-13 April reaching altitudes of 3.7-4.3 km (figure 57). They reported the plume on 8 April visible in satellite imagery at 7.3 km altitude drifting SE. An emission on 13 April was also visible in satellite imagery at 6.1 km altitude drifting NE.

Figure 57. Sentinel-2 satellite imagery captured a strong thermal anomaly and an ash plume drifting SE from Nevados de Chillan on 10 April 2020. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

During the second half of April 2020, SERNAGEOMIN reported that only one plume exceeded 2 km in height; on 21 April, it rose to 2.4 km above the crater (figure 58). The Buenos Aires VAAC reported isolated pulses of ash on 18, 26, 28, and 30 April. During the second half of April SERNAGEOMIN also reported that a pyroclastic flow traveled about 1,200 m from the crater rim down the SE flank. The ash from the pyroclastic flow drifted SE and S as far as 3.5 km. Satellite images showed continued activity from multiple emission centers around the crater. Pronounced scarps were noted on the internal walls of the crater, attributed to the deepening of the crater from explosive activity.

Figure 58. Tens of explosions were reported at Nevados de Chillan during the second half of April 2020 that produced dense ash plumes. The plume on 21 April rose 2.4 km above the Nicanor crater. Photo by Josefa Carrasco Acuña from San Fabián de Alico; posted by Noticias Valpo Express.

Intermittent explosive activity continued during May 2020. The plumes contained abundant particulate material and were accompanied by periodic pyroclastic flows and incandescent ejecta around the active crater, especially visible at night. The Buenos Aires VAAC reported several sporadic weak ash emissions during the first week of May that rose to 3.7-5.2 km altitude and drifted NE. SERNAGEOMIN reported that only one explosion produced an ash emission that rose more than two km above the crater during the first two weeks of the month; on 6 May it rose to 2.5 km above the crater and drifted NE. They also observed pyroclastic flows on the E and SE flanks that day. Additional pyroclastic flows traveled 450 m down the S flank during the first half of the month, and similar deposits were observed to the N and NE. Satellite observations showed various emission points along the NW-trending lineament at the summit and multiple erosion scarps. Major erosion was noted at the NE rim of the crater along with an increase in degassing around the rim.

During the second half of May 2020 most of the ash plumes rose less than 2 km above the crater; a plume from one explosion on 22 May rose 2.2 km above the crater; the Buenos Aires VAAC reported the plume at 5.5 km altitude drifting NW (figure 59). Continuing pyroclastic emissions deposited material as far as 1.5 km from the crater rim on the NNW flank. There were also multiple pyroclastic deposits up to 500 m from the crater directed N and NE during the period. SERNAGEOMIN reported an increase in steam degassing between Nuevo-Nicanor and Nicanor-Arrau craters.

Figure 59. Explosions produced dense ash plumes and pyroclastic flows at Nevados de Chillan multiple times during May 2020 including on 22 May. Courtesy of SERNAGEOMIN.

Webcam images during the first two weeks of June 2020 indicated multiple incandescent explosions. On 3 and 4 June plumes from explosions reached heights of over 1.25 km above the crater; the Buenos Aires VAAC reported them drifting NW at 3.9 km altitude. Incandescent ejecta on 6 June rose 760 m above the vent and drifted NE. In addition, pyroclastic flows were distributed on the N, NW, E and SE flanks. Significant daytime and nighttime incandescence was reported on 6, 9, and 10 June (figure 60). The VAAC reported emission pulses on 6 and 9 June drifting E and SE at 4.3 km altitude.

Figure 60. Multiple ash plumes with incandescence were reported at Nevados de Chillan during the first ten days of June 2020 including on 6 June, after which explosive activity decreased significantly. Courtesy of SERNAGEOMIIN and Sismo Alerta Mexicana.

SERNAGEOMIN reported that beginning on the afternoon of 9 June 2020 a tremor-type seismic signal was first recorded, associated with continuous emission of gas and dark gray ash that drifted SE (figure 61). A little over an hour later another tremor signal began that lasted for about four hours, followed by smaller discrete explosions. A hybrid-type earthquake in the early morning of 10 June was followed by a series of explosions that ejected gas and particulate matter from the active crater. The vent where the emissions occurred was located within the Nicanor crater close to the Arrau crater; it had been degassing since 30 May.

Figure 61. A tremor-type seismic signal was first recorded on the afternoon of 9 June 2020 at Nevados de Chillan. It was associated with the continuous emission of gas and dark gray ash that drifted SE, and incandescent ejecta visible after dark. View is to the S, courtesy of SERNAGEOMIN webcam, posted by Volcanology Chile.

After the explosions on the afternoon of 9 June, a number of other nearby vents became active. In particular, the vent located between the Nuevo and Nicanor craters began emitting material for the first time during this eruptive cycle. The explosion also generated pyroclastic flows that traveled less than 50 m in multiple directions away from the vent. Abundant incandescent material was reported during the explosion early on 10 June. Deformation measurements showed inflation over the previous 12 days.

SERNAGEOMIN identified a surface feature in satellite imagery on 11 June 2020 that they interpreted as a new effusive lava dome. It was elliptical with dimensions of about 85 x 120 m. In addition to a thermal anomaly attributed to the dome, they noted three other thermal anomalies between the Nuevo, Arrau, and Nicanor craters. They reported that within four days the base of the active crater was filled with effusive material. Seismometers recorded tremor activity after 11 June that was interpreted as associated with lava effusion. Incandescent emissions were visible at night around the active crater. Sentinel-2 satellite imagery recorded a bright thermal anomaly inside the Nicanor crater on 14 June (figure 62).

Figure 62. A bright thermal anomaly was recorded inside the Nicanor crater at Nevados de Chillan on 14 June 2020. SERNAGEOMIN scientists attributed it to the growth of a new lava dome within the crater. Image uses Atmospheric penetration rendering (bands 12, 11, 8a); courtesy of Sentinel Hub Playground.

A special report from SERNAGEOMIN on 24 June 2020 noted that vertical inflation had increased during the previous few weeks. After 20 June the inflation rate reached 2.49 cm/month, which was considered high. The accumulated inflation measured since July 2019 was 22.5 cm. Satellite imagery continued to show the growth of the dome, and SERNAGEOMIN scientists estimated that it reached the E edge of the Nicanor crater on 23 June. Based on these images, they estimated an eruptive rate of 0.1-0.3 m³/s, about two orders of magnitude faster than the Gil-Cruz dome that emerged between December 2018 and early 2019.

Webcams revealed continued low-level explosive activity and incandescence visible both during the day and at night. By the end of June, webcams recorded a lava flow that extended 94 m down the N flank from the Nicanor crater and continued to advance. Small explosions with abundant pyroclastic debris produced recurring incandescence at night. Satellite infrared imagery indicated thermal radiance from effusive material that covered an area of 37,000 m², largely filling the crater. DEM analysis suggested that the size of the crater had tripled in volume since December 2019 due largely to erosion from explosive activity since May 2020. Sentinel-2 satellite imagery showed a bright thermal anomaly inside the crater on 27 June.

Geologic Background. The compound volcano of Nevados de Chillán is one of the most active of the Central Andes. Three late-Pleistocene to Holocene stratovolcanoes were constructed along a NNW-SSE line within three nested Pleistocene calderas, which produced ignimbrite sheets extending more than 100 km into the Central Depression of Chile. The largest stratovolcano, dominantly andesitic, Cerro Blanco (Volcán Nevado), is located at the NW end of the group. Volcán Viejo (Volcán Chillán), which was the main active vent during the 17th-19th centuries, occupies the SE end. The new Volcán Nuevo lava-dome complex formed between 1906 and 1945 between the two volcanoes and grew to exceed Volcán Viejo in elevation. The Volcán Arrau dome complex was constructed SE of Volcán Nuevo between 1973 and 1986 and eventually exceeded its height.

Information Contacts: Servicio Nacional de Geología y Minería (SERNAGEOMIN), Observatorio Volcanológico de Los Andes del Sur (OVDAS), Avda Sta María No. 0104, Santiago, Chile (URL: http://www.sernageomin.cl/, https://twitter.com/Sernageomin); 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/); Buenos Aires Volcanic Ash Advisory Center (VAAC), Servicio Meteorológico Nacional-Fuerza Aérea Argentina, 25 de mayo 658, Buenos Aires, Argentina (URL: http://www.smn.gov.ar/vaac/buenosaires/inicio.php); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); #EmergenciasÑuble (URL: https://twitter.com/urgenciasnuble/status/1222943399185207296); T13, Channel 13 Press Department (URL: https://twitter.com/T13/status/1222951071443771394); Chillanonlinenoticia (URL: https://twitter.com/ChillanOnline/status/1240754211932995595); Noticias Valpo Express (URL: https://twitter.com/NoticiasValpoEx/status/1252715033131388928); Sismo Alerta Mexicana (URL: https://twitter.com/Sismoalertamex/status/1269351579095691265); Volcanology Chile (URL: https://twitter.com/volcanologiachl/status/1270548008191643651).

Kerinci is a stratovolcano located in Sumatra, Indonesia that has been characterized by explosive eruptions with ash plumes and gas-and-steam emissions. The most recent eruptive episode began in April 2018 which has included intermittent explosions and ash plumes. The previous report (BGVN 44:12) described more recent activity consisting of intermittent gas-and-steam and ash plumes which occurred during June through early November 2019. This volcanism continued through May 2020, though little to no activity was reported during December 2019. The primary source of information for this report comes from Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM) and the Darwin Volcanic Ash Advisory Centre (VAAC).

Activity during December 2019 consisted of white gas-and-steam emissions rising 100-500 m above the summit. White and brown emissions continued intermittently through May 2020, rising to a maximum altitude of 1 km above the summit on 14 April. During 3-6 and 8-9 January 2020, the Darwin VAAC and PVMBG issued notices reporting brown volcanic ash rising 150-600 m above the summit drifting S and ESE (figure 19). PVMBG published a VONA notice on 24 January at 0828 reporting ash rising 400 m above the summit. Brown emissions continued intermittently throughout the reporting period. On 1 February, volcanic ash was observed rising 300-960 m above the summit and drifting NE; PVMBG reported continuing brown emissions during 1-3 February. During 16-17 February, two VONA notices reported that brown ash plumes rose 150-400 m above the summit and drifted SW accompanied by consistent white gas-and-steam emissions (figure 20).

Figure 19. Brown ash plume rose 500-600 m above Kerinci on 4 January 2020. Courtesy of MAGMA Indonesia via Øystein Lund Andersen.

Figure 20. White gas-and-steam emissions rose 400 m above Kerinci on 19 February 2020. Courtesy of MAGMA Indonesia via Øystein Lund Andersen.

During 1-16 and 25-26 March 2020 brown ash emissions were frequently observed rising 100-500 m above the summit drifting in multiple directions. During 6-8 and 10-15, April brown ash emissions were reported 50-1,000 m above the summit. The most recent Darwin VAAC and VONA notices were published on 14 April, reporting volcanic ash rising 400 and 600 m above the summit, respectively; however, PVMBG reported brown emissions rising up to 1,000 m. By 25-27 April brown ash emissions rose 50-300 m above the summit. Intermittent white gas-and-steam emissions continued through May. The last brown emissions seen in May were reported on the 7th rising 50-100 m above the summit.

Geologic Background. Gunung Kerinci in central Sumatra forms Indonesia's highest volcano and is one of the most active in Sumatra. It is capped by an unvegetated young summit cone that was constructed NE of an older crater remnant. There is a deep 600-m-wide summit crater often partially filled by a small crater lake that lies on the NE crater floor, opposite the SW-rim summit. The massive 13 x 25 km wide volcano towers 2400-3300 m above surrounding plains and is elongated in a N-S direction. Frequently active, Kerinci has been the source of numerous moderate explosive eruptions since its first recorded eruption in 1838.

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/); 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/); MAGMA Indonesia, Kementerian Energi dan Sumber Daya Mineral (URL: https://magma.vsi.esdm.go.id/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Øystein Lund Andersen (Twitter: @OysteinLAnderse, https://twitter.com/OysteinLAnderse, URL: http://www.oysteinlundandersen.com, images at https://twitter.com/OysteinLAnderse/status/1213658331564269569/photo/1 and https://twitter.com/OysteinLAnderse/status/1230419965209018369/photo/1).

Tinakula is a remote stratovolcano located 100 km NE of the Solomon Trench at the N end of the Santa Cruz. In 1971, an eruption with lava flows and ash explosions caused the small population to evacuate the island. Volcanism has previously been characterized by an ash explosion in October 2017 and the most recent eruptive period that began in December 2018 with renewed thermal activity. Activity since then has consisted of intermittent thermal activity and dense gas-and-steam plumes (BGVN 45:01), which continues into the current reporting period. This report updates information from January-June 2020 using primary source information from various satellite data, as ground observations are rarely available.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed weak, intermittent, but ongoing thermal activity during January-June 2020 (figure 41). A small cluster of slightly stronger thermal signatures was detected in late February to early March, which is correlated to MODVOLC thermal alert data; four thermal hotspots were recorded on 20, 27, and 29 February and 1 March. However, observations using Sentinel-2 satellite imagery were often obscured by clouds. In addition to the weak thermal signatures, dense gas-and-steam plumes were observed in Sentinel-2 satellite imagery rising from the summit during this reporting period (figure 42).

Figure 41. Weak thermal anomalies at Tinakula from 26 June 2019 through June 2020 as recorded by the MIROVA system (Log Radiative Power) were intermittent and clustered more strongly in late February to early March.

Figure 42. Sentinel-2 satellite imagery shows ongoing gas-and-steam plumes rising from Tinakula during January through May 2020. Images with atmospheric penetration (bands 12, 11, 8a) rendering; courtesy of Sentinel Hub Playground.

Three distinct thermal anomalies were observed in Sentinel-2 thermal satellite imagery on 22 January, 11 April, and 6 May 2020, accompanied by some gas-and-steam emissions (figure 43). The hotspot on 22 January was slightly weaker than the other two days, and was seen on the W flank, compared to the other two that were observed in the summit crater. According to MODVOLC thermal alerts, a hotspot was recorded on 6 May, which corresponded to a Sentinel-2 thermal satellite image with a notable anomaly in the summit crater (figure 43). On 10 June no thermal anomaly was seen in Sentinel-2 satellite imagery due to the presence of clouds; however, what appeared to be a dense gas-and-steam plume was extending W from the summit.

Figure 43. Sentinel-2 thermal satellite images showing a weak thermal activity (bright yellow-orange) on 22 January 2020 on the W flank of Tinakula (top) and slightly stronger thermal hotspots on 11 April (middle) and 6 May (bottom) in at the summit, which are accompanied by gas-and-steam emissions. Images with atmospheric penetration (bands 12, 11, 8a) rendering; courtesy of Sentinel Hub Playground.

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. Similar to Stromboli, 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 satellitic cone of Mendana 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. Frequent historical eruptions have 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Ibu is an active stratovolcano located along the NW coast of Halmahera Island in Indonesia. Volcanism has recently been characterized by frequent ash explosions, ash plumes, and small lava flows within the crater throughout 2019 (BGVN 45:01). Activity continues, consisting of frequent white-and-gray emissions, ash explosions, ash plumes, and lava flows. This report updates activity through June 2020, using data from the Pusat Vulkanologi dan Mitigasi Bencana Geologi (PVMBG, also known as Indonesian Center for Volcanology and Geological Hazard Mitigation, CVGHM), Darwin Volcanic Ash Advisory Centre (VAAC), and various satellites.

Volcanism during the entire reporting period dominantly consisted of white-and-gray emissions that rose 200-800 m above the summit drifting in multiple directions. The ash plume with the maximum altitude of 13.7 km altitude occurred on 16 May 2020. Sentinel-2 thermal satellite imagery detected multiple smaller hotspots within the crater throughout the reporting period.

Continuous ash emissions were reported on 6 February rising to 2.1 km altitude drifting E, accompanied by a hotspot visible in infrared satellite imagery. On 16 February, a ground observer reported an eruption that produced an ash plume rising 800 m above the summit drifting W, according to a Darwin VAAC notice. Ash plumes continued through the month, drifting in multiple directions and rising up to 2.1 km altitude. During 8-10 March, video footage captured multiple Strombolian explosions that ejected incandescent material and produced ash plumes from the summit (figures 21 and 22). Occasionally volcanic lightning was observed within the ash column, as recorded in video footage by Martin Rietze. This event was also documented by a Darwin VAAC notice, which stated that multiple ash emissions rose 2.1 km altitude drifting SE. PVMBG published a VONA notice on 10 March at 1044 reporting ash plumes rising 400 m above the summit. PVMBG and Darwin VAAC notices described intermittent eruptions on 26, 28, and 29 March, all of which produced ash plumes rising 300-800 m above the summit.

Figure 21. Strombolian explosions recorded at the crater summit of Ibu during 8-10 March 2020 ejected incandescent ejecta and a dense ash plume. Video footage copyright by Martin Rietze, used with permission.

Figure 22. Strombolian explosions recorded at the crater summit of Ibu during 8-10 March 2020 ejected incandescent ejecta and ash. Frequent volcanic lightning was also observed. Video footage copyright by Martin Rietze, used with permission.

A majority of days in April included white-and-gray emissions rising up to 800 m above the summit. A ground observer reported an eruption on 9 April, according to a Darwin VAAC report, and a hotspot was observed in HIMAWARI-8 satellite imagery. Minor eruptions were reported intermittently during mid-April and early to mid-May. On 12 May at 1052 a VONA from PVMBG reported an ash plume 800-1,100 m above the summit. A large short-lived eruption on 16 May produced an ash plume that rose to a maximum of 13.7 km altitude and drifted S, according to the Darwin VAAC report. By June, volcanism consisted predominantly of white-and-gray emissions rising 800 m above the summit, with an ash eruption on 15 June. This eruptive event resulted in an ash plume that rose 1.8 km altitude drifting WNW and was accompanied by a hotspot detected in HIMAWARI-8 satellite imagery, according to a Darwin VAAC notice.

The MIROVA (Middle InfraRed Observation of Volcanic Activity) system detected frequent hotspots during July 2019 through June 2020 (figure 23). In comparison, the MODVOLC thermal alerts recorded a total of 24 thermal signatures over the course of 19 different days between January and June. Many thermal signatures were captured as small thermal hotspots in Sentinel-2 thermal satellite imagery within the crater (figure 24).

Figure 23. Thermal anomalies recorded at Ibu from 2 July 2019 through June 2020 as recorded by the MIROVA system (Log Radiative Power) were frequent and consistent in power. Courtesy of MIROVA.

Figure 24. Sentinel-2 thermal satellite imagery (bands 12, 11, 8A) showed occasional thermal hotspots (bright orange) in the Ibu summit crater during January through June 2020. Courtesy of Sentinel Hub Playground.

Geologic Background. The truncated summit of Gunung Ibu stratovolcano along the NW coast of Halmahera Island has large nested summit craters. The inner crater, 1 km wide and 400 m deep, contained several small crater lakes through much of historical time. The outer crater, 1.2 km wide, is breached on the north side, creating a steep-walled valley. A large parasitic cone is located ENE of the summit. A smaller one to the WSW has fed a lava flow down the W flank. A group of maars is located below the N and W flanks. Only a few eruptions have been recorded in historical time, the first a small explosive eruption from the summit crater in 1911. An eruption producing a lava dome that eventually covered much of the floor of the inner summit crater began in December 1998.

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/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); Martin Rietze, Taubenstr. 1, D-82223 Eichenau, Germany (URL: https://mrietze.com/, https://www.youtube.com/channel/UC5LzAA_nyNWEUfpcUFOCpJw/videos, video at https://www.youtube.com/watch?v=qMkfT1e4HQQ).

Suwanosejima is an active stratovolcano located in the northern Ryukyu Islands. Volcanism has previously been characterized by Strombolian explosions, ash plumes, and summit incandescence (BGVN 45:01), which continues to occur intermittently. A majority of this activity originates from vents within the large Otake summit crater. This report updates information during January through June 2020 using monthly reports from the Japan Meteorological Agency (JMA), the Tokyo Volcanic Ash Advisory Center (VAAC), and various satellite data.

During 3-10 January 2020, 13 explosions were detected from the Otake crater rising to 1.4 km altitude; material was ejected as far as 600 m away and ashfall was reported in areas 4 km SSW, according to JMA. Occasional small eruptive events continued during 12-17 January, which resulted in ash plumes that rose 1 km above the crater rim and ashfall was again reported 4 km SSW. Crater incandescence was visible nightly during 17-24 January, while white plumes rose as high as 700 m above the crater rim.

Nightly incandescence during 7-29 February, and 1-6 March, was accompanied by intermittent explosions that produced ash plumes rising up to 1.2 km above the crater rim (figure 44); activity during early February resulted in ashfall 4 km SSW. On 19 February an eruption produced a gray-white ash plume that rose 1.6 km above the crater (figure 45), resulting in ashfall in Toshima village (4 km SSW), according to JMA. Explosive events during 23-24 February ejected blocks onto the flanks. Two explosions were recorded during 1-6 March, which sent ash plumes as high as 900-1,000 m above the crater rim and ejected large blocks 300 m from the crater.

Figure 44. Surveillance camera images of summit incandescence at Suwanosejima on 29 January (top left), 21 (middle left) and 23 (top right) February, and 25 March (bottom left and right) 2020. Courtesy of JMA (Monthly bulletin reports 511, January, February, and March 2020).

Figure 45. Surveillance camera images of which and white-and-gray gas-and-steam emissions rising from Suwanosejima on 5 January (top), 19 February (middle), and 24 March 2020 (bottom). Courtesy of JMA (Monthly bulletin reports 511, January, February, and March 2020).

Nightly incandescence continued to be visible during 13-31 March, 1-10 and 17-24 April, 1-8, 15-31 May, 1-5 and 12-30 June 2020; activity during the latter part of March was relatively low and consisted of few explosive events. In contrast, incandescence was frequently accompanied by explosions in April and May. On 28 April at 0432 an eruption produced an ash plume that rose 1.6 km above the crater rim and drifted SE and E, and ejected blocks as far as 800 m from the crater. The MODVOLC thermal alerts algorithm also detected four thermal signatures during this eruption within the summit crater. An explosion at 1214 on 29 April caused glass in windows to vibrate up to 4 km SSW away while ash emissions continued to be observed following the explosion the previous day, according to the Tokyo VAAC.

During 1-8 May explosions occurred twice a day, producing ash plumes that rose as high as 1 km above the crater rim and ejecting material 400 m from the crater. An explosion on 29 May at 0210 produced an off-white plume that rose as high as 500 m above the crater rim and ejected large blocks up to 200 m above the rim. On 5 June an explosion produced gray-white plumes rising 1 km above the crater. Small eruptive events continued in late June, producing ash plumes that rose as high as 900 m above the crater rim.

MIROVA (Middle InfraRed Observation of Volcanic Activity) analysis of MODIS satellite data showed relatively stronger thermal anomalies in late February and late April 2020 with an additional six weaker thermal anomalies detected in early January (2), early February (1), mid-April (2), and mid-May (1) (figure 46). Sentinel-2 thermal satellite imagery in late January through mid-April showed two distinct thermal hotspots within the summit crater (figure 47).

Figure 46. Prominent thermal anomalies at Suwanosejima during July-June 2020 as recorded by the MIROVA system (Log Radiative Power) occurred in late February and late April. Courtesy of MIROVA.

Figure 47. Sentinel-2 thermal satellite images showing small thermal anomalies (bright yellow-orange) from two locations within the Otake summit crater at Suwanosejima. Images with “Atmospheric penetration” (bands 12, 11, 8A) rendering; courtesy of Sentinel Hub Playground.

Geologic Background. The 8-km-long, spindle-shaped island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two historically active summit craters. The summit is truncated by a large breached crater extending to the sea on the east flank that was formed by edifice collapse. Suwanosejima, one of Japan's most frequently active volcanoes, was in a state of intermittent strombolian activity from Otake, the NE summit crater, that began in 1949 and lasted until 1996, after which periods of inactivity lengthened. The largest historical eruption took place in 1813-14, when thick scoria deposits blanketed 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 the horseshoe-shaped Sakuchi caldera, which extends 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); Tokyo Volcanic Ash Advisory Center (VAAC), 1-3-4 Otemachi, Chiyoda-ku, Tokyo 100-8122, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); 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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground).

Bagana lies in a nearly inaccessible mountainous tropical rainforest area of Bougainville Island in Papua New Guinea and is primarily monitored by satellite imagery of ash plumes and thermal anomalies. After a state of elevated activity that lasted through December 2018 (BGVN 43:05, 44:06, 44:12), the volcano entered a quieter period that persisted through at least May 2020. This report focuses on activity between December 2019 and May 2020.

Atmospheric clouds often obscured satellite views of the volcano during the reporting period. When the volcano could be observed, light-colored gas plumes were often observed (figure 43). Based on satellite and wind model data, the Darwin Volcanic Ash Advisory Centre (VAAC) reported that during 29 February-2 March ash plumes rose to an altitude of 1.8-2.1 km and drifted SW and N. On 1 May an ash plume rose to an altitude of 3 km and drifted NW and W. According to both Darwin VAAC volcanic ash advisories, the Aviation Color Code was Orange (second highest of four hazard levels).

Figure 43. Sentinel-2 image of Bagana, showing a gas plume drifting SE on 13 March 2020, during a period when the Darwin VAAC had not reported any ash explosions (Natural Color rendering, bands 4, 3, 2). Courtesy of Sentinel Hub Playground.

During the reporting period, the MIROVA (Middle InfraRed Observation of Volcanic Activity) volcano hotspot detection system recorded only intermittent thermal anomalies, all of which were of low radiative power. Sulfur dioxide emissions detected by satellite-based instruments over this reporting period were at low levels.

Geologic Background. Bagana volcano, occupying a remote portion of central Bougainville Island, is one of Melanesia's youngest and most active volcanoes. This massive symmetrical cone was largely constructed by an accumulation of viscous andesitic lava flows. The entire edifice could have been constructed in about 300 years at its present rate of lava production. Eruptive activity is frequent and characterized by non-explosive effusion of viscous lava that maintains a small lava dome in the summit crater, although explosive activity occasionally producing pyroclastic flows also occurs. Lava flows form dramatic, freshly preserved tongue-shaped lobes up to 50 m thick with prominent levees that descend the flanks on all sides.

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/); Sentinel Hub Playground (URL: https://www.sentinel-hub.com/explore/sentinel-playground); 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/); 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/).

This report summarizes Japan Meteorological Agency (JMA) monthly reports (available in English since October 2010) covering the interval April 2011 to September 2012, with a separate subsection largely focused on aviation reports of Aso plumes emitted at Naka-dake crater during mid-2011. During this reporting interval Naka-dake continued to degas and emit small ash plumes. Eruptions of mud resumed after a hiatus of several years (February 2008 to April 2011).

Aso (also called Aso-san) is a caldera with dimensions ~17 km E-W by ~25 km N-S encompassing an area of ~350 km². Figure 28 indicates the location of Aso in relation to other Holocene Japanese volcanoes and landmarks in the region.

Figure 28. A map of the major volcanoes of Japan. Aso is shown on the left side, on the island of Kyushu. Courtesy of the U.S. Geological Survey.

Aso's most recent series of eruptions began in April 2011, with minor phreatic (mud-bearing) eruptions from Naka-dake's crater lake. These eruptions were accompanied by minor ash plumes, rock ejections, an increase in the temperature of fumaroles (BGVN 36:09), and continuous, small-amplitude tremor.

Field observations during April 2011-June 2011. In April 2011, a small phreatic (mud-bearing) eruption 5-10-m-high was observed in Naka-dake's crater lake; the lake's temperature was 67°C. Volcanic seismicity remained at a relatively low level. A photo from 21 April 2011 shows a white steam plume (figure 29).

Figure 29. (A) A photo taken by a field survey team on 21 April 2011 shows a white steam plume rising from the crater floor. (B) A photo taken on 16 May 2011 shows a grayish plume venting from the crater floor. Courtesy of JMA.

From 3 to 10 May, continuous small-amplitude tremor was detected. Seismicity, including isolated-pulse events, remained relatively low during this time. On 6 and 9 May, field surveyers observed a small 5-10-m-high phreatic eruption from the hot crater lake (locally called "Yudamari").

A camera installed by the Aso Volcano Museum detected a small volcanic ash emissions from within the crater beginning on 13 May. Six cameras provide live image feeds to the Aso Museum website. There are also many videos showing Aso and Naka-dake on YouTube.

On 13 May, a field survey found increased fumarole temperatures in the crater, and a video camera revealed incandescence on multiple nights. According to JMA, a small eruption occurred on 15 May followed by minor ashfall, which extended 2 km NE of the crater. A field survey on 15 May recorded a temperature of ~370°C at a fumarole in the crater.

Another eruption occurred on 16 May, producing a grayish plume that rose 500 m above the crater rim. As a result of this increased activity, the Alert Level was raised from 1 to 2 (on a scale from 1-5). A field surveyer later the same day saw a gray plume rise 800 m above the crater rim (figure 29). Small-scale eruptions occurred intermittently on the 17th. The lake water volume was low around this time, ~10-20% of its full volume.

A 9 June field survey revealed a decrease in fumarole temperatures from ~370°C on 15 May to ~160°C on 9 June. After 10 June, eruptions ceased and the lake water volume increased from 60% full on 12 June to 80% full on 17 June (figure 30). The rising lake level suggested a decrease in activity. Consequently, the Alert Level was lowered from 2 to 1 on 20 June. Seismicity, including isolated-pulse events, remained at relatively low levels.

Figure 30. (A) Photo taken on 9 June 2011 showing the bottom of Naka-dake crater. Note the absence (or near absence) of the crater's lake. (B) Photo taken on 22 June 2011 showing the presence of the steaming crater lake just about two weeks after the photo in (A) was taken. Courtesy of JMA.

Plume heights and drift directions during May-June 2011. We summarize reports from the Tokyo Volcanic Ash Advisory Center (VAAC) issued between 15 May and 9 June 2011 (table 10). Many plumes contained ash. Notice that the plume heights are stated as altitudes above sea level (as compared to heights above the crater rim, as in the other sections of this report).

Table 10. Summary of plumes at Aso between 15 May and 9 June 2011. Smaller plumes may not have been recorded or were omitted. In most cases, the presence of ash in the plume was noted; in other cases ash may have been present but not recorded. '-' indicates data not reported. Data provided by Tokyo VAAC and JMA.

Field observations during October 2011-June 2012. In October 2011, white plumes rose on average less than 200 m above the crater rim, with a maximum of 300 m. The lake water volume during September and October was at about 90% full, and the September and October lake-surface temperatures were 47-56°C and 49-58°C, respectively. Based on field surveys made on 3, 17, and 20 October, the sulfur-dioxide (SO₂) flux was ~300-500 tons/day, compared to ~300 tons/day in September. Volcanic seismicity remained low. Tremor, detected 13 times during September, was absent during October. The total magnetic intensity measured at the NW rim of the Naka-dake crater had increased since December 2010, but was static during June 2011 through October 2011. No change was detected by GPS measurements.

The next JMA monthly report on Aso discussed activity during May and June 2012. Because of heavy rains after 15 May, the lake water volume had increased to ~70% full, and during the course of the month the volume was in the range 60-80% full. Then in late May, the lake level begain to drop, and continued into at least mid-June.

The lake surface temperature was 63-72°C in May and 67-73°C in June. The highest temperature of fumaroles along the southern crater wall was 246-260°C, compared to 228-267°C in May. Scientists conducting a field survey at night on 22 June noted that part of the S crater wall was incandescent.

In June 2012, white plumes rose an average of 600 m above the crater rim. There were 621 isolated cases of tremor in June, approaching a 2-fold increase over some of the previous months, but only amounting to a duration of a few minutes per month. Isolated volcanic tremor and seismicity remained low but had slightly increased overall after February 2012, with most hypocenters located at shallow depths under Naka-dake. No change was detected by GPS measurements. The total magnetic intensity began to increase again in June 2012.

Lake levels during July-September 2012. In July, heavy rains caused the lake level to rise to 80-90% full (from 30-70% full in June). The volume remained high in August and September (90-100% full). During June-July the lake surface temperature decreased slowly, from 58-66°C in July to 57-61°C in August and to 54-59°C in September. Steam emissions from the crater occurred in July and August, but stopped by September.

Crater temperatures during July-September 2012. The highest temperature of the S wall of Naka-dake-Daiichi crater decreased in July, but rose slightly in August and September (213-250°C in July, 241-249°C in August, and 250-283°C in September). A field survey on 24 September revealed that the hot areas had not changed since the previous survey on 22 June. On 23-26 September, weak glow in the crater was recorded at night by a thermal camera. Officials assumed the glow was caused by the hot crater wall.

July-September 2012 seismicity. Both isolated volcanic tremor and other seismicity returned at low levels during July-September 2012. 621 volcanic tremors occurred in June, 669 in July, 1,025 in August and 867 in September. 669 volcanic earthquakes occurred in July, 951 in August, and 978 in September. Other seismic events occurred 369 times in June, 626 in July, and were not reported in August or September. Few short-term tremors occurred (4 in June, none in July, 2 in August, and 1 in September). Most hypocenters were located at shallow depths (2-4 km) and in an area ~6 km NE of Naka-dake.

Based on field studies, sulfur dioxide levels were elevated during May-September 2012 (600-800 t/d in May, ~400 t/d on 10 July, and 500-700 t/d on 19 and 24 September). The total magnetic intensity at the NW rim of Naka-dake-Daiishi crater increased between December 2010 and September 2012, which officials suggested might signify a temperature rise underneath the crater.

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: Japan Meteorological Agency (JMA), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/); Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Aso Volcano Museum (URL: http://www.asomuse.jp/); Volcano Discovery (URL: http://www.volcanodiscovery.com/); Earth Observation Research Center (Japan) (URL: http://www.eorc.jaxa.jp/en/index.php).

This report covers ongoing dome growth and other activity at Bezymianny since our previous report in January 2010 (BGVN 34:11) and extending into early September 2012. Multiple strong eruptions occurred during this reporting period. In one case, on 2 September 2012, an eruption generated a plume that rose to 10-12 km altitude and was later detected 1,500 km from the vent. In this and many other cases, fresh lava flows were extruded at the dome. Some intervals of the remainder of 2010 and early 2011 were chiefly characterized by intermittent thermal anomalies at the dome and fumarolic activity.

The data in this report come primarily from the Kamchatka Volcanic Eruptions Response Team (KVERT) and the Tokyo Volcanic Ash Advisory Center (VAAC). Portions of this report were initially synthesized and edited by Matthew Loewen, submitted as part of a graduate student writing assignment in a volcanology class at Oregon State University under the guidance of professor Shan de Silva.

The Kamchatka peninsula's low population density often thwarts confirmation of significant events, and seismic signals were likely obscured by activity at nearby Kliuchevskoi volcano. Seismic activity and other observations between 29 January 2010 and 3 September 2012 are summarized in table 5.

Table 5. Summary of activity at Bezymianny from 29 January 2010 through 3 September 2012. Data courtesy of KVERT, Tokyo VAAC, and Anchorage VAAC.

Several abstracts discussing the June 2010 explosive eruption were presented at the Fall 2010 American Geophysical Union conference in San Francisco. These studies were primarily the work of the U.S.-Russia Partnership for Volcanological Research and Education (PIRE). Part of the initiative was to install and monitor 14 GPS stations around Bezymianny (Serovetnikov and others, 2010; their figure 4). Over the course of the five-year project, the scientists noted precursory changes in GPS-measured surface velocity. The anomalies occurred 15-25 days before, and 25-30 days after, typical eruptions, suggesting relatively short periods of shallow magma storage before eruptions. Grapenthin and others (2010) also reported that during the December 2009 and May 2010 eruptions, the 12 available GPS stations showed little or no significant inflation before explosions, suggesting the magma was deeply sourced.

Izbekov and others (2010) reported that the December 2009 and June 2010 eruptive products contained abundant high-silica, amphibole-bearing enclaves. This was in contrast to all previous eruptions since 1956. Until December 2009, the juvenile products of Bezymianny were remarkably homogeneous; enclaves and xenoliths had been exceptionally rare.

Figures 13-15 show images and photos of Bezymianny that help document the 14 April 2011 eruption, which is also noted in table 5. Several other strong eruptions took place later in the reporting interval (discussed below).

Figure 13. A natural-color EO-1 satellite image of Bezymianny acquired 22 April 2011 showing evidence of the size of the 14 April eruption. Dark volcanic deposits (likely a combination of pyroclastic flows and lahars) extend more than 7.3 km SW into valleys. A light-colored plume of ash, steam, and SO2 rises above the summit and drifts W. Volcanic ash covers the upper slopes of the volcano, especially to the S and W. White snow, still deep in late April, blankets the surrounding landscape as seen in figure 15. These images were acquired on 22 April 2011 by the Advanced Land Imager (ALI) aboard the Earth Observing-1 (EO-1) satellite. Caption and figure courtesy of Jesse Allen and Robert Simmons, NASA Earth Observatory.

Figure 14. Bezymianny, as captured 22 April 2011 in an EO-1 false-color satellite image. At the summit, a red hot spot indicates where fresh lava extruded to the growing lava dome. To the SE, an active lava flow appears as a similar hot spot. In these wavelengths, bare rock and ash are gray; snow and ice appear cyan. These images were acquired near noon on 22 April 2011 by the Advanced Land Imager (ALI) aboard the Earth Observing-1 (EO-1) satellite. Caption and figure courtesy of Jesse Allen and Robert Simmons, NASA Earth Observatory.

Figure 15. Photographs depicting the ash from the 14 April 2011 eruption of Bezymianny mantling the snow base. Courtesy of KVERT.

On 8 March 2012, KVERT raised Bezymianny's Aviation Color Code to Red after a sharp and sustained increase in seismic activity. KVERT also noted a significant increase in both the size and temperature of a thermal anomaly at the summit, suggesting that new, hot magma was very close to or at the dome's surface. Therefore, the organization suggested that "strong ash explosions up to 13 km a.s.l. were possible at any time during the next 24 hours." The following day, 9 March, Bezymainny exploded; the magnitude of the volcanic tremor was 7.52 m/s. Ash plumes from pyroclastic flows rose to 8 km in altitude and drifted NE. According to later satellite data, the ash plume was distinguishable for ~700 km. In addition, gas-and-steam plumes containing ash rose to an altitude of 3.5-4.0 km and drifted NE. Seismologists reported that the explosion did not pose a threat to population centers in the area. After the strong explosive phase, the eruptive vigor decreased gradually and continued at a low level. Following the 8-9 March event, KVERT lowered the Aviation Color Code to Orange.

During 9-13 March, video captured strong gas-and-steam emissions; no ash was noted. Strong degassing accompanied the effusion of a viscous lava flow on the S flank of the lava dome, along with moderate-to-strong gas-and-steam emissions. Seismic activity was low after 10 March, although the volcano emitted gas-and-steam plumes during 14-15 March. Satellites continued to record thermal anomalies. KVERT lowered the Aviation Color Code to Yellow.

According to visual observations during 15-16 March, the length of the 8 March 2012 pyroclastic deposits was ~4 km. According to satellite data, a thermal anomaly continued to register at the volcano on 23 and 25-26 March. Clouds obscured the volcano on other days of week.

The viscous lava flow continued to effuse on the S flank of the lava dome, accompanied by degassing, well into May. KVERT noted thermal anomalies (detected by satellite) during 29-31 March, 3-4, 9-10, 13-17, 19, 28-29 April, and 3 May. Seismic activity remained low.

According to KVERT, seismicity increased during the middle of August 2012. On 28 August, 17 events were recorded; on 31 August, 71 events were detected. Observers noted weak-to-moderate fumarolic activity during 25-26 and 29 August; cloud cover prevented observations on other days. A thermal anomaly was detected in satellite imagery on 25 August.

On 2 September, an explosion sent ash plumes to an altitude of 10-12 km; plumes drifted more than 1,500 km ENE. A thermal anomaly observed in satellite imagery was very bright before the explosion. The Aviation Color Code was raised to Orange, then Red. Later that day, ash plumes rose to an altitude of 4 km and drifted NE before ash emissions ceased. The Aviation Color Code was then lowered to Yellow. On 3 September seismic activity was low, while a viscous lava flow effused on the lava-dome flank, accompanied by fumarolic activity and hot avalanches.

References. Grapenthin, R., Freymueller, J.T., and Serovetnikov, S., 2010. The December 2009 and May 2010 eruptions of Bezymianny volcano, Kamchatka: Interpretation of the GPS Record, American Geophysical Union, Fall Meeting 2010, abstract #V33D-04.

Izbekov, P.E., Neill, O.K., Shipman, J.S., Turner, S.J., Shcherbakov, V.D., and Plechov, P., 2010. Silicic Enclaves in Products of 2009-2010 Eruptions of Bezymianny Volcano, Kamchatka: Implications for Magma Processes, American Geophysical Union, Fall Meeting 2010, abstract #V33D-01.

Serovetnikov, S., Freymueller, J.T., Titkov, N., Bahtiarov, V., and Senyukov, S,2010. GPS Monitoring Bezimyany Volcano 2006-2010 (Kamchatka), American Geophysical Union, Fall Meeting 2010, abstract #V21B-2325.

Geologic Background. Prior to its noted 1955-56 eruption, Bezymianny had been considered extinct. The modern volcano, much smaller in size than its massive neighbors Kamen and Kliuchevskoi, was formed about 4700 years ago over a late-Pleistocene lava-dome complex and an ancestral edifice built about 11,000-7000 years ago. Three periods of intensified activity have occurred during the past 3000 years. The latest period, which was preceded by a 1000-year quiescence, began with the dramatic 1955-56 eruption. This eruption, similar to that of St. Helens in 1980, produced a large horseshoe-shaped crater that was formed by collapse of the summit and an associated lateral blast. Subsequent episodic but ongoing lava-dome growth, accompanied by intermittent explosive activity and pyroclastic flows, has largely filled the 1956 crater.

Information Contacts: Kamchatka Volcanic Eruptions Response Team (KVERT), Institute of Volcanology and Seismology (IV&S) Far East Division, Russian Academy of Sciences (FEDRAS), Kamchatka Branch of the Geophysical Service of the Russian Academy of Sciences (KBGS RAS), Piip Ave. 9, Petropavlovsk-Kamchatsky, 683006, Russia (URL: http://www.kscnet.ru/ivs/, http://www.emsd.ru/~ssl/monitoring/main.htm); Sergei Ushakov, IVS FED RAS; Tokyo Volcanic Ash Advisory Center (VAAC), Tokyo, Japan (URL: http://ds.data.jma.go.jp/svd/vaac/data/); Alaska Volcano Observatory (AVO), a cooperative program of the U.S. Geological Survey, the University of Alaska’s Geophysical Insitute, and the Alaska Division of Geological & Geophysical Surveys (URL: http://www.avo.alaska.edu/); Anchorage Volcanic Ash Advisory Center (VAAC), Alaska Aviation Weather Unit, NWS NOAA US Dept of Commerce, 6930 Sand Lake Road, Anchorage, AK 99502-1845 (URL: http://www.ssd.noaa.gov/).

219 low-magnitude earthquakes occurred at Campi Flegrei during September 2012, a comparatively large number with respect to the previous year (figure 22). The earthquakes chiefly were contained within two swarms (with events up to M_w 1.9; M_w indicates moment magnitude) occurring on 7 and 15 September. Peak ground accelerations (PGA) were non-trivial (up to ~0.5 g), and some earthquakes were widely felt by area residents. Analysis revealed that the strain release rate of the 7 September swarm fell within values seen for other swarms during the last 20 years. The observations reported by the Vesuvius Observatory (who provided the material for this report) were limited to those associated with the earthquakes and related seismic analysis. Other reporting on topics such as deformation appears on the Observatory's website (see Information Contacts, below). The observatory is part of Italy's National Institute of Geophysics and Volcanology (INGV).

Figure 22. Campi Flegrei earthquake count recorded between October 2011 and the end of September 2012. (A) The number of earthquakes recorded per month during October 2011-September 2012 (288 total events). (B) The number of earthquakes recorded during September 2012 alone (219 total events), highlighting the swarm of 188 events on 7 September. Courtesy of Vesuvius Observatory-INGV (Naples).

Almost all of the earthqaukes that occurred during September took place in two swarms (figures 22 and 23). The first swarm occurred in the area of Pozzuoli during 0715-0935 UTC on 7 September. The two largest events of that swarm were M_w 1.9 (a duration magnitude, M_d, value of 1.7; figure 24); these events were the largest recorded events of the prior year (figure 24A). The 7 September swarm was dominant over the 15 September swarm both in terms of the number and magnitude of events.

Figure 23. (A) Hypocentral locations registered at Campi Flegrei during October 2011-August 2012 (blue) and September 2012 (red). The size of the symbols is proportional to the magnitude, as shown in the lower right box. (B) A map with the seismic network at Campi Flegrei. The boxed area zooms in on the region where the two swarms occured. Courtesy of Vesuvius Observatory-INGV (Naples).

Figure 24. Magnitudes (duration magnitude, Md) of seismic events recorded at Campi Flegrei during October 2011-September 2012 (A). (B) shows the details of the computed magnitudes during the September 2012 seismic swarm. Courtesy of Vesuvius Observatory-INGV (Naples).

The second swarm of September 2012 took place between 0901 and 1012 UTC on 15 September (figure 22), with the strongest events (M_d -0.3) occurring at 0947 and 0954 UTC. This swarm was recorded by only one station (STH, Agnano, figure 23B) and thus was plausibly located in close proximity to that station at shallow depth. This swarm is absent on the depth plot in figure 25 (depth not available).

Figure 25. Time series plots of the hypocentral depths of seismic events recorded at Campi Flegrei during October 2011-September 2012 (A) and during September 2012 (B) showing details of the September 2012 seismic swarm. Courtesy of Vesuvius Observatory-INGV (Naples).

The hypocenters of 49 events were determined during September 2012; their depths were generally less than 4 km (figures 23 and 25). The seismological parameters did not show significant anomalies (figures 24 and 25). However, September 2012 was the most seismically energetic time period of the prior year (figure 26); seismicity during September produced >3 times the cumulative energy released during the preceding year.

Figure 26. Cumulative seismic energy released at Campi Flegrei during (A) October 2011-September 2012 and (B) September 2012. Courtesy of Vesuvius Observatory-INGV (Naples).

Analysis of the 7 September seismic swarm. For the two main events (0734 and 0825 UTC) on 7 September, source parameters were determined from S-wave displacement spectra (results shown in figure 27).

Figure 27. Displacement spectra (blue) for the S-waves of the largest events in the 7 September 2012 seismic swarm, occurring at 0734 (top) and 0825 UTC (bottom). The red curves represent the fit with a theoretical model. The displacement spectra were obtained from the records of the accelerometer CPOZ (Pozzuoli, figure 23B). The tabulated values display the computed source parameters for each event: Mw, moment magnitude; Md, duration magnitude; Fc, corner frequency; R (m), source radius, and stress drop (bars). For discussion of source parameters see Mooney (1989). Courtesy of Vesuvius Observatory-INGV (Naples).

The duration and strain release of the 7 September swarm were similar to other seismic swarms at Campi Flegrei since at least 1994 (figure 28).

Figure 28. A plot showing duration and strain release rate for Campi Flegrei seismic swarms since 1994. Courtesy of Vesuvius Observatory-INGV (Naples).

Some of the events in the swarm were widely felt in the urban area of Pozzuoli. Peak ground acceleration values (PGA, units of %g, the acceleration due to gravity) recorded by the accelerometer in Pozzuoli (CPOZ, figure 23B) show two prominent peaks corresponding to the two largest events that occurred at 0734 and 0825 UTC (figure 29).

Figure 29. Peak ground acceleration values (PGA, in units of %g, the acceleration due to gravity) recorded by the accelerometer CPOZ (Pozzuoli, figure 23B) between 0700 and 1100 UTC on 7 September 2012. The visible gap in the data between 0722 and 0733 was caused by technical problems in the data transmission system. The two largest events are labelled with their timestamps and PGA values. Courtesy of Vesuvius Observatory-INGV (Naples).

Reference. Mooney, W.D., 1989. Seismic methods for determining earthquake source parameters and lithospheric structure, in Pakiser, L.C. and Mooney, W.D. (eds), Geophysical framework of the continental United States, Geological Society of America Memoir 172.

Geologic Background. Campi Flegrei is a large 13-km-wide caldera on the outskirts of Naples that contains numerous phreatic tuff rings and pyroclastic cones. The caldera margins are poorly defined, and on the south lie beneath the Gulf of Pozzuoli. Episodes of dramatic uplift and subsidence within the dominantly trachytic caldera have occurred since Roman times. The earliest known eruptive products are dated 47,000 yrs BP. The caldera formed following two large explosive eruptions, the massive Campanian ignimbrite about 36,000 BP, and the over 40 km3 Neapolitan Yellow Tuff (NYT) about 15,000 BP. Following eruption of the NYT a large number of eruptions have taken place from widely scattered subaerial and submarine vents. Most activity occurred during three intervals: 15,000-9500, 8600-8200, and 4800-3800 BP. Two eruptions have occurred in historical time, one in 1158 at Solfatara and the other in 1538 that formed the Monte Nuovo cinder cone.

Information Contacts: Vesuvius Observatory, National Institute of Geophysics and Volcanology (INGV), Via Diocleziano 328, 80124 Napoli, Italy (URL: http://www.ov.ingv.it/ov/).

Our last report on Nevado del Ruiz (BGVN 37:07) summarized monitoring efforts by the Instituto Colombiano de Geología y Minería (INGEOMINAS) volcano observatory based in Manizales, highlighting the long records of geophysical and radon-gas data starting in 1988 and continuing through 2006. Here we follow up on volcanic activity from 2007 to 2012, including an escalation leading to explosions in February 2012. Elevated seismicity, wide-spread ashfall, and very high SO₂ fluxes (~30,000 tons/day) resulted in a Level I Red Alert announcement (on a scale from IV to I, Alert Level I is the highest, "Red Alert") in June 2012 and public notices of evacuations. Activity subsided in July 2012 and remained low through the remainder of this reporting period ending 9 September 2012.

Seismicity from 2007-August 2010. From 2007 to August 2010, INGEOMINAS reported numerous volcano-tectonic (VT) and long-period (LP) events originating at depths of 1-12 km below Nevado del Ruiz. Rare hybrid and tremor earthquakes were detected, and seismic swarms occurred intermittently (19-78 events per swarm; figure 54). Seismicity was frequently concentrated within the crater and to the SE, S, SW, and W (table 3).

Figure 54. Maps of located earthquakes at Nevado del Ruiz during the month of April 2010. (Left) This map shows the distribution of VT events and cross-sections for depths in 1 km intervals; the 15 April 2010 swarm is circled. (Right) This map shows 209 registered LP events (M 0.09-2.15); frequencies were below 5 Hz with average event durations of 0.3 s. LP events were concentrated in a zone to the W of the crater, a characteristic observed in records since 2006. Courtesy of INGEOMINAS.

Table 3. Seismicity types and counts at Nevado del Ruiz registered from 2006 to September 2012 compiled from INGEOMINAS reports. The LP Total column accounts for all forms of LPs including hybrid and tornillo when present; tornillo earthquakes are described by Narváez and others (1997). The TR/TO column contains tremor ("TR") and tornillos ("TO"). Epicenter Clustering refers to directions relative to the crater, and to epicenters occurring within the immediate crater region "C". Notable Seismicity includes swarms with dates and the number of events provided when known in parentheses; seismicity interpreted as possible explosions is listed as "ES" (explosion signature); multi-events ("ME") refer to seismicity that is described in figure 56; pseudo-tornillo events are listed ("PT"), a class of earthquakes also detected at Galeras volcano (BGVN 37:04) and illustrated in figure 55. For all entries with "na," this represents seismicity that has been recorded but only tallied within the LP Total column. The "x" indicates values not currently available. Shading (yellow, orange, and red) corresponds to the alert announcements released by authorities according to the level of hazardous conditions. Courtesy of INGEOMINAS.

Geodesy, 2007-August 2010. Deformation monitoring expanded in late 2007 when INGEOMINAS installed additional electronic tilt stations, augmenting their dry tilt datasets. Dry tilt measurements had been recorded since at least 1986 (see the station distribution map, figure 10 in BGVN 37:07). While the term "dry tilt" is pervasive in volcano monitoring literature, this can cause confusion as it was originally adopted to differentiate measurements made with water leveling techniques (Yamashita, 1992). Alternative terms are "single-setup leveling" or "tilt leveling" however, the term "inclinómetro seco," has been used consistently throughout INGEOMINAS monthly technical reports since March 2006. Tilt measurements collected with site occupation techniques are manually intensive, requiring extensive field time, reliable benchmark pairs, a spirit level, and leveling rods. In August 2010, dry tilt values were available from three stations and electronic tilt values were available from five operating stations; results were reported in the INGEOMINAS technical bulletin (available online).

In August 2008, electronic distance meter (EDM) base stations and reflectors were installed on the W flank of the volcano. Site occupations at Olleta and Refugio recorded stable conditions from September 2008 through August 2010.

Gas emissions, 2007-August 2010. Frequent steam plumes were visible reaching 50-850 m above the crater from January 2007 through August 2010. On 17 July 2010, the Washington Volcanic Ash Advisory Center (VAAC) was alerted to a spike in seismicity detected at Nevado del Ruiz. Several aviation alerts were released; however, no volcanic ash was detected in satellite imagery and advisories were canceled that same day. Several peaks in diffuse soil CO₂ emissions were detected in mid-2008 from two geochemical stations, Gualí and Cajones (N and S of the summit, respectively).

Radon-gas emissions measured at Gualí and Cajones also showed peaks in early 2010. INGEOMINAS had maintained emission records since 1995 and was investigating links between radon emissions and earthquakes (Garzón and others, 2003). Radon hazard investigations had been conducted in Manizales (located ~30 km NW of the volcano) by INGEOMINAS that determined water supply and household levels of radon (Salazar and others, 2003). This baseline data was mapped for SE Manizales and showed low levels of radon in water supplies and also low levels at the 43 indoor sites where passive sampling detected an average of 1.9 pCi/L.

During fieldwork on 30 November-1 December 2009, INGEOMINAS installed two scanning Differential Optical Absorption Spectrometer (DOAS) systems within 5 km W of the edifice. Stations Bruma and Alfombrales were telemetered to send SO₂ flux data to the Manizales observatory where results were analyzed with NOVAC software. The Network for Observation of Volcanic and Atmospheric Change (NOVAC), designed by the European Commission's Sixth Framework Program, supported this installation. Colombia was one of seven countries participating in the program that sought to monitor and assess SO₂ emissions from active volcanoes (Galle and others, 2009). During 2-29 December, SO₂ flux ranged 195-554 t/d at Bruma and 41-140 t/d at Alfombrales.

Escalating seismicity from September 2010 to 2011. Seismicity notably increased in September 2010 and prompted authorities to raise the alert to Level III (Yellow, on the four-level scale) on 30 September (table 3). Within four months, pseudo-tornillo earthquakes (figure 55) and possible explosive signatures appeared in the seismic record. From September 2010 through December 2011, an average of more than 890 VT earthquakes per month were recorded, almost eight times as many events as recorded during the previous 12 months. A similar increase in LP events was also observed during this time period; however, epicenters were clustered in the same regions as previous years: within the crater, to the SE, S, SW, and W (as in figure 54).

Figure 55. This long-period earthquake (described as a pseudo-tornillo) was recorded on 6 January 2011 at 1343 from Nevado del Ruiz on seven seismic stations (appearing strongest on station BISz, the trace second from the top). BISz is the closest seismic station to the volcano, located ~2 km W of the crater. The spectra (right) show a dominant frequency of ~6.25 Hz; this characteristic, in addition to the relatively short coda, classified the event as a pseudo-tornillo (Narváez and others, 1997). Courtesy of INGEOMINAS.

A type of earthquake classified as "multi-event" began to appear in February 2011 (see ME events in table 3). These events frequently occurred from February through August and were attributed to small explosions and degassing (figure 56). Tremor and tornillo earthquakes were recorded in March of 2011 and, over the next six months, occurred more frequently with time.

Figure 56. Seismic traces of a "multi-event" registered at 1351 on 6 October 2011 as recorded at five stations around Nevado del Ruiz. The earthquake appeared strongest at BISz, the closest station to the volcano, and much weaker-to-unrecognizable at other stations. Courtesy of INGEOMINAS.

Geodesy, September 2010-2011. During September 2010-2011, INGEOMINAS recorded stable conditions with minor fluctuations from the EDM stations Refugio and Olleta. Both stations were surveyed in February, October, and November 2011, and only Refugio was surveyed in September and December.

INGEOMINAS noted an increasing trend at the electronic tilt station LISA that began in October 2010 and continued through 2011; the two components registered a cumulative increase of 20 µrad. RECIO had been recording stable conditions until May 2011; from May through December 2011, the N component increased by 23 µrad and the E component decreased by 10 µrad. Corrective measures had been taken to protect the BIS and REFUGIO tilt stations from thermal effects, however, cyclical changes persisted in their datasets. By December 2011, seven electronic tilt stations were online and were recording minor fluctuations primarily due to temperature change.

Permanent GPS stations Gualí and Nereidas were installed on the lower W flanks between May and August 2011 and a third station, Olletas, was online by November 2011. GPS instrumentation and continuous data processing were part of a collaborative effort between INGEOMINAS and the University of Wisconsin, Madison.

SO₂ emissions, 2010-2012. Since installation of the two scanning DOAS stations in late 2009, background levels of SO₂ were rarely higher than 1,000 t/d until September 2010. INGEOMINAS recorded increased SO₂ emissions in late 2010 (figure 57), while plumes rose to heights of 220-1,000 m above the crater (averaging ~700 m) through 2011. An increase was observed from November 2010 through much of 2011; maximum daily values of SO₂ flux frequently exceeded 1,500 t/d. Occasional peaks above 3,000 t/d were recorded from November 2010 to January 2011 (a), June-July 2011 (b), and November 2011 to February 2012 (c). Beginning in February 2012, emissions dramatically increased during a period of escalated seismicity (table 3). SO₂ flux peaked during May and June; the three strongest peaks were greater than 33,000 t/d. By late June, emissions were declining.

Figure 57. (Top) The map of the geochemical network for Nevado del Ruiz shows sites for thermal springs, scanning Differential Optical Absorption Spectrometer (DOAS) stations (white triangles show coverage area directed toward the crater), alkaline sampling, and radon gas sampling. (Bottom) The histogram summarizes maximum daily SO2 flux from scanning DOAS stations from January 2010 through August 2012. Following a period of low emissions during January-September 2010 (highlighted in yellow), three periods of increased SO2 flux occurred (a, b, c) and significant escalation was observed during February-March 2012 and May-June 2012 (vertical yellow bars). Annotated areas are approximations of time periods. Courtesy of INGEOMINAS.

Explosive activity in 2012. In late January 2012, while SO₂ flux began to increase dramatically (figure 57), explosion signatures (also described as strong degassing events) and multi-events continued to appear in the seismic records. On 8 March an overflight of the summit provided INGEOMINAS scientists a view of ash-covered snow on the E flank and near the crater rim (figure 58); in their monthly report, INGEOMINAS suggested this ash may have fallen during an explosion detected on 22 February 2012.

Figure 58. This photo was taken during a flight past Nevado del Ruiz's active crater at 0705 on 8 March 2012. Viewed from the Azufrado sector (NE of the summit crater), a column of gas was rising to a maximum height of ~1,400 m above the crater. A thin layer of ash was visible on the snow near the crater (in the foreground of the image). Courtesy of INGEOMINAS.

On 29 March authorities raised the alert to Level II (Orange) when LP seismicity underwent a ~100-fold increase and banded tremor persisted (table 3).

Based in part on information captured by webcameras around the volcano (including one in Manizales located 30 km NW of Nevado del Ruiz), INGEOMINAS reported that plume heights had increased significantly in March 2012 (figure 59). Reports from local populations around the volcano also alerted INGEOMINAS of sulfur odors. Residents smelled these odors during March; April, May, and August reports were from Manizales, Lebanon, Palocabildo, and Chinchiná.

Figure 59. (Top) The map of Nevado del Ruiz's geophysical monitoring network includes webcameras, meteorological stations, mudflow stations with acoustic flow sensors, and infrasound. (Bottom) Plot of plume height above the crater as measured from webcameras located near the flanks (including sites Piraña (PIRA), Gualí (GUAL), and Manizales (OVSM)) from January through June 2012. Courtesy of INGEOMINAS.

The national park surrounding the volcano, Los Nevados National Park, closed in April 2012 due to possible ashfall and lahar hazards. The rainy season (March-June) had begun and mass wasting on the steep slopes, especially of remobilized ash, was a major concern. "Most lahars are initiated as dilute, subcritical flows high on volcanic slopes, but quickly increase their volumes as they incorporate sediment along travel paths (Lockwood and Hazlett, 2010)."

On 16 and 19 April 2012, INGEOMINAS observed ash emissions from the summit and on 22 April, Washington VAAC announced possible ash in the steam plume. Volcanic ash was detected later with satellite imagery, spreading ~110 km NE of the summit on 29 May.

Seismicity decreased in early May 2012 to levels observed before the escalation began in February, and fewer explosions and multi-events were recorded. On 3 May authorities lowered the alert to Level III (Yellow). Conditions at Nevado del Ruiz continued to change, however, and when seismicity abruptly increased, the Alert Level was raised to Level II (Orange) on 29 May (table 3, figure 60). That day, explosions from the crater generated ash plumes that dispersed over more than 20 communities located to the WNW, NW, and NNW. Washington VAAC released four notices on 29 May describing ash up to 11 km altitude. News media reported that three primary airports in the region (Manizales, Pereira, and Armenian) collectively canceled ~20 flights that affected ~700 passengers on 29 May.

Figure 60. A seismic record from Nevado del Ruiz starting just prior to 29 May 2012 and ending slightly past noon on 1 June 2012. The notes explain the start of ash emissions (top shaded bar), alert announcement (orange diamond), and intervals of tremor (shaded bars with orange connected lines). Translation of text: Initial pulse of ash emission at 0397 on 29 May. Throughout the seismogram, volcanic tremor is present and in parts, appears as banded tremor that increases in amplitude. Courtesy of INGEOMINAS.

Widespread ashfall in early June 2012 required field maintenance by INGEOMINAS to clear ash from solar panels and equipment (figure 61). Imagery captured by the NASA satellite EO-1 revealed a two-toned summit disclosing partial ash cover over the white summit glacier (figure 62). The seismic station INDERENA, acoustic flow station MOLINOS, and the radio repeater that served Nevado del Ruiz, Tolima, and Santa Izabel volcanoes were disabled due to ash cover. Washington VAAC released advisories regularly until 24 June; ash reached altitudes in the range of ~5.5-7.6 km. Plumes tended to drift N, NW, WNW, and W; however, an ash plume on 8 June drifted ~28 km SE. The range of plume lengths was 28-110 km until a period of quiescence during 25 June-2 July.

Figure 61. Ash covered several solar panels as well as field equipment located near Nevado del Ruiz's W flank in June 2012. Here, at near-equatorial latitude (~5° N), the panels are typically oriented near-horizontal for effective solar exposure which also makes it easy for ash to collect and not wash away. Courtesy of INGEOMINAS.

Figure 62. (Left) This image was taken by the NASA Expedition 23 crew on 23 April 2010, with a Nikon D3S digital camera fitted with an 800 mm lens. A steam plume drifts SW from the summit crater, blending in with the snow-cover. The summit crater is indicated with a black arrow and the neighboring features, Cráter de Olleta and Altas de Piraña correspond with the outlined field of view in yellow in the left image. Note the scale is approximate and there is some skew to this image as it was taken from a shuttle flight as opposed to the orbiting satellite. Courtesy of NASA. (Right) This satellite image of Nevado del Ruiz was taken during significant ash explosions on 6 June 2012. The summit glacier displays the sharp contrast of muted gray on the NW due to ash cover and bright white on the SE where ash had not fallen. The black arrow points to the summit crater and white clouds are concentrated in the NW and SE corners of the image that also partially cover the peak Altas de Piraña. Image courtesy of NASA by Jesse Allen and Robert Simmon using EO-1 Advanced Land Imager data.

On 30 June 2012, seismicity increased and large plumes of ash vented from the summit (figure 63). At 1700 that day, authorities raised the alert to Level I (Red). Local news media reported the preventative evacuation notice provided by the Emergency Committee of Caldas; Caldas is the department of Colombia encompassing Nevado del Ruiz and six districts, 27 municipalities, and the capital, Manizales. An estimated 300 families were ordered to evacuate from the rural zones of districts Chinchiná (30 km WNW), Villamaría (28 km NW), Palestina (40 km WNW), and Manizales (30 km NW) due to both escalated explosions and also the potential for flooding along the rivers Chinchiná and Río Claro. In the Department of Tolima, located S of Caldas there was a recommendation to evacuate 1,500 families in risk zones in eight municipalities.

Figure 63. A snapshot of the seismic record from Nevado del Ruiz on 30 June 2012 and annotated to mark when officials announced the maximum Alert Level (Level I). Colored circles indicate events associated with fracturing (red), gas and fluid movement (yellow), and tremor resulting from gas or ash emissions (blue). Note that time stamps are not included except for the 1740 arrow. Courtesy of INGEOMINAS.

On 2 July 2012, Washington VAAC announced a 7.5-km-wide plume visible in satellite imagery that had drifted ~75 km W. Seismicity was decreasing, however, and that same day, authorities lowered the Alert Level to II (Orange). Airborne ash remained visible in satellite images until 8 July and continued to be observed at low elevations based on webcamera images. Ashfall was reported in Pereira (40 km WSW) on 11 July, and on 31 July a plume of ash and gas was observed rising 300 m above the crater.

Low levels of tremor had been detected in late July and throughout much of August 2012. Seismic swarms were detected on 12 and 13 August (table 3) with ~140 low-magnitude events under 5 km deep concentrated WSW of the Arenas Crater. On 6 August, ashfall was reported in Manizales and Chinchiná; on 12 August there were reports of ash in Manizales and Brisas (50 km SW). Through the end of August, plumes (ranging 200-800 m above the crater) were visible from the summit. Field measurements by INGEOMINAS and remote sensing with OMI determined that SO₂ emissions remained high (figure 64) through August and early September. On 5 September 2012 authorities reduced the Alert Level to III (Yellow).

Figure 64. A Nevado del Ruiz SO2 plume was detected by the Ozone Monitoring Instrument (OMI) on NASA's AURA satellite on 9 September 2012 from 1328-1507 (local time), extending well over the Pacific Ocean. The mass of SO2 was 1.28 kt, covering an area of 44,199 km2, and the maximum was 4.23 Dobson Units (DU) at 1331 local time. Courtesy of Simon Carn, Michigan Technological University and Joint Center for Earth Systems Technology, University of Maryland Baltimore County.

Recalling 1985 and additional hazard mitigation efforts. Nevado del Ruiz's most deadly natural disaster was a lahar that, on 13 November 1985, scoured the Lagunillas River (E flank drainage system) and suddenly flooded the towns of Armero, Chinchiná, Mariquita, and Honda (figure 65). Armero was completely destroyed and more than 23,000 residents died. Light ashfall had been reported that day and a seismic network was in place, but no early warning system had been established to initiate evacuations (Lockwood and Hazlett, 2010).

Figure 65. Released in 2007, this hazard map of Nevado del Ruiz is dominated by lahar and pyroclastic flow scenarios. Highest risk areas are shaded red with lower risk areas in yellow; note that the town of Armero (Antiguo Armero, 48 km E of the summit) is in a region of high risk. A topographic assessment augmented with substantial field evidence determined flow paths and inundation probabilities within the major drainages of Gualí, Azufrado, Lagunillas, Recio, and Chinchiná (listed clockwise starting with the NE drainage). Pyroclastic flow, ashfall, and lava inundation were also considered and the radial sectors directed NE attribute hazards to lateral explosions based on crater morphology and geologic mapping of tephra units. Names highlighted in green indicate major towns. Courtesy of INGEOMINAS.

Since 1985, realtime geophysical monitoring greatly increased, including acoustic flow sensors designed to detect impulsive flooding in local drainages. Other advances included mobile gas monitoring (mini-DOAS) that augmented routine geochemical sampling at Nevado del Ruiz and recent hazard map revisions that emphasized inundation scenarios with zoning that clearly communicates areas at highest risk (figure 65). International collaborations with universities and agencies (for example, the University of Wisconsin and the European Union mentioned previously) have focused on mitigation efforts through training and technical resources.

Following the disastrous 1985 lahars, the USGS and the U.S. Office of Foreign Disaster Assistance (OFDA) developed the Volcano Disaster Assistance Program (VDAP) to respond to selected volcanic crises around the world (Ewert and others, 1997). The VDAP mission is to work with international counterparts to reduce fatalities and economic losses in those countries experiencing a volcano emergency. The VDAP website states that "Between crises, VDAP scientists focus on building and improving volcano monitoring systems and conduct joint activities to reduce volcanic risk by improving understanding of volcanic hazards [figure 66]."

Figure 66. The USGS/OFDA Volcano Disaster Assistance Program sent a team of scientists to aid INGEOMINAS and local authorities mitigating risk at Nevado del Ruiz on 28 May 2012. Courtesy of The Columbian.

References. Ewert, J.W., Miller, C.D., Hendley, J.W., and Stauffer, P.H., 1997. Mobile Response Team Saves Lives in Volcano Crises, USGS Fact Sheet: 064-97.

Galle, B. and the NOVAC Team, 2009. NOVAC - A global network for volcanic gas monitoring, 6th Alexander von Humboldt International Conference, Abstract AvH6-34-1, 2010.

Garzón, G., Serna, D., Diago, J., and Morán, C., 2003. Radon soil increases before volcano-tectonic earthquakes in Colombia, Proceedings of ICGG7: 6-7.

Lockwood, J.P., and Hazlett, R.W., 2010. Volcanoes: Global Perspectives, Wiley-Blackwell, Hoboken, NJ, ix, p.539.

Narváez, L.M., Torres, R.A., Gómez, D.M., Cortez, G.P., Cepeda, H.V., and Stix, J., 1997. 'Tornillo'-type seismic signals at Galeras volcano, Colombia, 1992-1993, Journal of Volcanology and Geothermal Research, 77: 159-171.

Salazar, S., Carvajal, C., and Garzón, G., 2003. Radiological geohazard survey in the south east of Manizales city (Colombia), Proceedings of ICGG7: 3-5.

Yamashita, K.M., 1992. Single-Setup Leveling Used to Monitor Vertical Displacement (Tilt) on Cascades Volcanoes, in Ewert, J. and Swanson, D. (Eds.), Monitoring volcanoes; techniques and strategies used by the staff of the Cascades Volcano Observatory, 1980-90, U.S. Geological Survey Bulletin 1966, pp. 143-149.

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: Instituto Colombiano de Geologia y Mineria (INGEOMINAS), Volcanological and Seismological Observatory, Avenida 12 Octubre 15-47, Manizales, Colombia (URL: https://www2.sgc.gov.co/volcanes/index.html); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); Ozone Monitoring Instrument (OMI), Sulfur Dioxide Group, Joint Center for Earth Systems Technology, University of Maryland Baltimore County (UMBC), 1000 Hilltop Circle, Baltimore, MD 21250, USA (URL: https://so2.gsfc.nasa.gov/); El Colombiano (URL: http://www.elcolombiano.com/); The Columbian (URL: http://www.columbian.com/photos/2012/may/28/44870/).

When last active in October 2011, San Cristóbal produced ash plumes accompanied by elevated seismicity (BGVN 36:12). This report covers the January-September 2012 monitoring efforts (seismic, gas, thermal, and visual observations) and the onset of a volcanic crisis during 8-15 September 2012. Seismicity remained high through early 2012 and tremor was frequently detected. Explosions of ash and gas began impulsively from the summit crater on 8 September causing heavy ashfall, evacuations of local populations, and aircraft deviations.

January-September 2012 seismicity. Instituto Nicaragüense de Estudios Territoriales (INETER) detected seismic tremor every day in January 2012 and throughout much of February, March, and April. A station outage took place during 1-14 June, but when the data stream returned, it recorded significant tremor. INETER reported a generally increasing trend in earthquake counts from January through April (figure 22).

Figure 22. Total earthquakes detected from San Cristóbal during January-April 2012. Courtesy of INETER.

In January, tremor persisted for 1-12 hours per day for a total of 118 hours. In February, tremor duration averaged 4 hours/day (131 hours); in March, 6 hours/day (166 hours); in April, 2 hours/day (38 hours); and in June, 5 hours/day (23.5 hours). No estimates were available for May.

From January through April 2012, a class of seismic events considered "degassing earthquakes" (DE) were detected throughout the seismic records. These events were characterized in spectrograms as events in the range of 4-10 Hz. INETER described the events as resulting from gas moving through the conduit, causing displacements and, after building pressure in confined spaces, the pressure was released impulsively, generating low-amplitude shockwaves and arriving as emergent seismic signals with low energy. These conditions suggested that the volcanic system was partially open (as opposed to a closed system that would be expected to pressurize). Individual DEs occurred with durations of ~60 seconds, and up to 1,379 DE events were recorded in April 2012 with dominant frequencies of 5-10 Hz.

Volcano-tectonic (VT) earthquakes were a minor part of San Cristóbal's seismicity during January-June. Typically occurring 6-15 km deep, the maximum number of VT events occurred in March; 39 earthquakes were detected with dominant frequencies in the range of 10-20 Hz.

Long-period (LP) earthquales dominated the seismic record in June; 1,413 events were recorded (22-707 monthly events were noted in the records during February-April). The duration of these signals ranged from 40-90 seconds with dominant frequencies of 1-5 Hz. Depths of these events were not announced, but in March and April, LPs occurred at depths of 6-25 km.

Reports from INETER during the volcanic crisis in September highlighted sporadic signals indicating eruptions in the seismic records along with tremor and the appearance of shallow, low-magnitude events (microseismicity). Elevated seismicity on 8 September decreased dramatically by 10 September. Seismic tremor increased on 14 September, however, by 16 September, seismicity had returned to normal levels.

SO₂ monitoring. In January 2012 INETER reported that three miniature Differential Optical Absorption Spectrometer (Mini-DOAS) stations were installed in the field around the flanks of San Cristóbal. These stations stored SO₂ flux data locally and telemetered it to the INETER network through the El Chonco repeater. These installations were part of the Network for Observation of Volcanic and Atmospheric Change (NOVAC), a collaboration supported by the European Union's Natural Disasters Program (Galle and others, 2009).

Employing a mobile DOAS, INETER collected SO₂ data on traverses in March; five traverses were made between the junction of Chinandega and Corinto and the town of Las Grecias (for town locations, see BGVN 36:12 figure 20). The average SO₂ flux recorded on 30 March 2012 was 542 t/d; the reported wind velocity was 5 m/s to the E. Previous measurements from this region (10 January 2011) yielded an average SO₂ flux of 436 t/d.

Thermal data and visits to the summit. INETER technicians noted regular gas emissions from San Crisóbal's summit from January through August 2012. During field investigations to the summit (April-August 2012), loud jetting was heard one day (22 April) coming from the central crater. That day, gas emissions were relatively low and there was evidence of numerous rockfalls from the W side of the crater. Vapor plumes drifted mainly W and E of the crater depending on wind direction.

Fumarole temperatures measured from April through August show small variations in the range of 50-93°C (figure 23). These measurements were taken from five sites located within the SE sector of the crater rim. The previous temperature from the central crater was last measured on 3 December 2011 (382°C); the most recent measurement, on 20 June 2012, was 543.7°C.

Figure 23. (Top) Site locations of fumaroles found along the SE summit rim of San Cristóbal and visited in 2012. The white color is due to heavy steam emissions which have conformed with the topography due to GoogleEarth 3D rendering. (Bottom) Fumarole temperatures (grouped by fumarole) during April-August 2012. INETER noted that lowest temperatures were obtained from Fumarole 2 (50-73°C) while other sites showed variations between 70 and 93°C (the maximum measured from Fumarole 5). Courtesy of INETER.

Heavy rain in May restricted field operations, however, on 24 May INETER technicians visited the lower flanks of San Cristóbal to maintain seismic and gas instrumentation. They encountered evidence of a lahar that had covered the main trail between the Hacienda Las Rojas and Pedro Marín to the SW of the summit. The lahar had reached a maximum height of 0.8 m and was up to 15 m wide.

Field investigations to the summit on 20 June determined that deep channels had been eroded in the W flank of the volcano, exposing loose soil (figure 24). INETER advised vigilance for this region since the soil could easily remobilize as a mudflow with heavy rainfall. The W flank was particularly at risk due to a forest fire that, in April 2012, removed significant vegetation that would otherwise have provided some stability for the steep slopes. Particularly vulnerable locations would be the areas of Las Rojas and Pedro Marín, farming areas within the drainage network on the W flank.

Figure 24. Views from the summit of San Cristóbal volcano on 20 June 2012. (Left) A diffuse plume of vapor that reached ~200 m above the summit crater, drifting NE. (Right) With the El Chonco peak in the distance, INETER staff photographed fresh signs of erosion on the W flank of the volcano attributed to recent heavy rainfall. Courtesy of INETER.

Ash explosions in September 2012. At 0845 local time on 8 September, a substantial ash plume erupted suddenly from San Cristóbal's summit, followed by a second plume 10 minutes later. Later that day, INETER confirmed GOES-13 satellite observations of a wide-spreading ash plume from the summit of San Cristóbal (figure 25). Three explosions produced ash-and-gas plumes that day and were observed rising up to 1.5 km above the crater and drifted 9 km/hr NW (figure 26).

Figure 25. Distribution of ash plumes from the 8 September summit explosion of San Cristóbal. The top left image (at 1645 UTC) represents a best fit polygon of the ash plume based on satellite imagery while the other images are the forecasted distributions of the plume for the following 6, 12, and 18 hours. Courtesy of Washington VAAC.

Figure 26. A still-shot from a video taken of the dense ash plume from San Cristóbal on 8 September 2012. The location of this recording was not disclosed but the view is directed S with the El Concho peak to the far right-hand side. Time of the video was undisclosed. Courtesy of YouTube contributor A Callejas.

On 8 September INETER released special online reports announcing observations and volcanic crisis incidents. Residents reported ashfall at El Viejo (18 km WSW of San Cristóbal), El Chonco, and Ranchería. Sporadic explosions later that day generated ash plumes that rose 1.5-5 km and drifted 50 km WNW. The sporadic explosions appeared in the seismic records but microseisms (a category of shallow, small-magnitude earthquakes) dominated the record.

Between 0900 and 1000 local time on 8 September, SO₂ flux was 3,221 t/d, well above the normal range of 550-700 t/d. Residents in Versalles Arriba, a zone near the crater, reported seeing a fissure-like feature, however, INETER did not report follow-up site visits for this observation. Rockfalls were observed on the N flank; on the NW flank, ash mixed with incandescent rock fell in an area occupied by livestock. Field investigators noted that six animals were burned from this event.

According to a news article, emergency officials evacuated ~3,000 people by 1857 local time. The national emergency agency of Nicaragua (Sistema de Prevención, Mitigación y Atención de Desastres, SINAPRED) reported that airplanes were diverted around San Cristóbal to other routes.

Rainfall was closely monitored on 8 September. By 1600 local time, 26.1 mm of rain had fallen and INETER warned of possible mudflows resulting from remobilized ash. Thunderstorms were expected on 9 September in the region of Chinandega and INETER warned that acid rain could result from the mixture of volcanic gases.

During 9 September, INETER coordinated field teams that investigated ashfall within the region. These teams determined that ash fell in an area covering 2,438 square kilometers, including the communities of El Viejo, La Grecia, La Joya, Santa Catalina, El Piloto, Las Banderas, Las Rojas, Carlos Fonseca, Jiquilillo, Mechapa, and Cosiguina (figure 27). Ashfall was 5 cm thick in areas near the crater and up to 3 mm thick in more distant places.

Figure 27. Several towns and roads were blanketed with ash from San Cristóbal on 8 September. This Nicaraguan police officer wears a protective mask to prevent inhaling the fine volcanic ash. Courtesy of the Associated Press/Esteban Felix.

By 10 September, INETER reported that seismicity decreased after the 8 September eruption. A traverse between Chinandega and El Guasaule during 0700-0830 with a mobile DOAS measured an SO₂ flux of 1,626 t/d. This emission rate was significantly lower compared to the previous day.

During 10-11 September, steam plumes rose 200-300 m above the crater and drifted W. Three small explosions on 11 September generated ash-and-gas plumes that rose 300 m above the crater and drifted W. An explosion and ash venting was observed a few hours later; a plume drifted S and ash fell on the flanks. Microseismicity continued; at 0900 on 11 September, 63 small events had been recorded so far that day.

Abundant gas emissions were observed on the morning of 12 September. RSAM was notably higher (by 35 to 70 RSAM units compared to the previous day). At the time of the Special Report on 12 September at 1100 local time, 86 microseismic events had been recorded.

On 13 September, INETER reported that the seismic network continued to detect small, sporadic explosions. Sulfur dioxide gas emissions were above normal (1,360 t/d), similar to levels detected on 8 September. RSAM calculated since the release of the last INETER Special Report was considered normal, 40-60 RSAM units, and microseismicity appeared to have decreased (only 17 events had been detected).

Fieldwork was conducted on 13 September as a joint venture between INETER and the El Salvadoran agency Servicio Nacional de Estudios Territoriales de El Salvador (SNET). The scientific team reached the summit crater of San Cristóbal to measure temperatures, collect rock samples, and observe current conditions. They noted that portions of the crater had collapsed (N and S sectors) and found blocks and ejecta on the flanks, 850 m from the crater. Changes had also occurred in the summit fumarolic areas. Three of the five fumarolic sites no longer emitted gas; these sites appeared to be sealed. Fumaroles 1 and 2 had measurably elevated temperatures (85°C), broadly similar to previous values recorded (figure 23). Based on the field assessment of ejecta, INETER warned that mudflows remained a hazard during heavy rainfall.

Increased seismic tremor was recorded at 0340 on 14 September. Low levels of summit emissions were visible drifting in a plume to the SW. Elevated SO₂ flux continued (2,490 t/d). The following day, abundant gas emissions were visible drifting NE and SO₂ emissions had increased (3,054 t/d). RSAM had increased to 120 on 15 September. A small explosion was detected at 0817 local time; however, there was no visual confirmation due to cloud cover.

Early in the morning on 16 September, minor tremor was recorded and few earthquakes were recorded. The seismic events were too small to be located and INETER reported that, based on RSAM, seismicity had returned to normal levels (40 RSAM units). Low level emissions were visible and less SO₂ was detected compared to the previous two days (2,053 t/d). By 17 September, no tremor was recorded and minor emissions were visible drifting N of the crater.

References. A Callejas, 2012, Volcan San Cristobal en erupción - Nicaragua Sept 8, 2012 (from YouTube), Uploaded on 10 September 2012, Accessed on 3 October 2012, http://www.youtube.com/watch?v=hQStun1FF3o&feature=related.

Galle, B. and the NOVAC Team, 2009. NOVAC - A global network for volcanic gas monitoring, 6th Alexander von Humboldt International Conference, Abstract AvH6-34-1, 2010.

Geologic Background. The San Cristóbal volcanic complex, consisting of five principal volcanic edifices, forms the NW end of the Marrabios Range. The symmetrical 1745-m-high youngest cone, named San Cristóbal (also known as El Viejo), is Nicaragua's highest volcano and is capped by a 500 x 600 m wide crater. El Chonco, with several flank lava domes, is located 4 km W of San Cristóbal; it and the eroded Moyotepe volcano, 4 km NE of San Cristóbal, are of Pleistocene age. Volcán Casita, containing an elongated summit crater, lies immediately east of San Cristóbal and was the site of a catastrophic landslide and lahar in 1998. The Plio-Pleistocene La Pelona caldera is located at the eastern end of the complex. Historical eruptions from San Cristóbal, consisting of small-to-moderate explosive activity, have been reported since the 16th century. Some other 16th-century eruptions attributed to Casita volcano are uncertain and may pertain to other Marrabios Range volcanoes.

Information Contacts: Instituto Nicaragüense de Estudios Territoriales (INETER), Apartado Postal 2110, Managua, Nicaragua (URL: http://www.ineter.gob.ni/); Washington Volcanic Ash Advisory Center (VAAC), Satellite Analysis Branch (SAB), NOAA/NESDIS E/SP23, NOAA Science Center Room 401, 5200 Auth Rd, Camp Springs, MD 20746, USA (URL: http://www.ospo.noaa.gov/Products/atmosphere/vaac/); La Prensa de Nicaragua (URL: http://www.laprensa.com.ni/2010/07/04/nacionales/30240); La Prensa de Honduras (URL: http://www.laprensa.hn); BBC: Latin America & Caribbean (URL: http://www.bbc.co.uk/news/world-latin-america-19533933).

Our last report covered beharior at Suwanose-jima through July 2011 (BGVN 36:07). This report, compiling translated material from the Japan Meteorological Agency (JMA), covers ongoing activity through June 2012, with minor magnitude venting at Otake crater and the tallest plume rising to 1 km over the crater rim. Throughout the reporting period, the volcano's crater produced weak glow at night that was imaged by a high-sensitivity camera. The Alert Level remained at Level 2 (on a scale from 1-5, access to the crater area prohibited due to threat of eruption). As summarized in the text, numbers of A- and B-type events were in the ranges of 11-24 and 62-205, respectively. There were multiple cases of ashfall at [the village 4 km SSW] from the summit crater.

The table below summarizes some other information reported by JMA, including a tally of small eruption heights. Tremor duration extended to over 50 hours during several months and to 132 hours in June 2012.

Monthly coverage. Volcanic earthquakes and tremor continued during July and August 2011 (table 10). In August, seismic activity decreased; A- and B-type events occurred 24 and 62 times, respectively. A-type earthquakes are generally considered to have shallow focal depths; B-type earthquakes, deeper focal depths.

Table 10. A compilation of data on Suwanose-jima during July 2011 through June 2012. "--" indicates data not reported. Data courtesy of JMA.

Explosive eruptions from Otake crater occurred on 9 and 12 September 2011. A temporal increase in seismicity, including intermittent tremor, was observed during 9-14 September, later dropping to background level. Ash fell [in the village] on 7, 9, 12, 15, and 18 September.

Small-scale eruptions were observed in October and November 2011. Ashfall was reported [in the village] on 15 November.

Aerial observations were conducted in cooperation with the Japan Maritime Self Defense Force (JMSDF) on 19 December 2011. They revealed a high temperature area at the center of Otake crater.

GPS measurements showed no remarkable crustal change between January and June 2012. GPS data from Tongama ceased starting in mid-May due to a technical failure.

No explosive eruptions occurred in April 2012. Instruments detected 21 A-type events and 85 B-type events.

During May, there were 11 A-type events and 205 B-type events. Noteable volcanic tremor occurred on 5 and 25-26 May. [Residents in the village] registered ashfall on 25 and 28-30 May.

[Village residents] again reported ashfall on 11 and 13-14 June 2012. During June instruments detected 21 A-type events and 116 B-type events. Volcanic tremor was registered during 2?22 June 2012 (table 10).

Geologic Background. The 8-km-long, spindle-shaped island of Suwanosejima in the northern Ryukyu Islands consists of an andesitic stratovolcano with two historically active summit craters. The summit is truncated by a large breached crater extending to the sea on the east flank that was formed by edifice collapse. Suwanosejima, one of Japan's most frequently active volcanoes, was in a state of intermittent strombolian activity from Otake, the NE summit crater, that began in 1949 and lasted until 1996, after which periods of inactivity lengthened. The largest historical eruption took place in 1813-14, when thick scoria deposits blanketed 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 the horseshoe-shaped Sakuchi caldera, which extends 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), Otemachi, 1-3-4, Chiyoda-ku Tokyo 100-8122, Japan (URL: http://www.jma.go.jp/).