Report on Izu-Oshima (Japan) — November 1987
Scientific Event Alert Network Bulletin, vol. 12, no. 11 (November 1987)
Managing Editor: Lindsay McClelland.
Izu-Oshima (Japan) Explosions after increased seismicity and new fumaroles
Please cite this report as:
Global Volcanism Program, 1987. Report on Izu-Oshima (Japan) (McClelland, L., ed.). Scientific Event Alert Network Bulletin, 12:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198711-284010.
34.724°N, 139.394°E; summit elev. 758 m
All times are local (unless otherwise noted)
Small explosions began in the crater of the summit cinder cone (Mihara-yama) at 1047 on 16 November. Bombs were ejected to 1.6 km distance, instruments in the summit area were damaged, and nearby windows were cracked. The sounds were heard from as far as the Honshu mainland (at least 30 km away). The crater was obscured by clouds, but within the next hour aerial observers saw eruption columns rising to 5,000 m height. Tephra fell primarily to the E, and totaled a few thousand metric tons. Explosions were heard at irregular intervals until 1546. At about 1700, a lava fountain a few meters high was observed within the crater, but no flows formed. The next day, observations indicated that the crater floor had subsided 25 m since before the eruption.
Summit glow was observed at 0329 on 18 November, and ashfall to the SW soon followed. At about 0500, a gray ash column rose an estimated 1,600 m. Five hours later, at 1004, there were a few small explosions and a small amount of ash fell to the NW. Small eruption columns were observed until evening. The next day, helicopter observations indicated that the crater floor had subsided an additional 150 m [but see 13:1]. From 19 to 27 November, plumes a few hundred meters high occasionally rose from Mihara-yama. On 21 November, 5,200 of the 10,000 Oshima Island residents participated in evacuation drills that used six ships, 20 planes, and 145 motor vehicles. A white plume was the only activity at the volcano that day.
The groundmass of the ejecta contained abundant microphenocrysts of pyroxene and plagioclase. This suggested to volcanologists that the rock was not fresh magma, but was derived from the lava lake that has been cooling inside Mihara-yama crater since the November 1986 eruption.
Seismicity. Seismicity in the summit region had been gradually increasing since January 1987 (figure 12). A significant peak at the end of September 1987 was not accompanied by eruptive activity. Seismicity rapidly increased in the middle of November, reaching 543 events/day on 13 November and 586 on 14 November. Within a few days after the eruption on 16 November, seismicity rapidly declined to <50 events/day.
|Figure 12. Number of earthquakes per day in the summit region of Izu-Oshima island, May-November 1987. Courtesy of Earthquake Research Institute, Univ of Tokyo.|
Seismographs installed in the summit area two weeks before the eruption showed that the earthquakes were very shallow (less than ~400 m depth), and almost entirely confined to the area underneath the summit crater. Periods of both episodic and continuous tremor occurred in the months before the eruption. Tremor became continuous just before the eruption.
Ground deformation. Levelling and tilt data suggested that almost the entire summit region of Oshima has been subsiding since the November 1987 eruption. No clear ground deformation precursors were seen prior to the eruption. A stepwise change at all tiltmeters around the island occurred almost simultaneously with the start of the second series of eruptions at 0329 on 18 November (figure 13). The change is consistent with a center of inflation roughly 5 km NW of Mihara-yama. The volumetric strainmeter in the NW part of the island recorded an expansion consistent with the tilt (figure 13).
|Figure 13. Map showing monitoring instrumentation on Izu-Oshima Island. Vectors show downward direction and magnitude of stepwise change in tilt, 18 November at 0326.|
Fumarolic activity. A clear increase in fumarolic activity was noted before the eruption. A ring of fumaroles around the summit cone was observed at the end of July, and the activity of this ring increased with time. A second ring formed around the first ring in November. Fumarolic activity increased rapidly 5-10 days before the eruption.
Further References. Abe, K. and Takahashi, M., 1987, Description of the November 21, 1986 fissure eruption on the caldera floor of Izu-Oshima volcano, Japan: analysis of a series of photographs: Bulletin of the Earthquake Research Institute, Tokyo, v. 62, p. 149-162.
Aramaki, S., ed., 1988, The 1986-1987 eruption of Izu-Oshima volcano: Earthquake Research Institute, University of Tokyo, 61 p. (9 papers).
Endo, K., Chiba, T., Taniguchi, H., Sumita, M., Tachigawa, S., Miyahara, T., Uno, R., and Miyaji, N., 1988, Izu-Oshima 1986-1987 eruptions and the eruptive products: Proceedings, Kagoshima International Conference on Volcanoes, p. 119-122.
Special Issue — Oshima volcano, Izu: Journal of Geomagnetism and Geoelectricity (Terra Science Publishing Co.), v. 42, p. 139-363 (in English).
Special Issue — The 1986 eruption of Isu-Oshima volcano: Bulletin of the Geological Survey of Japan, v. 38 (1987), p. 601-753 (12 papers) (in Japanese with English abstracts and captions).
Special Issue — The 1986 eruption of Isu-Oshima volcano: Bulletin of the Volcanological Society of Japan, v. 33 (2nd series) (1988), (in Japanese with English abstracts and captions).
Watanabe, H., the 1986-1987 eruption of Izu-Oshima volcano: Proceedings, Kagoshima International Conference on Volcanoes, p. 37-40.
Geologic Background. Izu-Oshima volcano in Sagami Bay, east of the Izu Peninsula, is the northernmost of the Izu Islands. The broad, low stratovolcano forms an 11 x 13 km island and was constructed over the remnants of three dissected stratovolcanoes. It is capped by a 4-km-wide caldera with a central cone, Miharayama, that has been the site of numerous historical eruptions. More than 40 cones are located within the caldera and along two parallel rift zones trending NNW-SSE. Although it is a dominantly basaltic volcano, strong explosive activity has occurred at intervals of 100-150 years throughout the past few thousand years. Historical activity dates back to the 7th century CE. A major eruption in 1986 produced spectacular lava fountains up to 1600 m height and a 16-km-high eruption column; more than 12,000 people were evacuated from the island.
Information Contacts: Y. Ida, H. Watanabe, K. Yamaoka, S. Aramaki, and H. Glicken, Earthquake Research Institute, Univ of Tokyo; Kyodo radio, Tokyo.