Report on Izu-Oshima (Japan) — April 1987
Scientific Event Alert Network Bulletin, vol. 12, no. 4 (April 1987)
Managing Editor: Lindsay McClelland.
Izu-Oshima (Japan) Steam emission; tremor and strain changes; earthquake swarms
Please cite this report as:
Global Volcanism Program, 1987. Report on Izu-Oshima (Japan). In: McClelland, L. (ed.), Scientific Event Alert Network Bulletin, 12:4. Smithsonian Institution. https://doi.org/10.5479/si.GVP.SEAN198704-284010.
34.724°N, 139.394°E; summit elev. 758 m
All times are local (unless otherwise noted)
Since the 18 December explosion, no additional eruptive activity has been reported. Steam jets rose daily to 10 m above some of the November fissures and white clouds usually extended 50 m above the summit.
Seismicity declined steadily after the November eruptive event and continued to decline through March. There was no change in the distribution of epicenters, which were concentrated in a zone trending NNW-SSE on the island. Volcanic tremor resumed on 1 January, after 10 days of quiet, occurring at almost regular 1-hour intervals (figure 6) [but see 12:1]. Periodic tremor had declined by February but continued at [January] amplitudes. Durations of tremor episodes ranged from 10 to 30 minutes. The tremor source (estimated from amplitude at each seismometer) appeared to be below the central cone (Mihara-yama).
Periodic tremor was accompanied by step-like strain changes, recorded by a borehole strainmeter 108 m below sea level, 5 km N of Mihara-yama. Step-like expansion began on 1 January, reversing to contraction on 22 January [see also 12:1]. Other strainmeters had shown gradual contraction since the 21 November eruptive activity, before strain virtually ceased.
UPI reported that earthquake swarms began 6 May off the NW coast of the island, 10-20 km below the ocean floor. Of the 1,571 total recorded earthquakes 6-12 May [but see 12:6], 895 occurred on the 11th, the day of the largest event (M 5.1).
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: JMA; UPI.