Geologic Background:
Oshima stratovolcano lies in Sagami Bay, forming
the northernmost of the Izu Islands. The broad, low
stratovolcano is constructed over the remnants of three
dissected stratovolcanoes and is capped by a 4-km-wide
caldera with a central cone, Mihara-yama, that has been
the site of numerous historical eruptions. More than 40
parasitic cones are located within the caldera and along
two parallel rift zones trending NNW-SSE. Although Oshima
is a dominantly basaltic volcano, strong explosive
activity has occurred at regular intervals throughout the
past few thousand years.
Historic Activity:
- At least 50 eruptions between 0684 and 1974.
Recent Activity:
- February to June 1974, weak strombolian activity.
Since 1974, activity has been limited to earthquake
swarms in 1977, 1983, and 1985. Volcanic tremor began on
July 20, 1986 and continued till November 15, 1986 when
an eruption began. Eruption ended by end of November
1986. Main crater and three fissures fed four lava flows
with a total volume of 25 million cubic yards. Short 2
hour eruption on December 18, 1986. Volcanic tremor and
earthquake swarms from February to November1987. Renewed
explosions with ash erupted to 2500 meters during
November 1987. Small erruptions on January 25, 1988
ejected ash clouds to 300-400 meter above summit
crater.
- Continued seismic activity through March 1990 when
tremor began to decrease. No tremor from May thru August
1990. Small eruption on Oct. 4, 1990 formed small
collapse pit.
- Continuous volcanic tremor began on the evening of
May 30, 1993 at the active Mihara-yama cone. The next
night, an increase in shallow earthquake activity began
in the summit area. Both the earthquake swarm and tremor
had stopped by June 5. This was the first episode of
volcanic tremor recorded since April 1990. Weak and
steady steaming from the Mihara-yama crater continued
without change throughout the period of increased
seismicity.
- Increased seismic activity in August 1996.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 11, No. 8 to V. 13,
No. 8)
- Global Volcanism Network (V. 15, No. 6 to V. 21, No.
8).
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