Geologic Background:
The southernmost Ryukyu Islands volcano is a
submarine volcano north of Nishi-Omote-jima island.
However, no volcanic landform or ejecta were found on the
sea floor during a 1990-91 survey, and the site of the
1924 eruption was not confirmed. Iriomote-jima island (25
x 20 km) has 1,800 residents and is composed of
metamorphic and sedimentary rocks.
Historic Activity:
- A major submarine eruption took place at this volcano
on 31 October 1924. It produced pumice rafts with an
estimated volume of about 1 km3 that were carried by
currents along both coasts of Japan as far north as
Hokkaido. However, no volcanic landform or ejecta were
found on the sea floor during a 1990-91 survey, and the
site of the 1924 eruption was not confirmed.
Recent Activity:
- High levels of seismicity at Iriomote-jima Island
began on Jan. 23, 1991 and peaked in mid February.
Seismic activity suddenly declined in late April 1991. A
total of 670 high-frequency earthquakes were felt by the
end of April, including a M 4.3 event on March 31. The
swarm was centered on the NW coast of the island. No
surface phenomena (steaming, bubbling, or water
discoloration) were found despite frequent patrolling
over the island and adjacent sea area by Japan Maritime
Safety Agency aircraft.
- An earthquake swarm occurred at the western tip of
Iriomote-jima Island on August 24-31, 1992, including a M
5.2 shock at 9 km depth on Aug. 24. A total of 24
earthquakes was detected.
- A more severe swarm began on Sept. 17, centered a few
kilometers N of the island and about 20 km NE of the
August swarm. Epicenters migrated towards the island, an
by November, the source area encompassed the northern
coast. Depths ranged from 0 to 10 km, concentrated at
around 5 km. Seismicity gradually declined toward the end
of 1992, after a peak in mid-October. A total of 1316
earthquakes were recorded from Sept. 17 to Dec. 31.
Data Sources:
- Global Volcanism Network (V. 16, No. 3 to V. 17, No.
12).
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