Report on Hachijojima (Japan) — 21 August-27 August 2002
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 21 August-27 August 2002
Managing Editor: Gari Mayberry
Please cite this report as:
Global Volcanism Program, 2002. Report on Hachijojima (Japan). In: Mayberry, G (ed.), Weekly Volcanic Activity Report, 21 August-27 August 2002. Smithsonian Institution and US Geological Survey.
33.137°N, 139.766°E; summit elev. 854 m
All times are local (unless otherwise noted)
Seismicity that began to increase at Hachijo-jima on 13 August (when 16 earthquakes were recorded) began to decrease on 20 August (less than 10 earthquakes were recorded). The earthquake's hypocenters migrated from NW to SE, roughly parallel to the long axis of the island. Hypocenters of low-frequency earthquakes that occurred on the morning of 21 August were located near the 13-15 August swarm's hypocenters.
Geologic Background. Hachijojima, in the central Izu Islands about 300 km S of Tokyo, consists of two small Quaternary dominantly basaltic stratovolcanoes forming an elongated NW-SE-trending island. The eroded Pleistocene-to-Holocene Higashiyama volcano occupies the SE end of the 14-km-long island, and the symmetrical Holocene Nishiyama volcano the NW end. Parasitic cones occur on the SE flank of Nishiyama. The small volcanic island of Kojima lies several kilometers W of Hachijojima. Growth of Higashiyama began several tens of thousands of years ago, and included the formation of two small calderas. The initial submarine and early subaerial eruptions of Nishiyama took place from 10,000 to 8000 years before present (BP). Its latest major activity, from the early Holocene until about 4000 BP, was restricted to flank eruptions. Historical eruptions of Hachijojima, recorded since the 15th century, have been restricted to the summit of Nishiyama and a submarine vent of unknown location.