Report on Hachijojima (Japan) — 4 September-10 September 2002
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 4 September-10 September 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, 4 September-10 September 2002. Smithsonian Institution and US Geological Survey.
33.137°N, 139.766°E; summit elev. 854 m
All times are local (unless otherwise noted)
In late August, fewer than ten earthquakes occurred at Hachijo-jima per hour and ground deformation had returned to normal levels. On 27 August, very long-period signals were detected near Hachijo-jima. The signals were felt widely on the southern coast of Honshu. The hypocenter of a high-frequency earthquake preceded by a very long-period signal was located NW offshore of the volcano.
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 4,000 BP, was restricted to flank eruptions. Eruptions recorded since the 15th century have been restricted to the summit of Nishiyama and a submarine vent of unknown location.