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.

Geological Summary. 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.

Source: Japan Meteorological Agency (JMA) via the Volcano Research Center