Hourly number of earthquakes has been less than 10 in the late August. The ground deformation on Hachijojima island, detected by GPS of Geographical Survey Institute, was being recovered to the previous state. JMA reported on August 27 that very long-period signals sourced near Hachijojima volcano were observed widely in the southern coast of Honshu (Tokai and Kanto districts). These were detected by broad-band seismometers, tilt-meters and strain meters. The hypocenter of a high-frequency earthquake preceded by a very long-period signal was located NW offshore of the volcano.
According to JMA, the seismic activity around Hachijojima volcano had decreased. Hourly numbers of earthquakes since the August 20 afternoon had kept less than 10. Hypocenters of them had moved to NW from the first place of earthquake nesting. The migration direction (NW-SE) is roughly in parallel to the long axis of the island. However, low-frequency earthquake events occurred in early moring and evening of this day; the hypocenter of the latter event was located near the first swarms of Aug. 13-15.
The seismic activity began to decrease in the Aug. 16 evening: since then, hourly numbers mostly less than 20 (JMA data). JMA installed two seismometers temporally in the island this day. JMA released the news on the volcanic activity at Hachijojima volcano on the Aug. 16 evening. The comment said that the present seismic activity might not lead directly to eruption, though it may continue a little while in this level. Simultaneously the Geographical Survey Institute reported the GPS measurement result on the present activity; the GPS station located in the center of the island had migrated 5-6 cm to E, and had been uplifted by 10 cm till 3 p.m. on Aug. 16.
High seismic activity had continued at Hachijojima volcano. Volcanic tremor-like waveforms were also included. According to JMA, the hourly number of erthquakes had been over 20 since the Aug. 13 evening: the peak was about 250 on the Aug. 15 morning. Cumulative number of felt-quakes amounted to 22 as of 3 p.m., Aug. 16.
Hypocenters of volcanic earthquakes during August 13 to 15 (as of 20:00, Aug. 15).
Sites of seicmic stations are shown on the map in the upper left. Courtesy of S. Sakai (ERI, U-Tokyo).
Information contact: Dr. Shin'ichi Sakai (Earthquake Research Institute, University of Tokyo); firstname.lastname@example.org
JMA repeatedly reported the seismic activity in Hachijojima volcano since the August 13 evening; seismic swarms began in the Hachijojima island at about 4 p.m., August 13 (JST). As of 5 p.m., August 15, totally 16 felt-quakes were observed. The maximum magnitude reached 3.0 on August 15. The hypocenters locate about 2 km off the western shore in the central part of the island, and in the depth 15-5 km, depending on assumed seicmic-wave velocity models. They started to migrate to N, to the direction of Mt. Nishiyama on the August 15 morning, being deepened.
Five seismic stations of Tokyo Metropolitan Government, JMA, and National Research Institute for Earth Science and Disaster Prevention (NIED) had been settled on this island.
Information contact: JMA, H. Yamasato; email@example.com
Hachijojima is a basaltic volcanic
island elongated in NW-SE, a gourd-shape of 14 km long and 7.5 km
wide, about 290 km south of Tokyo. Two stratovolcanoes, Nishiyama
(younger than 10, 000 years ago) and Higashiyama (30,000 to 4, 000
years ago) in NW and SW respectively, are comprised of this island.
Small eruptions were recorded in 1487, 1518, 1522-23, 1605 and 1606,
though the eruption sites were not known clearly. Seismic swarms
occurred in 1690-91 and 1697-98.
The population in this island is about 9,200. They are living mainly in a lowlands of a saddle between two volcanoes and partly in the southern slope of Higashiyama.
Map of Hachijojima
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