November 10, 2023
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* In the ocean area near Torishima Island (about 100 km southwest of Torishima Island), since October 2 till October 8, seismic activity was active such that there were 4 earthquakes of M6.0 or over. The largest one in the series of seismic activity is an M6.5 earthquake occurring on October 5 at a depth of approximately 10 km (due to CMT solution). The focal mechanism showed a normal fault type with a tension axis in an ENE-WSW direction. This event occurred within the Philippine Sea plate. This event caused tsunami of 0.2 m observed at Yaene, Hachijojima Island of the Izu Islands. Also, an M6.0 earthquake on October 6 at 10:31 (JST) caused tsunami of 0.2 m observed at Yaene, Hachijojima Island and feeble tsunami observed at Kaminato, Hachijojima Island and in Kagoshima prefecture.
* Also, between 04 and 07 o'clock (JST) on 9th, there occurred at least 14 earthquakes which accompanied waves which were considered as T phases (#). The generating source of T phases was estimated to be in the west of Sofugan Island. Although earthquakes in this seismic activity had M4~5 or so, tsunami higher than expected to those M was observed along the coasts of the Izu-Ogasawara Islands and the area extending from Chiba to Okinawa prefectures, such as of 0.7 m observed at Yaene, Hachijojima Island. Also, in Miyazaki and Kagoshima prefectures, was observed shaking of seismic intensity 1~2 considered to be due to T phases accompanying the seismic activity.
* Then the seismic activity has lowered.
* In the ocean area near Torishima Island (about 110 km north-northwest of Torishima Island), different from the recent activity area, an M5.9 earthquake on May 3, 2015 caused tsunami of 0.6 m observed at Yaene, Hachijojima Island: this below M6.0 or so earthquake caused tsunami observed similarly as those on October 9.
* Note that on October 20 in the ocean area about 50 km west of Torishima Island, Japan Coast Guard confirmed that there were scattered floating materials which appeared to be pumices. Also, Earthquake Research Institute, University of Tokyo and National Institute of Advanced Industrial Science and Technology analyzed that the pumices collected by Japan Meteorological Agency in the ocean area about 100 km southwest of Torishima Island on October 27 have similar characteristics with the rock distributed in the back-arc rift zone continuing to the west of Izu-arc volcanic front and are considered as the pumices generated in the recent volcanic activity. But the relation of the pumices and the recent seismic activity is unknown.
#: Seismic wave, converted to sonic wave at the ocean bottom plane, then propagates in the sea.