October 11, 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 as there were 4 earthquakes of magnitude (M) 6.0 over. The largest one in the series of seismic activity is the 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 0.3 m (preliminary) observed at Yaene, Hachijojima Island of the Izu Islands.
* Also, from 04 o'clock to 06 o'clock (JST) on 9th, there occurred at least 14 earthquakes which had too small magnitudes for determining hypocenters, still accompanied waves which were considered as T phases (#). This seismic activity caused tsunami observed along the coasts of the area extending from Izu-Ogasawara Islands to Kyushu-Shikoku regions, especially of 0.6 m (preliminary) observed at Yaene, Hachijojima Island.
* Then the seismic activity seems to have 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 small tsunami observed along the Pacific coasts of the area extending from Chiba to Okinawa prefectures, especially of 0.6 m observed at Yaene, Hachijojima Island. In addition, an M5.9 earthquake on June 13, 1984, an M5.9 earthquake on January 1, 2006, and an M5.7 earthquake on May 6, 2018 caused tsunami: these M6.0 below earthquakes caused tsunami similarly as those on October 9.
* In this area rather small magnitude earthquakes cause tsunami, and therefore we should pay attention for the time being.
#: Seismic wave, converted to sonic wave at the ocean bottom plane, then propagates in the sea.