Seismic Activity in the Ocean near Izu-Oshima Island
From December 4, 2021, in the ocean near Izu-Oshima Island (Area a), there began a cluster of earthquakes and till December 17 there were 25 earthquakes with maximum seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 2: 7, 1:18). All these earthquakes occurred within the crust of the Philippine Sea plate.
Of these the largest were an M3.2 earthquake (maximum seismic intensity 2) occurring on December 6 at 13:46 (JST) at a depth of 6 km and an M3.2 one (maximum seismic intensity 2) on 7th at 10:48 (JST) at a depth of 7 km.
For the seismic activity since October 1997, in the vicinity of the recent cluster (Area a), there have sometimes been clusters of earthquakes. Recently from March to May, 2021, there was a cluster: there occurred 21 earthquakes with maximum seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 3: 3, 2: 2, 1: 13).
For the seismic activity since 1919, in the region around the recent cluster (Area b), on January 14, 1978, there was an M7.0 earthquake (maximum seismic intensity 5, "the 1978 Izu-Oshima-Kinkai Earthquake") which caused damage with 25 fatalities, 211 injured persons, 96 completely collapsed houses and so on (Materials for Comprehensive List of Destructive Earthquakes in Japan). Also, tsunami of 70 cm (peak to trough amplitude) was observed at Okada, Izu-Oshima.
Japan Meteorological Agency
[Evaluation of Seismic Activities for December 2021 (January 13, 2022)]