Earthquake in the ocean area near Niijima and Kozushima on December18
"The ocean area near Izu-Oshima Island" is the region name used for the earthquake on December 18, 2020 at 18:09 in the announcement of Japan Meteorological Agency.
On December 18, 2020 at 18:09 (JST), there was a M5.0 earthquake (maximum seismic intensity 5 Lower, ① in the figure) at a depth of 11km under the ocean area near Niijima and Kozushima Islands. The focal mechanism (CMT solution) showed a strike-slip fault type with a compression axis in an NNW-SSE direction. Note that on the same day at 20:36 (JST), there was an M4.6 earthquake (maximum seismic intensity 4, ② in the figure) at a depth of 8 km. The focal mechanism also showed a strike-slip fault type with a compression axis in an NNW-SSE direction. In addition, on January 1, 2021 at 01:53 (JST), there was an M4.7 earthquake (maximum seismic intensity 4, ③ in the figure). The focal mechanism showed a strike-slip fault type with a compression axis in an NW-SE direction. All 3 earthquakes occurred within the crust of the Philippine Sea plate.
In the vicinity of the recent events (Area a), there has been a seismic activity since mid-December and till January 3, there were 39 earthquakes with observed seismic intensity 1 or over.
For the seismic activity since October 1997, in the area extending from Niijima and Kozushima Islands to Miyakejima Island, i.e. Area b, there were 6 earthquakes of M6.0 or over in the period from July to August 2007 when the volcanic activity was high in Miyakejim Island.
Japan Meteorological Agency
[Evaluation of Seismic Activities for December 2020 (January 13, 2021)]