October 8, 2021
Earthquake Research Committee,
Headquarters for Earthquake Research Promotion
* On October 7 at 22:41 (JST), there was a magnitude (M) 5.9 (provisional) earthquake at a depth of approximately 75 km under the northwestern Chiba prefecture. Maximum seismic intensity 5 Upper was observed in Saitama and Tokyo prefectures and some damage was caused. Also, in Chiba and Tokyo prefectures long-period seismic intensity 2 was observed. The focal mechanism showed a reverse fault type with a compression axis in an E-W direction. This event occurred at the boundary between the Pacific and the Philippine Sea plates.
* The seismic activity continues in the area including the M5.9 event, extending about 10 km length in E-W and N-S directions, and till October 8 at 19 o'clock there occurred 2 earthquakes with maximum seismic intensity 1 or over. The largest aftershock till October 8 at 19 o'clock was that of M3.6 (preliminary) which occurred on October 8 at 05:11 (JST).
* According to the result of GNSS observation (quick solution), no significant crustal deformation associated with this event was observed.
* In the vicinity of the recent event, on July 23, 2005, there was an M6.0 earthquake and maximum seismic intensity 5 Upper was observed in Tokyo prefecture. There have sometimes been earthquakes of M6 or so, such as an M6.0 one on September 25, 1980. The largest was an M6.3 earthquake on September 30, 1956.
* In the area of strong shaking, we should pay attention for a week or so, to the possibility of an earthquake which causes maximum seismic intensity 5 Upper or so. Especially for 2 or 3 days after the earthquake, there is a possibility that a large scale earthquake occurs.
* In the southern Kanto region where the recent event occurred, the Pacific plate is subducting from the Japan Trench in the east under the Philippine Sea plate which is subducting in the south, and there have occurred many earthquakes of M7 or so. In the "Long-Term Evaluation of the Seismic Activity Along the Sagami Trough (2nd ed.) (Published on April 25, 2014)" the probability of the occurrence of an earthquake of M7 or so (M6.7~M7.3) within 30 years was evaluated as rank III (#), classified as the group of the high probability in the trench-type earthquakes.
# : Rank of the probability of occurrence of a trench-type earthquake within 30 years is expressed as follows; rank: probability, III: greater than 26%, II: 3% ~ 26%, I: smaller than 3%, X: unknown (an immediate occurrence of an earthquake cannot be denied).
Note: GNSS is a general name of satellite positioning system such as GPS.