Earthquake Under the Northwestern Chiba Prefecture on October 7 - (1) Summary

(1) Summary
On October 7, 2021 at 22:41 (JST), there was an M5.9 earthquake at a depth of 75 km under the northwestern Chiba prefecture. In addition to maximum seismic intensity 5 Upper observed at Kawaguchi City and Miyashiro Town, both Saitama prefecture and Adachi ward, Tokyo prefecture, seismic intensity 5 Lower ~ 1 was observed in the area extending from Tohoku to Kinki regions. Also long-period seismic intensity 2 was observed in the northwestern Chiba prefecture and 23 wards, Tokyo prefecture, and long-period seismic intensity 1 was observed in Kanto region. 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 Japan Meteorological Agency announced Earthquake Early Warning (Warnings) at 22:41:28.5, 3.7 sec. after the detected time of the first seismic wave.
This event caused damage with 6 heavily injured, 32 lightly injured persons and so on (as of October 15 at 17:00 (JST), Fire and Disaster Management Agency). Table 1-1 shows the situation of the damage.
After the occurrence of the event till October 31 there were 8 earthquakes with seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 5 Upper: 1, 2: 3, 1: 4)#.
The Japan Meteorological Agency performed the inspection of the seismic intensity observation points which observed seismic intensity 5 Upper, and confirmed that the observing environments of these points were not changed by the event. Also, the situation of the damage and shaking was confirmed.

# Includes the earthquakes under the Tokyo Bay on October 8 at 05:11 and on October 9 at 11:16 (both have a maximum seismic intensity 2).

Japan Meteorological Agency
[Evaluation of Seismic Activities for October 2021 (November 10, 2021)]