Earthquake Offshore Ibaraki Prefecture on August 4

On August 4, 2021 at 05:33 (JST), there was an M6.0 earthquake (maximum seismic intensity 3) at a depth of 18 km, offshore Ibaraki prefecture. The focal mechanism (CMT solution) showed a reverse fault type with a compression axis in a WNW-ESE direction. This event occurred at the boundary between the Pacific and the continental plates. In the vicinity of the recent event (Area a), from August 3 till 4, 2021, there were 14 earthquakes with seismic intensity 1 or over (maximum seismic intensity: number of earthquakes, 3: 1, 2: 4, 1: 9).
For the seismic activity since October 1997, in the vicinity of the recent event (Area a), there have sometimes been earthquakes of M5.0 or over. On May 8, 2008, there was an M7.0 earthquake (maximum seismic intensity 5 Lower) which caused damage with 6 injured persons and so on (Fire and Disaster Management Agency).
For the seismic activity since 1919, in the area around the recent event (Area b), there have sometimes been earthquakes of M7.0 or over. Of these the M7.6 earthquake (maximum seismic intensity 6 Upper) on March 11, 2011 at 15:15 (JST), offshore Ibaraki prefecture is the largest aftershock of the Tohoku-Oki Earthquake.

Japan Meteorological Agency
[Evaluation of Seismic Activities for August 2021 (September 9, 2021)]