21 July 2000
Headquarters for Earthquake Research Promotion
Earthquake Research Committee

Seismic activity on July 21st in the sea off Ibaraki Prefecture

*At around 3:39 a.m. on July 21st, there was an M6.0 event at a depth of approximately 50 km in the sea off Ibaraki Prefecture. The event recorded a maximum seismic intensity of 5 lower.

*It is though that this event occurred near the juncture of the Continental Plate and the subducting Pacific Plate. The focal mechanism was a reverse fault type with a northwest-southeast pressure axis, similar to events occurring in this area in the past. Subsequent seismic activity took the form of aftershocks to this event, and at 7:29 a.m. the strongest aftershock to date, at M4.2, occurred, but aftershock activity is now subsiding.

*The results of GPS observation show no marked changes.

*Since 1926, there have been 13 M6.0 or higher events (including the present event) within a 30 km radius of the hypocenter of the present event.