26 July 2003
Headquarters for Earthquake Research Promotion
Earthquake Research Committee

Seismic Activity in northern Miyagi prefecture on July 26, 2003

*On May 26, at 0:13 (JST) there was a M5.5 (preliminary magnitude) earthquake at a depth of approximately 10km in northern Miyagi prefecture. This event had a maximum seismic intensity 6 Lower in Miyagi prefecture. Also, at 7:13 (JST) there was a M6.2 (preliminary magnitude) earthquake with a maximum seismic intensity 6 Upper in Miyagi prefecture. Each earthquake caused some damage and injuries to people.

There were a number of aftershocks following these events and they occurred mainly in a north-south direction over a length of 15km. It is thought that the seismic activity was a foreshock - main shock - aftershock sequence. The main shock was a shallow event in the crust with a focal mechanism that showed a reverse fault with a compression axis in an E-W direction.

As of 17:00 on July 26, the largest aftershock was a M5.4 (quick preliminary magnitude) event at 16:56.

* According to the GPS data, some changes have been observed corresponding to the recent earthquakes. No changes above the noise level were observed before this recent seismicity.

*The recent earthquakes occurred within the crust, which means they are different from the 1978 off-shore Miyagi prefecture earthquake that occurred along the plate boundary. The recent earthquakes are also different from the earthquake off-shore Miyagi prefecture on May 26 of this year that occurred within the subducting Pacific plate. The Earthquake Research Committee has expected the repeat of the 1978 Off-shore Miyagi prefecture earthquake to occur in the near future, however, the recent seismic activity is thought to have little direct effect on that earthquake.

*There has been active shallow seismicity since the earthquake on May 26 of this year in a wide area onshore from the focal region of this event that occurred off-shore of Miyagi prefecture.

*It is estimated that there is a 50 percent chance of occurrence of M4.5 or greater aftershocks, and a 20 percent of occurrence of M5.0 or greater aftershocks within the 3 days from 17:00 on July 26. If an aftershock with a magnitude of about 4.5 occurs, there will presumably be shaking with a maximum seismic intensity 5 Lower, and if an aftershock with a magnitude of about 5.0 occurs, there will presumably be shaking with a maximum seismic intensity 5 Upper.