August 17, 2005
Headquarters for Earthquake Research Promotion
Earthquake Research Committee

Earthquake Off-shore of Miyagi Prefecture on August 16, 2005

* On August 16, at 11:46 (JST) there was a M7.2 (preliminary) earthquake at a depth of approximately 40km off-shore of Miyagi prefecture. This event had a maximum seismic intensity 6 Lower, and caused some damage. Also, a small tsunami was observed on the Pacific coast in the Tohoku region, with a tsunami height of 0.1m in Ayukawa, Ishinomaki city. The focal mechanism showed a reverse fault with a compression axis in a WNW-ESE direction. It is thought that this event occurred at the boundary between the Pacific and the continental plates. The seismic activity was a main shock - aftershock sequence. The largest aftershock, as of 14:00 on August 17, was a M4.6 (preliminary) earthquake that occurred at 11:52 on August 16, with a maximum seismic intensity 2.

* According to the GPS data, crustal movements associated with the main shock were observed centered on Miyagi prefecture, with a displacement of approximately 5cm to the east at the Oshika observation station (Ishinomaki city, Miyagi prefecture), which is consistent with the focal mechanism.

* This event occurred near the location of the 1978 Off-shore Miyagi Prefecture Earthquake. Most aftershocks of the recent event occurred within the aftershock region of the 1978 earthquake, however there is a concentration within a relatively narrow range in the southern part. Compared to the 1978 event, the magnitude, the observed tsunami and the estimated source region from the tsunami, are all smaller.

* 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 event is not thought to be this earthquake. This is because the magnitude of the earthquake was small, and the source area, which was estimated from the aftershock distribution and seismic waves, did not cover the whole expected source region. Although, the recent event ruptured a part of the focal region of the expected earthquake.

* It is estimated that there is a 10 percent probability of occurrence of M6.0 or greater aftershocks (seismic intensity 5 Lower in some areas) within the three days from 14:00 on August 17.