9 October 2003
Headquarters for Earthquake Research Promotion
Earthquake Research Committee

Seismic Activity off-shore of Tokachi

*On September 26, at 04:50 (JST) there was a M8.0 earthquake* at a depth of approximately 40km off-shore of Tokachi (maximum seismic intensity 6 Lower). (see "Off-shore Tokachi Earthquake on September 26, 2003" in the 118th Earthquake Activity Evaluation by the Earthquake Research Committee) This earthquake caused a tsunami on the Pacific coast from Hokkaido to the Tohoku region. According to the surveys so far, tsunamis were observed with a height of 2.5 m at the Tokachi Port (tidal observatory), and tsunami run-up of 4.0 m at Hyakunin-hama (Erimo-cho, Horoizumi-gun, Hokkaido prefecture). The seismic activity had a main shock ? aftershock sequence. The aftershock activity has been declining. At 6:08 (JST) on September 26, there was a M7.1 aftershock (maximum seismic intensity 6 Lower) that occurred near the southwestern edge of the aftershock area, which was the largest aftershock, as of 16:00 (JST) on October 9.

According to the GPS data, crustal movements associated with this event, were observed over a large area in Hokkaido prefecture. There were very large movements toward the southeast in the area around Cape Erimo. Displacements of approximately 97 cm in a southeastern direction at the Hiroo station, and subsidence of approximately 28 cm at the station Taiki 2, were observed. The size of the fault model determined from the GPS data is generally consistent with the size of the aftershock area. Following the main shock through October 8, postseismic movements have been seen in the large area in Hokkaido prefecture centered on Cape Erimo. At the stations Erimo 1 and Erimo 2, movements of 5-6 cm in a southeastern direction were observed. The pattern of crustal movements is similar to that of the main shock, so it is thought that the plate boundary in the focal region has been slipping slowly on a small scale following the main shock.

* It is thought that this event is the M8-class plate boundary earthquake off-shore of Tokachi that has been expected by the Earthquake Research Committee. From the analysis of the focal process, it is estimated that there was a region with large slip located in nearly the same place as that of the 1952 Off-shore Tokachi Earthquake.

Following this earthquake, the shallow seismic activity in the crust has increased in most areas in the eastern half of Hokkaido prefecture.

*It is estimated that there is about a 20 percent chance of occurrence of M6.5 or greater aftershocks within the 3 days, and 40 percent chance within 7 days from 18:00 on October 9. If an aftershock with a magnitude of about 6.5 occurs, there will presumably be shaking with a maximum seismic intensity 5 Upper. Also it is estimated that aftershocks of M4.3 or greater (approximately corresponding to events with intensity equal to or greater than1) may occur about once per day until the end of October.

*: The Japan Meteorological Agency named this event the "2003 Off-shore Tokachi Earthquake".