26 September 2003
Headquarters for Earthquake Research Promotion
Earthquake Research Committee

Off-shore Tokachi Earthquake* on September 26, 2003

*On September 26, at 04:50 (JST) there was a M8.0 (preliminary magnitude) earthquake at a depth of approximately 40km off-shore of Tokachi. This event had a maximum seismic intensity 6 Lower in Hokkaido prefecture, and caused some damage. This earthquake caused a tsunami on the Pacific coast from Hokkaido to the Tohoku region, with tsunami heights of 1.2 m in Kushiro, 1.3 m in Urakawa, and 1.0 m in Hachinohe. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. This event occurred on the boundary between the Pacific and the continental plates.

The activity had a main shock - aftershock sequence. At 6:08 (JST) on September 26, there was a M7.1 (preliminary magnitude) aftershock with a maximum seismic intensity 6 Lower, which was the largest aftershock, as of 18:00 (JST) on September 26.

*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 or east in the area around Cape Erimo. At the station Erimo 2, displacements of approximately 90 cm in a southeastern direction and subsidence of approximately 20 cm were observed. These crustal deformations are consistent with the fault movement for a reverse fault on the plate boundary.

In addition, no changes above the noise level were recognized before the seismic activity.

*This event is a M8-class plate boundary earthquake which occurred at nearly the same location as the Off-shore Tokachi Earthquake on March 4, 1952 (M8.2). According to the magnitude, the hypocenter location, and the focal mechanism, it is thought that this event is the M8-class Off-shore Tokachi Earthquake that has been expected by the Earthquake Research Committee (expected magnitude was around M8.1).

In the long-term evaluation that the Earthquake Research Committee announced on May 24, 2003, there was 10-20 percent chance of occurrence of a M8-class Off-shore Tokachi Earthquake over the next 10 years starting from January 1, 2003, and a 60 percent chance over the next 30 years.

*It is estimated that there is an about 20 percent chance of occurrence of M7.0 or greater aftershocks within 3 days from 18:00 on September 26. If a magnitude of about 7.0 aftershock occurs, there will presumably be shaking with maximum seismic intensity 6 Lower.

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