November 29, 2004 |
Seismic Activity Off-shore of Kushiro
On November 29, at 03:32 (JST) there was a M7.1 (preliminary) earthquake off-shore of Kushiro, with a maximum seismic intensity 5 Upper observed in Kushiro-cho, Teshikaga-cho, and Bekkai-cho. This earthquake caused small tsunamis on the eastern Pacific coast of Hokkaido with a tsunami height of 0.1m at Hanasaki, Nemuro city. The focal mechanism showed a reverse fault with a compression axis in a NW-SE direction. It is thought that this event occurred on the plate boundary. By 17:00 on November 29, there were 11 aftershocks equal to or larger than seismic intensity 1, with the largest aftershock a M6.0 event at 03:36 (maximum seismic intensity 4).
According to the GPS data, crustal movements associated with this event were observed in eastern Hokkaido, with a displacement of approximately 2cm in a SE direction at the Shibecha observation station, and a displacement of approximately 2cm in a NE direction at the Nemuro 4 observation station. These are consistent with the focal mechanism of the earthquake.
In this vicinity, there was a M7.2 earthquake on August 12 in 1961. It is thought that this event also occurred on the plate boundary similar to the recent earthquake.
It is thought that the recent earthquake is one of the “smaller interplate earthquakes” that were mentioned in the “Long term evaluation on the seismic activity along the Chishima Trench” that the Earthquake Research Committee announced on March 24, 2003 (corrected on November 12, 2003). The evaluation says that there has been an interplate earthquake of magnitude M7.0 to M7.2 about every 20 years since 1900 in the area off-shore of Tokachi or off-shore of Nemuro. It is estimated that there is an 80 percent probability of earthquake occurrence over the next 30 years, and it should have a magnitude of approximately M7.1.