2004 年 57 巻 2 号 p. 115-130
The Tokachi-oki earthquake (MJMA8.0) occurred on September 26, 2003 (JST) off southeastern Hokkaido, Japan. We investigate the seismic activity before and after the mainshock using hypocenter catalogues produced by Japan Meteorological Agency and Hokkaido University. A decline of seismic activity and a seismic gap in the asperity of the 2003 earthquake had been clearly recognized from the early 1990's to just the mainshock faulting. Similar low seismicity had been observed before the former 1952 Tokachi-oki earthquake (MJMA 8.2). These facts may reflect that the plate boundary fixed loosely has become tight coupling during one seismic cycle. A relatively low seismicity patch during the interseismic phase corresponds to the asperity. This suggests that interseismic seismic activity is strongly controlled by existence of the asperity. The aftershock region of the 2003 earthquake is approximately 160km by 160km, which is slightly smaller than that of the 1952 earthquake. This feature is in good agreement with the slip distributions of the 1952 and 2003 earthquakes estimated from tsumani waveform inversions. Large aftershocks are located out of the asperity, which may reflect the stress status in and around the asperity. Remarkable triggered seismic activity has begun just after the mainshock along the volcanic front in the eastern Hokkaido.