抄録
The 2003 Tokachi-oki earthquake (Mj 8.0) occurred in the area off the southeastern coast of Hokkaido in northern Japan at 19:50 UTC, 25 September 2003. Five days after the mainshock, we constructed a dense seismic array using pop-up type ocean bottom seismometers (OBSs) to obtain a detailed aftershock distribution. The observation lasted one and half months. We deployed in total 47 OBSs at 38 sites. The observation array covered a 150km×100km area, where a high aftershock activity was estimated from a land seismic network. For accurate determination of aftershock distributions, we deployed OBSs with a spacing of 15km near the trench in contrast to 20km in the landward region. Hypocenters were determined by using a three-dimensional velocity structure based on the seismic refraction study. From an epicentral distribution, aftershocks occurred within the small slip region of the mainshock rather than the large slip region. In addition, the epicentral distribution of aftershocks except near the large slip region, is similar to that of the earthquakes occurring before the mainshock. The hypocentral distribution forms a dipping plane toward the land and we infer that this plane shows the upper boundary of the Pacific Plate. The position of the plate boundary estimated by the aftershock distribution is consistent with those estimated by the past seismic surveys.