Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network

The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of aftershocks near the mainshock. From the data overall OBS, seventy-four aftershocks were located with high spatial resolution. Most of the aftershocks were located in a depth range of 15–20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15–20 km from the aftershock distribution.