Permalink : http://hdl.handle.net/2115/38143

JaLCDOI : 10.14943/gbhu.72.23

Observation of micro-earthquakes using Ocean Bottom Seismometers (OBSs) was carried out from May 17 to July 2, 2007 to obtain seismic activities beneath the area off Cape Erimo, Hokkaido. In this neighboring region, large earthquakes with magnitude around 8 occurred at intervals of 40̃60 years. In this study area, clusters of micro-earthquakes were detected immediately before the 2003 Tokachi-oki earthquake (M8.0) by a previous OBS observation (Takanami et al., 2005). The purpose of this study is to investigate the seismic activity after the 2003 Tokachi-oki earthquake by comparing with the results by Takanami et al. (2005). Most of the hypocenters determined in this study are located at shallower depth than those by the Japan Meteorological Agency (JMA). This means that it is difficult to obtain detailed micro-seismicity in offshore area by using land stations only. Therefore, ocean bottom seismographic observations are necessary to detect micro-earthquakes in the region far from the land. According to the depth of the subducting Pacific plate, which is about 10km beneath this study area, it is interpreted that most of these earthquakes occurred at the plate boundary. Seismic activities in this study area were lower than those by Takanami et al. (2005), but some concentrated earthquakes were detected. However, there was no temporal concentration. This suggests that cumulated stress was released by the 2003 Tokachi-oki earthquake and the following post-seismic slip and that the seismicity of this area returns to the steady state.