Similarities and Differences in the Rupture Process of the M ∼ 4.8 Repeating-Earthquake Sequence off Kamaishi, Northeast Japan: Comparison between the 2001 and 2008 Events

Earthquakes of M similar to 5 have repeatedly occurred at the same location on the plate boundary off Kamaishi, northeast Japan, with a mean recurrence interval of about 5.5 years. The latest two events (M 4.8 on 13 November 2001, and M 4.7 on 11 January 2008) were successfully observed by the broadband seismic network of Tohoku University covering the Tohoku District of northeast Japan. We estimated the source processes of the 2001 and 2008 events by carefully picking the onsets of P and S waves and by inverting seismic waveforms recorded by the network. The results show that both events were caused by the rupture of the same asperity patch (diameter, similar to 1 km). As the previous 1995 event was also reported to have ruptured the source area of the 2001 event, at least the last three events (1995, 2001, and 2008) in this earthquake sequence are thought to have been caused by repeated ruptures of the same asperity. A closer examination, however, reveals a small discrepancy in the slip distribution between the last two events, which explains the difference in the high-frequency components of the seismograms. The regions in which slip was smaller during the 2001 event than during the 2008 event nearly coincide with the source areas of the smaller repeating earthquakes that occurred just before the 2001 event. This finding suggests that the activity of smaller events immediately before the mainshock can influence the slip distribution of the mainshock.