The rupture process of an earthquake on March 20, 2011 in the source area of the repeating earthquakes off Kamaishi, NE Japan, and its relation to the M9.0 Tohoku earthquake

On March 20, 2011, nine days after the M9.0 Tohoku earthquake, a M5.9 earthquake occurred on the plate boundary off Kamaishi, northeast Japan. Near the hypocenter, M4.8-4.9 earthquakes have repeatedly occurred with a mean recurrence interval of about 5.5 years (the off-Kamaishi repeating earthquakes), but no events larger than M5.0 had occurred in the area. In order to clarify whether this event is belonging to the repeating earthquakes, we estimated the rupture process of the M5.9 event using a seismic waveform inversion method. Before the waveform inversion analysis, we relocated the hypocenter of the M5.9 event with those of the 2001 and 2008 off-Kamaishi repeating earthquakes. The result shows that the hypocenter of the M5.9 event located about 1-2km southeast of the cluster of the off-Kamaishi repeating earthquakes. Using this hypocenter and waveforms from K-NET, we conducted a waveform inversion to estimate the rupture process of the M5.9 event. The result shows that the M5.9 event mainly ruptured toward northwest (deeper) and northeast (shallower) of the hypocenter and the diameter of the ruptured area is around 5km. The northwest rupture area corresponds to the location of the off-Kamaishi repeating earthquake cluster. From our results, we judge that the M5.9 event was one of the off-Kamaishi repeating earthquakes because the event ruptured the asperity of the off-Kamaishi events. On the other hand, the magnitude is 1.0 larger than those of the previous repeating events. This is mainly because the diameter of the rupture area is around 5 km which is five times larger than previous events. We also calculated the spectral ratios of the waveforms between the 2011 and 2001 events and between the 2011 and 2008 events. The results show the corner frequency of the 2011 event is around 0.6 Hz while those of the 2001 and 2008 are around 3Hz. Thus, the source size surely became five times larger than usual. The main reason of the source area expansion is that the 2011 event ruptured not only the asperity of the previous events but also the area east of the asperity. The eastern area is thought to have had slipped aseismically, probably as afterslip of the M4.8-4.9 events, because no earthquakes had been observed there. The change from aseismic to seismic was probably caused by the stress perturbation due to the large afterslip and/or seismic slip of the M9.0 Tohoku earthquake, which occurred on March 11, 2011.