Comparison of source areas of M4.8±0.1 repeating earthquakes off Kamaishi, NE Japan: are asperities persistent features?

M4.8±0.1 earthquakes have occurred regularly since 1957 with a recurrence interval of 5.52±0.68 yr at the same location of the plate boundary off Kamaishi, Iwate Prefecture. The last event (M4.7) occurred on 11/13/2001 16:44 (JST) and the previous one (M4.8) on 3/11/1995 10:30 (JST). Waveforms of these events are similar to each other. To examine the hypothesis that this characteristic earthquake sequence is caused by repeating ruptures of an asperity (frictionally locked patch) surrounded by creeping areas, we compared the rupture area of the 2001 event with that of the 1995 event by a waveform inversion method. Estimated spatial extents of the rupture areas of the 2001 and 1995 events are almost the same and are estimated to be about 1.5×1.5 km2. The rupture area of the 2001 event is almost completely overlapped with that of the 1995 event, although the ruptures of the two events were initiated from different points. The present observations clearly show that the 1995 and 2001 events are caused by the repeating ruptures of the same asperity patch, and support the hypothesis of persistent asperities.