Recurrence intervals of characteristic M4.8±0.1 earthquakes off-Kamaishi, NE Japan-Comparison with creep rate estimated from small repeating earthquake data

We examined the cause of the fluctuation in recurrence intervals of an M4.8±0.1 'characteristic earthquake' sequence off-Kamaishi, NE Japan by comparing the recurrence intervals with creep (quasi-static slip) rates around the asperity estimated from small repeating earthquake data. The eight recurrence intervals (5.52 yrs on average) of the sequence are almost constant but have small fluctuations (standard deviation of 0.68 yrs) characterized by a longer recurrence interval following a shorter one. From the analysis of small repeating earthquakes from 1990 to 2003, we found a significant acceleration of creep on the plate boundary during and after an earthquake swarm in 1992 to the east of the M4.8±0.1 event. The acceleration seems to have migrated from east (shallower part of the plate boundary) to west (deeper part) over about 80 km in 2 yrs to reach the asperity of the M4.8±0.1 events. The 1995 event that occurred with the shortest recurrence interval was probably advanced by this acceleration of creep in around 1994. We also found similar earthquake swarms prior to the 1962 and 1973 'characteristic' events that occurred after the second and third shortest recurrence intervals respectively. Therefore it is possible that the 1962 and 1973 events were also advanced by creep accelerations. The three pairs of longer and shorter recurrence intervals we observed suggest the existence of creep deceleration following the acceleration. We conclude that the recurrence interval fluctuations of the M4.8±0.1 events are mainly controlled by temporal changes of creep around its asperity.