Quiescence of repeating earthquakes preceding M-6 class interplate earthquakes

Understanding of processes preceding large earthquake generation is important. Interplate repeating earthquakes are considered as repetition of ruptures on the small patches distributed on the plate interface and can be used to monitor relative slips on the plate boundary and determine configuration of plate interfaces. At the Kanto district, M-6 class interplate earthquakes often occurred on the subducting Pacific plate (PAC). We studied temporal evolution of plate slips based on small repeating earthquakes. We conducted tentative repeating earthquake analysis for events from 2003 to 2006 in addition to the previous result of Kimura et al. (2003). Adding new results, we calculated cumulative slip history for the past 27 years from 1979 to 2006. At the eastern Kanto district, five M-6 class interplate earthquakes occurred on the PAC for the past 27 years (1989, 1990, 2000, 2005a, and 2005b event). These events and their aftershocks are distributed in a seismic cluster with size of approximately 17 x 17 km. In this cluster, numerous repeating earthquakes have been found and we calculated cumulative slip history by using them. In the slip history, deceleration of plate slip with time period of 2 to 4 years preceding M-6 class events are clearly observed for 3 of 5 events (1989, 2000, 2005a events). Deceleration of plate slip corresponds to quiescence of repeating earthquakes. Our results indicate that repeating earthquakes within 17 x 17 km region show quiescence preceding M-6 class earthquake. For the other 2 events, time interval from the previous event is not long enough, and it is difficult to judge whether slip rate decelerates or not. We interpret this phenomenon of quiescence of repeating earthquakes as that degree of locking on the plate interface increases with time before occurrence of the M-6 class earthquake. As the degree of plate locking increases, plate boundary slip is likely to decelerate near the source region.