Relationship Between Quasi-static Slips and Rupture of Asperities in the Northeastern Japan Subduction Zone: Insights From Small Repeating Earthquake Data Analyses

We have examined relationships between quasi-static slips and rupture of asperities on the plate boundary off Sanriku in the northeastern Japan subduction zone. We assumed that cumulative slips for small repeating earthquakes expressed the quasi-static slip histories on the plate boundaries surrounding them (Igarashi et al., 2003; Uchida et al., 2003) and compared them with occurrence times of larger events. We found that repeating earthquakes near the epicenters of 1989 (M7.1), 1992 (M6.9) and 1994 (M7.6) events had been activated since 6 days, 2 days, and 8 months before the respective events. These results indicate that the areas near the hypocenters of those earthquakes had already slipped quasi-statically before the main shocks. The stresses on the asperities for the main shocks concentrated by the quasi-static slips are probably responsible to the rupture of the asperities. We also found prominent repeating earthquake activities during earthquake swarms including the 1989 and 1992 events and aftershock activities of the interplate events with M>=6. Moreover, the quasi-static slip accelerations migrated during the 1989 and 1992 earthquake swarms and after the 1994 event. These results show that the quasi-static slips play an important role for the generation of earthquake swarms and the expansion of aftershock area off Sanriku. After the 1992 earthquake swarm, the quasi-static slip acceleration further migrated toward west to reach the asperity of the M4.8 _echaracteristic earthquake_f sequence off Kamaishi (Matsuzawa et al., 2002) about 2 years later. After the arrival of the acceleration, the next _echaracteristic earthquake_f occurred in 1995. The recurrence interval before the last event in this case is the shortest among the 8 recurrence intervals ever known. Thus, the 1995 event was probably advanced by the quasi-static slip acceleration. These observations obtained in this study indicate that the quasi-static slips affect the occurrences of larger interplate events as well as the small repeating earthquakes.