Pre- and post-seismic slow slip on the plate boundary off Sanriku, NE Japan associated with three interplate earthquakes as estimated from small repeating earthquake data

The spatio-temporal variation of quasi-static (slow) slip on the plate boundary before and after three major earthquakes (Mj 7.1 in 1989, Mj 6.9 in 1992, and Mj 7.6 in 1994) off Sanriku, northeastern Japan, is estimated using small repeating earthquake data. The small repeating earthquake sequence employed is considered to correspond to repeated rupture of a small isolated asperity surrounded by a quasi-static slipping area, and the activity of the source reflects the quasi-static slip rate. Small repeating earthquakes accompanying moderate-sized events appear to have been activated prior to the major events under investigation, with activity starting 6 days, 2 days, and 8 months before the occurrence of the 1989, 1992, and 1994 events. These results indicate that the quasi-static slip near the hypocenters of these major earthquakes was accelerated prior to the main shocks, causing stress concentration at the asperities responsible for the main shocks. This idea is supported by the fact that both the epicenters (locations of initial breaks) and areas where quasi-static slip was accelerated were located on the same side of the main-shock asperities. Migration of quasi-static slip acceleration in the off-Sanriku region was also identified, apparent in the aftershock activity of the 1994 event and the earthquake swarms including the 1989 (Mj 7.1) and 1992 (Mj 6.9) events. The present results reveal that quasi-static slip may have played a crucial role in the occurrence of these large earthquakes, as well as contributing to aftershock and swarm activity on the plate boundary.