[S-SS11] Statistical seismology and underlying physical processes

[SSS11-08] A near-trench slow slip event in the asperity of the 1975 Kurile tsunami earthquake inferred from seismic quiescence and earthquake swarm activity

*Kei Katsumata¹ (1.Institute of Seismology and Volcanology, Hokkaido University)

Keywords:seismic quiescence, earthquake swarm, Kurile Islands

In the present paper, I report unusual changes in seismic activity in and around the focal area of two megathrust earthquakes in the southwestern Kurile Islands. A swarm activity of earthquakes occurred in 2003 in the focal area of the 1969 earthquake and immediately after that seismic quiescence started and has been lasting for 16 years. The combination of the two observations can be explained qualitatively by a slow slip event in the asperity of the 1975 Kurile tsunami earthquake. The focal areas of the 1969 and the 1975 earthquakes partly overlap. The 1975 tsunami earthquake is located near the trench axis and the 1969 earthquake is further from the trench axis than the 1975 event (Ioki and Tanioka, 2016). The earthquake swam in 2003 was caused by a slow slip event in the focal area of the 1975 event. The slow slip event is a sliding motion of a fault on the upper surface of the subducting Pacific plate and the down-dip-extensional stress decreases around the down-dip edge of the fault of the slow slip event. The long-term seismic quiescence following the earthquake swarm might have been caused by the decrease in down-dip-extensional stress within the Pacific slab.