Mechanisms of separation of rupture area and variation in time interval and size of great earthquakes along the Nankai Trough, southwest Japan

Abstract

According to the earthquake history along the Nankai Trough more than 1000 years, two great inter- plate earthquakes have occurred east and west off Kii Peninsula within a few years and such earthquakes have occurred repeatedly with 100-200 years intervals. Recent seismic structure surveys along the Nankai Trough reveal that there are some heterogeneous structures in several tens of kilometer scale at the boundaries of the separated rupture areas. For example, high velocity and high density doming body exists beneath Kii Peninsula, which is the main segmentation boundary. We consider that such large-scale heterogeneous struc- tures may control the rupture area separation. The heterogeneity should be also related to the mechanism of variation in time interval and size of great earthquakes occurred there, because such mechanism is closely related how rupture propagation stops. To investigate the mechanisms of rupture area separation and also time- and size-variation of great earthquakes along the Nankai Trough, we conducted large-scale numerical simula- tion of earthquake generation cycles. Heterogeneous distribution in frictional properties is assumed based on the heterogeneous structures obtained by seismic surveys. The results show that rupture area separation can be reproduced within the reasonable range in frictional parameters. The significant variation in recurrence time and earthquake size occurs only when large fracture energy area exists in the deeper portion of the seismogenic zone. The existence of such area causes heterogeneous stress distribution after one earthquake cycle. The varia- tion pattern obtained here is similar to that of the last three earthquake cycles in the historical data.