A numerical simulation of earthquake cycles along the Nankai Trough in southwest Japan: lateral variation in frictional property due to the slab geometry controls the nucleation position

Abstract

The occurrence pattern of historical great earthquakes along the Nankai Trough indicates the existence of rupture segments that seem to correlate with 100-km-scale lateral variation in the dip angle of the subducting Philippine Sea plate. For instance, the latest earthquakes, the 1944 Tonankai, and the 1946 Nankai earthquakes initiated their ruptures off Kii Peninsula, where the dip angle of the slab is steeper than in adjacent areas. To investigate how such heterogeneity in slab geometry affects the occurrence of earthquakes, we conducted a three-dimensional (3D) numerical simulation of earthquake cycles for a large source area, about 700 km long along the trough and 300 km wide in the dip direction, based on a rate- and state-dependent friction law. We used a simple flat dipping plane to model the plate boundary. Frictional parameters and effective normal stress were assumed to depend on the estimated depth of the real plate boundary based on seismic surveys and seismicity data. The depth-dependent frictional parameters were mapped on the flat model plane. This made lateral variation in frictional property due to the variation in the dip angle of the slab. The plate convergence rate in the model decreased eastward along the trough, consistent with a recent Global Positioning System (GPS) analysis. In the simulation results, preslip occurred, and rupture started off Kii Peninsula. Both effects of a narrower locked area due to the higher dip angle and a higher plate convergence rate caused the highest stress accumulation rate to be in this area, promoting slip nucleation. Our simple simulation suggests that lateral variation in the dip angle of the slab is one of the factors controlling the nucleation position of great interplate earthquakes.