Hypocenter depths of large interplate earthquakes and their relation to seismic coupling

Abstract

We examine locations of hypocenters of large interplate earthquakes (M≥7.5) at subduction zones relative to their rupture areas to obtain a relation between the hypocentral depth and the seismic coupling coefficient, which is the ratio of the long-term slip rate estimated from cumulative seismic moment of large earthquakes to the relative plate motion. We find: (1) that the hypocentral depth is close to the bottom of the depth extent of the rupture area when the seismic coupling coefficient is nearly equal to 1.0; and (2) that the hypocentral depth relative to the depth extent of the rupture area shows a large scatter from shallow to deep when the seismic coupling coefficient is smaller than about 0.5. To interpret these observations, we conduct a numerical simulation of seismic cycles of interplate earthquakes where the frictional stress on the plate boundary is assumed to obey a laboratory-derived rate- and state-dependent friction law. The observed result (1) is reproduced in the simulation when the critical fault length, which is the size of the fault where the slip nucleation process takes place, is much shorter than the seismogenic zone of the plate boundary. In this case, the plate boundary is strongly locked during an interseismic period, and accordingly the calculated seismic coupling coefficient is close to 1.0. Seismic slip starts near the bottom of the seismogenic zone, because the maximum shear-stress concentration is generated near the bottom of the seismogenic zone due to deeper steady plate motion. On the other hand, when the critical fault length is large, appreciable aseismic sliding occurs in the seismogenic zone and, therefore, the calculated seismic coupling coefficient becomes significantly smaller than 1.0. The hypocentral depths of simulated large earthquakes tend to be shallow for large critical fault lengths. Heterogeneous frictional properties on the plate boundary may produce non-uniform aseismic sliding during an interseismic period. In this case, the seismic coupling is small and seismic slip starts in various portions of the seismogenic zone. This can explain the observed result (2).