A 3-D Simulation of Slow Slip Event Occurring at a Subduction Zone Based on Rate and State Dependent Friction Law

Recently, several slow slip events occurred at subduction zones, which were detected by geodetic measurements. These events were located near strong coupling regions, where large earthquakes have occurred, on subducting plate interfaces. It is important to know what produces such a variety of slip behaviors for understanding the physical process of earthquake initiation. In this study, we present a possible model for producing observed slow slip events on plate interfaces. Our model is a 3-dimensional one and simulates slip motion, which obeys a laboratory derived rate and state dependent friction law, on a plane interface of a subducting plate. Namely, we take into consideration the finiteness of the fault and simulate what type of slip behavior occurs on a subducting thrust fault, where the rupture propagates not only vertically but also horizontally. We use a slowness version of the friction law and extend the method of Kato and Hirasawa [1997] (KH) to a 3-D plain fault, where the slip with dip component can be calculated in a quasi-static manner. We model a thrust fault which has a dip angle of 20^o and 200~km length in dip direction. To examine the effect of fault length in strike direction (H) on slip behavior, we run several cases which have different H ranging from 100~km to 1000~km. We assume the same distribution of friction parameters as that of case 1 in KH. This means there is no variation in friction parameter values in a strike direction. However, we impose a side-boundary condition, which assumes a stable slip outside of the both lateral ends in our model. In case with smaller H, a fast slip event occurs at the center in strike direction of the model region. This slip propagates in strike direction and reaches both lateral edges of the region. This slip behavior recurs periodically. This result is consistent with that of KH. In case with larger H, however, slip events occur not only at the center of the lateral extent of the region, but also near the edges. These slip events which occur near the edges have slower slip velocity than the ones at the center. Because the outside of the model region slides stably, the area near the edges behaves as a transition zone of interplate coupling, where the coupling changes laterally from strong to weak. Our result shows slower slip events tend to occur at such a transition area.