Slow slip events controlled by the slab dip and its lateral change along a trench

Several slow slip events (SSEs) have recently been globally detected at subduction zones by analyses of high precision GPS data. They occur at the transition zone from seismogenic to stable sliding ones, but seem to occur at restricted areas. Though researchers have proposed some generation mechanisms of SSE, they made no mention of such restricted occurrence areas of SSE. To explain why SSEs occur in some restricted area, we compile the features of globally detected SSEs and we find that the slab dip and its lateral change are important factors for controlling their occurrences. Through simulations of earthquake cycle including SSEs using a simplified cell model, we examine the effect of dip angle at the transition zone on the interseismic SSEs. The remarkable SSEs occur several times in a seismic cycle at the cell with a smaller dip, while at the cell with a larger dip, few small SSEs occur and attenuate in the interseismic term. Actually, SSEs occur only at the area with the smallest dip at the transition zone. Moreover in the case where cells are placed at the neighbor of the existing cell in the lateral direction, SSEs are restrained to become smaller if the dips of cells are the same. To the contrary, the cells with different dips do not restrain SSEs. This result well explains the observed feature that SSEs occur at the area where the dip of the plate is laterally changing. In addition, it is also reproduced that a cell with a smaller dip angle has SSEs with a longer interval and longer duration. These simulation results are understood in terms of the frictional condition related to the dip-dependent stiffness of plate interface, that is, the stress accumulation due to the plate subduction, relative to the critical stiffness determined by frictional parameters and the stress interaction which is also dependent on the dip angle.