A possible model for large preseismic slip on a deeper extension of a seismic rupture plane

I develop a numerical model for preseismic slip and episodic aseismic slip events (slow earthquakes) on a deeper extension of a seismogenic plate interface, using a rate- and state-dependent friction law. The deep preseismic slip and episodic slow slip were indicated by models based on geodetic observations, though they cannot be explained by common rate- and state-dependent laws. I investigate whether this incongruity can be removed by including use of additional known constraints on the frictional property. The steady-state friction on a model fault is taken to be velocity weakening at low velocities and velocity strengthening at high velocities. The transition velocity from velocity-weakening friction to velocity-strengthening is assumed to be the seismic slip velocity at shallow seismic depths and decreases with depth at deeper parts. Simulation results for a 2D model of interplate earthquakes at a subduction zone are as follows: (1) During an interseismic period between successive large earthquakes at shallow seismogenic depths, some episodic aseismic slip events with durations of a few years occur on the plate interface just below the seismogenic depths. (2) Preseismic slip occurs at nearly the same location where the episodic aseismic slip events occur. These may explain some characteristics of observed preseismic slip of the 1944 Tonankai earthquake (M=7.9) along the Nankai trough, central Japan, and episodic aseismic slip events in the same area.