Modeling the activity of short-term slow slip events along deep subduction interfaces beneath Shikoku, southwest Japan

We developed a model of short-term slow slip events (SSEs) on the 3-D subduction interface beneath Shikoku, southwest Japan, considering a rate-and state-dependent friction law with a small cutoff velocity for the evolution effect. We assume low effective normal stress and small critical displacement at the SSE zone. On the basis of the hypocentral distribution of low-frequency tremors, we set three SSE generation segments: a large segment beneath western Shikoku and two smaller segments beneath central and eastern Shikoku. Using this model, we reproduce events beneath western Shikoku with longer lengths in the along-strike direction and with longer recurrence times compared with events beneath central and eastern Shikoku. The numerical results are consistent with observations in that the events at longer segments have longer recurrence intervals. The activity of SSEs is determined by nonuniform frictional properties at the transition zone. We also attempt to model the very low frequency (VLF) earthquakes that accompany short-term SSEs, on a 2-D thrust fault. We consider a local patch in which the friction parameters are varied. In the case that critical displacement is very small at the patch, fast multiple slips occur at the patch. In the case that the effective normal stress is high at the patch, the patch acts as a barrier to SSEs; when it ruptures, however, rapid slip occurs. Because the source time functions of these cases are somewhat different, it would be possible in the future to assess if either case is an appropriate model for VLF earthquakes.