Numerical modeling of slow slip events in the seismic cycles of megathrust earthquakes in southwestern Japan

We numerically simulate slow slip events (SSEs) along the Eurasian-Philippine sea plate boundary in southwestern Japan, to examine the behavior of SSEs in the seismic cycles of megathrust earthquakes within a single model. In our previous study (Matsuzawa et al., 2013), long- and short-term SSEs were reproduced in the Shikoku region, considering the distribution of tremor and the configuration of subducting plate. However, the variation in a seismic cycle was not discussed, because calculated duration is short and modeled region is not sufficiently large to simulate seismic cycles. In this study, we model SSEs and megathrust earthquakes along the subduction zone from the Shikoku to the Tokai region in southwestern Japan. In our numerical model, the rate- and state-dependent friction law (RS-law) with cut-off velocities is adopted. We assume low effective normal stress and negative (a-b) value in the RS-law at the long- and short-term SSE region. We model the configuration of plate interface by triangular elements based on Baba et al. (2006), Shiomi et al. (2008), and Ide et al. (2010). Our numerical model reproduces recurrences of long- and short-term SSEs, and the segments of short-term SSEs. The recurrence intervals of short-term SSEs slightly decrease at the late stage of a seismic cycle, reflecting the increase of long-term averaged slip rate in the short-term SSE region as found in a flat plate model (Matsuzawa et al., 2010). This decrease is more clearly found in the Kii region than those in the Shikoku region. Perhaps, this difference may be attributed to the width between the short-term SSE region and the locked region of megathrust earthquakes, as the stress disturbance from transient SSEs, which occur between the locked region and the short-term SSE region (e.g. Matsuzawa et al., 2010, 2013), seems to be relatively small and infrequent due to the narrow width in the Kii region. In addition, as the plate configuration is relatively flat in the Kii region, the decrease of intervals may be easily recognized due to the small disturbance from the curved plate configuration.