Numerical Model of Long- and Short-Term Slow Slip Events during Seismic Cycles in the Shikoku Region, Japan

We numerically simulate long- and short-term SSEs in the seismic cycles of megathrust earthquakes, incorporating the actual shape of subducting plate in the Shikoku region, Japan. SSEs in the Nankai subsudction zone, Japan, are classified to short- and long-term depending on their duration. Short-term SSEs are found with episodic tremor activity. Long-term SSEs are found only in the Bungo channel and the Tokai region. It is reported that long-term SSEs in the Bungo channel interact with tremor, short-term SSEs, and very low frequency earthquakes (Hirose and Obara, 2005; Hirose et al., 2010). As SSEs are slip events at the rim of the locked region of megathrust earthquakes, modeling the SSEs in seismic cycles is important to understand not only the behavior of SSEs, but stress accumulation to the locked region. In the numerical simulations, the rate- and state-dependent friction law (RS-law) with cut-off velocities is adopted to reproduce SSEs (Shibazaki and Shimamoto, 2007). We assume low effective normal stress and negative (a-b) value in the RS-law inside of the SSE region. For short-term SSEs, SSE regions are given by the actual distribution of tremor sources (Obara et al., 2010). A long-term SSE region is assumed in the Bungo channel west of Shikoku, based on the geodetic observations (e.g., Hirose and Obara, 2005). Effective normal stress in the long-term SSE region is assumed to be higher than that in the short-term SSE region, but lower than that in the surrounding locked region, as in Matsuzawa et al. (2010). The other part below the locked region of megathrust earthquakes has positive (a-b) value. Plate interface is modeled by small triangular elements in a semi-infinite elastic medium, based on Shiomi et al. (2008) and Baba et al. (2006). We calculated the temporal evolution of slip velocity on these elements with the above frictional property. Our numerical model reproduced the recurrence of long- and short-term SSEs and megathrust earthquakes, as observed in the Shikoku region. Short-term SSEs periodically recur in segments. The recurrence interval of short-term SSEs in the segment of western Shikoku is shorter than that in central Shikoku. During long-term SSEs, recurrence intervals of short-term SSEs become shorter around the long-term SSE region. These results are also similar to the observations of tremor and SSEs. In our result, transient long-term SSEs occur between the locked region and the short-term SSE region in central Shikoku. The event is less periodic than long-term SSEs in the Bungo channel. As no long-term SSE region is assumed in this region, these events are caused by the effect of plate configuration which has a ridge-like shape in central Shikoku. Very recently, using leveling and tide gauge data, Kobayashi (2011) reported that similar transient long-term SSE occurred between 1978 and 1980 in this region. It is suggested that the newly found SSE is explained by the effect of plate configuration.