Modeling short-term silent slip events along the deeper parts of the Nankai subduction zone

Recent high-resolution observations of crustal movements have revealed short-term silent slip events (SSEs) with propagation velocities of around 10--15 km/day and with intervals of 3--14 months along the deeper parts of the Cascadia and Nankai subduction zones (Dragert et al., 2001; Obara et al., 2004). Shibazaki and Shimamoto (2007) have developed a model for these short-interval SSEs by considering the frictional behavior that was experimentally confirmed by Shimamoto (1987) using halite for the unstable-stable transition regime. They have modeled the SSEs considering a rate- and state-dependent friction law with a small cutoff velocity to an evolution effect. In their model, under conditions where the pore-fluid pressure is nearly equal to the lithostatic pressure and the critical weakening displacement is very small, short-interval SSEs with propagation velocities and slip velocities of 10 km/day and 5×10-7m/s, respectively, can be reproduced. They have found that the propagation velocity of short-interval SSEs is in proportion to the slip velocity. The activity of short-term SSEs in western Japan has been well investigated by Obara (1997). To understand the loading processes for great thrust earthquakes along the Nankai subduction zone, it will be important to model these short-term SSEs considering a realistic 3D geometry of the subduction interface. Based on the study by Shibazaki and Shimamoto (2007), we have developed a model of short-term SSEs on the 3D subduction interface beneath Shikoku, western Japan. The occurrence of SSEs is very complex: the generation zone of SSEs is divided into several segments in the horizontal direction. The mode of segmentation depends on the width of the generation zone. When the width of the generation zone is sufficiently small, significant segmentation occurs. On the other hand, when the width of the generation zone is large, larger events can occur that extend horizontally over the entire region. From the epicentral distribution of deep low-frequency tremors, we set the width of the generation zone of SSEs such that it is wider beneath the western part of Shikoku than beneath the eastern part. We can reproduce events with longer lengths in the horizontal direction beneath the western part of Shikoku with longer recurrence times. The numerical results are consistent with the observation by Obara (2007) that the events at longer segments have longer recurrence intervals. We have also attempted to model the very-low-frequency earthquakes observed by Ito et al. (2006) that are accompanied by short-interval SSEs. To model low-frequency earthquakes, we are required to consider a nonuniform fault zone structure, where local rupture with a high slip velocity occurs together with the propagation of short-interval SSEs. We consider a local patch where the critical displacement is very small. We then confirm that high-speed slips occur in the local patch with a small critical displacement. In some cases, slips occur at the same patch repeatedly within a short interval. We consider that, even in the transition zone, there are heterogeneities in the frictional properties and the critical displacement is scaled with the size of the events.¡¡We report the conditions for the friction parameters for which we can reproduce the low-frequency earthquakes that satisfy the scaling law proposed by Ide et al. (2007).