Slip History for Short-Term Slow Slip Events With Low-Frequency Tremor in Southwest Japan

Slow slip events (SSE) which coincide with nonvolcanic deep low-frequency tremor have been detected in southwest Japan subduction zone (Obara et al., 2004). This coupling phenomenon has similar characteristics to the similar to the episodic tremor and slip (ETS) in the Cascadia subduction zone (Rogers and Dragert, 2003). The SSEs have been found not only in the western Shikoku region (Obara et al., 2004; Hirose and Obara, 2005), but also in the Tokai area. This activity could happen on the deeper part of a source region of a megathrust earthquake which recurs on the shallower part of the subduction plate interface. Hence the knowledge about the generation history of the activity is of decisive importance in understanding the nature of the megathrust earthquake cycle. In this study, we investigate a spatio-temporal history of moment release by SSEs which are accompanied by the tremor in southwest Japan. We use the NIED Hi-net tiltmeter records for monitoring the crustal deformation due to SSE. Tilt change during the episode of the coupling activity is inverted to find a fault model for a SSE, based on Okada's (1992) expression. In the western Shikoku region, 12 episodes are observed since 2001. We estimate a fault model for each episode. These results are compiled to draw a diagram of spatio-temporal distribution of interplate moment release (Kawasaki et al., 2001) as SSE. We found that the interplate coupling ratio at the source region of SSE with tremor is 30-50% in 2001--02, while from 2003 to the early 2004, most of the strain accumulated by the relative plate motion is released by the SSEs. This temporal change in the coupling ratio is possibly related to the occurrence of the 2003 Bungo channel long-term SSE (Ozawa et al., 2004; Hirose and Obara, 2005). The slip history and the similarity in the spatial pattern of the tilt changes suggest that there may be at least four `SSE patches' in the western Shikoku region. We also found the repeating activity of the same coupling phenomenon in the Tokai region, where there seems to be at least four SSE patches. These results indicate that the phenomenon must be a universal activity along the tremor belt zone which has the internal structure, the SSE patch.