Re-estimation of average slip rate at the transition zone on the plate interface beneath southwest Japan

In southwest Japan, short-term slow slip events (SSE) are accompanied by deep low-frequency (DLF) tremors at the transition zone between the unstable and stable slip zones on the plate interface [e.g. Obara, 2004]. The long-term average slip rate at the transition zone is estimated from DLF tremors [e.g. Hiramatsu et al., 2008] and consistent with the difference between the convergence rate of Philippine Sea (PHS) plate and the slip deficit rate in southwest Japan [e.g. Hirose et al., 2010], except in the southern Kii Peninsula [Ishida et al., 2013]. In this study, we use SSEs detected by Hi-net and GNSS, expand the analyzed period, and re-estimate the average slip rate from the re-analyzed DLF tremors in southwest Japan. We analyzed continuous waveform data recorded by Hi-net, from April 2002 to July 2013 in southwest Japan. We searched DLF tremors using Hybrid catalog [Maeda and Obara, 2009] and Hybrid Clustering catalog [Obara et al., 2010]. We follow the procedure of Hiramatsu et al. [2008] to estimate the slip rate. The slip rate at the transition zone is coincident with the difference between the convergence rate of the PHS plate and the slip deficit rate in southwest Japan. In the southern Kii Peninsula, we obtained the slip rate, 3.7±0.3 [cm/yr], and resolved the mismatch. We interpret that the accumulated strain at the transition zone on the plate interface attributable to subduction of the PHS plate is partially stored as the slip deficit and partially released by short-term SSEs in the interseismic period beneath southwest Japan. The excitation efficiency of DLF tremors by short-term SSEs is distributed heterogeneously along the strike of the subducting PHS plate, especially small in the Kii Peninsula. Beneath the Kii Peninsula, Ji et al. [2016] reported a large subduction angle of the PHS plate and a cold thermal anomaly on the plate interface. This suggests that the interplate and intraslab temperatures may affect the excitation efficiency.