The slow slip event in the Tokai region, central Japan, since 2013 as seen from GPS data

In the Tokai region, cetral Japan, the previous long-term slow slip event (SSE) occurred on the subducting Philippine Sea plate from 2000 to 2005, the longest SSE ever found. In addition, many short-term SSEs have been observed in the Tokai region since 2004. Ozawa et al. (2016) reported that a slow slip event seems to have started in the similar area of the previous Tokai long term SSE in the beginning of 2013. We analyzed GPS data in the Tokai region to estimate the temporal evolution of the current event. GPS data from 1 Jan. 2008 to 30 Apr. 2015 were used in this study. The GIPSY-OASIS II software was used to estimate daily coordinates of 226 GPS stations from the GEONET in the Tokai district. The 2011 Tohoku-oki earthquake (Mw9.0) occurred on 11 Mar. 2011 and post-seismic deformation has also been observed in the Tokai region since then. Therefore, the effects of the post-seismic deformation due to the Tohoku-oki earthquake were removed by fitting the data from 11 Mar. 2011 to 31 Dec. 2012 with simple mathematical functions. We approximate the post-seismic deformations by the sum of exponential and logarithmic functions and subtract the estimated post-seismic deformation from the GPS time series. After removing this compornent, we applied a time-dependent inversion method to the data to obtain the spatio-temporal evolution of slip on the Philippine Sea plate beneath the Tokai region. For this purpose, we used a modified Network Inversion Filter (NIF) (Fukuda et al., 2008). The original NIF (Segall and Matthews, 1997) assumes a constant hyperparameter for the temporal smoothing of slip rate and thus results in oversmoothing of slip rate. The modified NIF assumes a time variable hyperparameter, so that changes in slip rate are effectively extracted from GPS time series. The results indicate that maximum slip for the long-term SSE from 1 Jan. 2013 to 30 Apr. 2015 was estimated to be about 6 cm and the large slip was located in nearly the same area as or slightly to the south of the previous event. This long term SSE is still continuing at the end of Apr. 2015. In addition, the results suggest that four short-term SSEs were embedded in the slow and steady long-term SSE during the time interval from the beginning of 2013 to the end of Apr. 2015. We will discuss the relationship between the long-term SSE and short-term SSEs in this presentation.