Spatiotemporal Evolution of Long- and Short-Term Slow Slip Events in the Tokai Region, Central Japan, Estimated From a Very Dense GNSS Network During 2013-2016

We analyzed Global Navigation Satellite System (GNSS) data from 1 January 2008 to 31 January 2016 for the Tokai region of the Nankai subduction zone, in order to study long- and short-term slow slip events (SSEs) on the subduction interface. The dataset included our new temporary stations to obtain high resolution observations with station spacing of about 5 km. Long- and short-term SSEs have been reported in the Tokai region, over the past two decades. It is difficult to directly compare those SSEs because long-term SSEs were observed by GNSS, while short-term SSEs were mainly observed by tiltmeters and strainmeters. We applied a time-dependent inversion with improved temporal resolution to GNSS data and successfully obtained the spatiotemporal evolution of long- and short-term SSEs. Our results show the very slow and stable slip of a long-term SSE with M-w 6.6, along with several episodes of accelerated slip associated with short-term SSEs (M-w 6.0-6.1). Our results showed the details of the migrating slip patterns and slip rates for the short-term SSEs and possible relation with low frequency tremor (LFT). LFT is more associated with smaller magnitude short-term SSEs rather than the larger long-term SSEs. However, the maximum daily slip rates for short-term SSEs are an order of magnitude larger than those for the long-term SSEs. This suggests an explanation that the LFT may be directly related to the loading of the plate interface by the short-term SSEs.