The slip deficit rate, slow and fast earthquake at the Nankai subduction zone.

The Nankai subduction zone is considered as a region with a high seismic risk. Large earthquakes with magnitudes larger than 8 have occurred and will recur in the future. Additionally, several observations of slow earthquakes at the shallow and deep plate interface have continuously been being happening. Large and slow earthquakes originate from different types of frictional characteristics, mostly driven by an accumulation of interplate slip deficit. Here, we present results from geodetic observations to estimate the slip deficit rate at the Nankai subduction zone based on displacement rate data of the dense onshore GNSS array as well as that of the offshore GNSS-Acoustic stations during the period 2002 to 2016 (Nishimura et al. 2018). After removing co-seismic and post-seismic deformations due to earthquakes in other regions, we estimated the slip deficit rate at the plate interface by using a trans-dimensional reversible jump Markov chain Monte Carlo algorithm (Tomita et al. 2021). To obtain a smooth slip distribution without imposing smoothing constraints we used a Voronoi partitioning for slip parameters, in which the number of cells is automatically set during the inversion, based on the spatial resolution of data. In contrast to previous slip deficit inversions at the Nankai region, we included scaling weight factors for different types of data. The scaling factors were parameterized to be adjusted in the inversion procedure. Furthermore, we used elastic Green Functions estimated from a three-dimensional heterogeneous structure of the Nankai regions (Hori et al. 2021), instead of a homogeneous medium as previously done. We accomplished a new model of slip deficit rate for the Nankai subduction zone that complements previous models. Regions with high slip deficits agree with the rupture areas of historic large earthquakes. Lastly, the location of deep and shallow slow earthquakes agrees with estimated intermediate values of slip deficit. The slow earthquake region defines the transition between a locked and unlocked plate interface, while shallow slow earthquakes can also be generated by shallow heterogeneous patches or subsurface structures at the subducting plate.