Mechanical locking distributions along the Nankai Trough in SW Japan estimated by onshore and seafloor geodetic observation data

Abstract:

Interplate earthquakes occur at locking areas where the interplate strain has been accumulated by plate convergence. Many previous studies tried to evaluate the seismic potential by estimating the spatial distribution of interplate coupling, that is defined as the ratio between slip deficit and plate convergence (e.g., Nishimura et al., 2018; Watanabe et al., 2018; Kimura et al., 2019). However, since their model is based on kinematic framework, they could not estimate substantial interplate mechanical locking distributions. To estimate the mechanical locking distributions from geodetic observation data, we construct the model that relates mechanical locking and crustal deformation. Utilizing this model to the onshore and seafloor crustal deformation data, we estimated the mechanical locking distributions along the Nankai Trough in SW Japan.

We express the surface crustal deformation as sum of the block motion, elastic response from the slip deficit due to interplate mechanical locking, and internal strain. We consider two kinds of slip deficit. One is given in mechanical locking areas and equivalent to the plate convergence rate. Another one is induced in creeping areas surrounding the mechanical locking areas to relieve the shear strain around the mechanical looking areas and can be calculated analytically considering the constant stress states. Model parameters are Euler vectors, up- and down-dip depths of mechanical locking areas, and internal strain rates. We obtain the probability density functions of them by Replica Exchange Monte Carlo method (e.g., Swendsen & Wang, 1986). We used seafloor GNSS/Acoustic observation data conducted by Japan Coast Guard and Nagoya University as well as onshore GEONET data; total number of sites is 877.

We found four locking patches in Bungo channel, Kii peninsula, off Shikoku and eastern Tokai. Locking patches off Shikoku and eastern Tokai are ranging 0–20 km in depths, whereas those in Bungo channel and Kii peninsula are ranging 15–55 km in depths. The locking patch off Shikoku has the largest area among four locking patches and consists of the source region of 1946 Nankai Earthquake. We will discuss about the relations between mechanical locking distributions and past interplate earthquakes, asperity distributions (e.g., Ichinose et al., 2003), and plate geometry.