3-D cell model simulation of the inland earthquake generation pattern in Southwest Japan during the Nankai earthquake cycles in a layered viscoelastic medium

In southwest Japan, there are a lot of inland active faults, such as Median Tectonic Line, the Neodani, Atotsugawa, and Rokko-Awaji faults. The earthquakes in these faults are mainly generated by the east-west compressive stress due to the Pacific plate subduction. However, because the activity of inland earthquakes increases in the period from 50 years before to 10 years after the great interplate earthquakes (Hori & Oike, 1999), earthquake generations in these faults are affected by the interplate earthquakes and collision of the Izu volcanic arc due to the Philippine Sea (PHS) plate subduction. To evaluate the effects quantitatively, we model the stress accumulation/release processes at the inland active faults in southwest Japan. For this problem, Pollitz & Sacks (1997), Hyodo & Hirahara (2004), and Hirahara (2007) evaluated the viscoelastic effect of great interplate earthquakes at the PHS plate subduction by examining Coulomb Failure Function ΔCFF. We here simulate earthquake generation pattern at inland active faults in southwest Japan by solving the boundary value problem. The governing equations are the slip response function and the friction constitutive law. The boundary conditions are east-west compressive stress due to the Pacific plate subduction, the interplate earthquakes and collision of the Izu volcanic arc due to the PHS plate subduction, and the geometry of plate interfaces and inland active faults. We here compute the slip response function in an elastic-viscoelastic stratified medium. We employ quasi-static viscoelastic slip response functions for point sources by Fukahata & Matsu’ura (2006). To obtain accurate slip response functions for rectangular sources effectively, we apply the Gauss-Legendre integration scheme. We use the approximate solutions under the assumption that slip response exponentially decays with time, since it is difficult to calculate viscoelastic slip response functions for all time steps. To approximate quasi-dynamic slip behavior during earthquakes, the radiation damping term (Rice, 1993) is introduced. As for the friction constitutive law, we employed the slowness law (Dietrich, 1979) among the rate- and state-dependent friction law. We set the history of the interplate earthquakes at the PHS plate subduction as the boundary condition; the occurrence time is from historical record and the amount of slip is from time or slip predictable model (Shimazaki & Nakata, 1980). We assume stick-slip cells and its downward aseismic cells to represent inland active faults in southwest Japan. The negative values of a-b friction parameter at stick-slip cells are set on the basis of recurrence time and stress accumulation rate obtained by paleoseismological studies and seismic stress drop. We first evaluate the ΔCFF for each inland fault, including interaction between inland faults in addition to the subduction of PHS and interplate earthquakes. We then simulate inland earthquake generation pattern in the southwest Japan with several initial conditions of simulation parameters on each inland faults.