Simulation of recurring earthquakes along the Nankai trough and their relationship to the Tokai long-term slow slip events taking into account the effect of locally elevated pore pressure and subducting ridges

Many earthquakes of moment magnitudes greater than 8 associated with subduction of the Philippine Sea plate under Japan have occurred along the Nankai trough with a recurrence interval of 90-150 years. A large-scale rupture of the Tokai fault segment is believed to be imminent because that segment did not rupture during the most recent Tonankai earthquake in 1944. Recurring long-term slow slip events (LSSEs) have recently been observed in the Tokai region. To investigate the current stress state in the Tokai region, we numerically simulated earthquake cycles using a three-dimensional model consisting of triangular cells based on the rate- and state-dependent friction law. Our numerical simulations successfully modeled the Tokai segment not ruptured during the 1944 Tonankai earthquake as well as the recurring LSSEs in the Tokai region. We found that a large characteristic displacement (L) and effective normal stress (sigma) in areas of subducted ridges are essential to avoid rupture of the Tokai segment during the 1944 Tonankai earthquake. We also demonstrated that we can reproduce the recurring LSSEs by assigning low values of sigma and L to the area beneath Lake Hamana without introducing a velocity-strengthening friction law at high slip rate. Our simulation showed that the amplitudes of the LSSEs increased following the earthquakes that did not rupture the Tokai segment, which suggests that the recent LSSEs may be related to the accumulation of stresses in the Tokai segment because it did not rupture during the 1944 earthquake.