Energy-Based Scenarios for Great Thrust-Type Earthquakes in the Nankai Trough Subduction Zone, Southwest Japan, Using an Interseismic Slip-Deficit Model

Abstract

We propose a new energy-based method to construct rupture scenarios for anticipated megathrust earthquakes by using an interseismic slip-deficit model. Our method bridges the gap between two well-known approaches, namely, kinematic and dynamic modeling. We construct the source model as a static-slip distribution, greatly reducing the computational load, and then examine whether the model follows the mechanics of faulting from the perspective of the energy balance of fault rupture. To quantify the energy balance, we introduce “residual energy,” which is defined as the difference between the strain energy released by the fault slip and the energy dissipated on the fault. Positive residual energy is a necessary condition for earthquake generation, which means that even if slip deficits accumulate, an earthquake does not occur unless the strain energy accumulation exceeds the dissipated energy. We apply the method to interplate earthquakes in the Nankai trough subduction zone, southwest Japan. Assuming a stress accumulation time of 150 years, 5 out of 10 scenarios meet the necessary condition, and one of these models can explain the crustal deformation of a historical Nankai earthquake. The significant amount of strain energy released by a rupture off Cape Muroto makes the scenario involving rupture of the whole subduction zone possible. The estimation of residual energy suggests that fault ruptures reaching the Nankai trough require more strain energy than buried ruptures. All of the scenarios eventually meet the condition, because the strain energy increases more with time than the fracture energy predicted from empirical scaling relationships.