[SSS15-29] Mechanics-based scenarios for the Nankai trough subduction earthquakes: A necessary condition for earthquake generation
Keywords:Earthquake scenarios , Interplate earthquakes , The Nankai trough subduction zone , Fault mechanics, Energy balance of shear faulting
In this study, we propose a new mechanics-based method to bridge the gap between the kinematic and dynamic modeling. The method greatly reduced the computational load by constructing the source model as a static slip distribution and then examined whether each scenario actually happens from the viewpoint of fault mechanics. We applied the method to large interplate earthquakes in the Nankai trough subduction zone.
First, we calculated shear-stress accumulation rates at the plate boundary from the estimated interplate slip-deficit rates (Noda et al. 2018 JGR). Assuming that the shear stress accumulated during interseismic period is completely released by the earthquake, we determined the stress drop distribution by multiplying the shear-stress change rates by the accumulation time. Next, we estimated the coseismic slip distribution which causes the assumed stress drop by solving an inverse problem. We constructed various scenarios by applying this procedure to different accumulation times and different rupture areas.
We then examined whether each scenario is consistent with fault mechanics in view of energy balance. The strain energy released by an earthquake is partially dissipated on the fault plane. We therefore introduced residual energy which is obtained by subtracting the dissipated energy from the strain energy. The residual energy should be positive to generate earthquakes. We estimated the strain energy (available energy) and dissipated energy (fracture energy) for each scenario assuming a slip-weakening friction law used in Hok et al. (2011 JGR). The available energy increased in proportion to the square of time, and the dissipated energy increased in proportion to time (seismic moment). While the dissipated energy is larger than the strain energy (negative residual energy) immediately after the previous event, as time passes the strain energy becomes dominant over the dissipated energy (positive residual energy) and earthquakes occur. This result suggests that the accumulation of strain energy (slip deficit) is not directly related to the earthquake generation unlike simple kinematic modeling and that a certain level of energy accumulation is required for the generation of great earthquakes.