Mechanically Coupled Areas on the Plate Interface in the Nankai Trough, Japan and a Possible Seismic and Aseismic Rupture Scenario for Megathrust Earthquakes

Quantifying the stress distribution or finding mechanically coupled areas on the plate interface is fundamentally important for conjecturing megathrust earthquakes that may occur in the future. Kinematically coupled areas, or slip deficit distributions, on plate interfaces were commonly estimated by geodetic‐data analyses. However, mechanically coupled areas are not identical to the kinematically coupled areas. The present study develops an inversion method to estimate the stress rate distribution as mechanically coupled areas. We apply this method to the Nankai trough subduction zone, southwestern Japan to detect mechanically coupled areas. Some of the estimated coupled areas correspond to the rupture areas of historical earthquakes. Others are in a deeper part, which may release the stress as aseismic slip. We then construct a rupture scenario that can occur in the Nankai trough in the future based on the estimated mechanical coupling, assuming that an effective stress accumulation period is 100 years. The scenario consists of a foreshock of MW 8.0 followed by an afterslip of MW 7.7 and a mainshock of MW 8.2. Although the moment magnitude of the afterslip is slightly smaller than the foreshock, the strain energy released by the foreshock is significantly larger than the afterslip. In order to compare events when the stress drops are significantly different, it is useful to use the minimum released strain energy, or available energy, to characterize the magnitude of the events. Plain Language Summary The driving force that generates huge earthquakes originates from the frictional locking between overriding continental plate and subducting oceanic plate. How strong and which parts are locked along the plate interface provide us with critical information for megathrust earthquakes that may occur in the future. Significantly coupled areas along the plate interface can be detected by the analysis of ground deformation. However, it is difficult to obtain correct information about the stress of the locking. To quantify the coupling along the plate interface, this study develops a method to estimate the stress rate based on elastic mechanics. We apply the method to the Nankai trough, in southwestern Japan, to estimate the plate locking. Then, by using the estimated locking information, we create a scenario for a sequence of earthquakes. This consists of a foreshock, a mainshock, and a slow slip between the foreshock and the mainshock. This study provides a metho