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 thestress 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 M_W 8.0 followed by an afterslip of M_W 7.9 and a mainshock of M_W 8.2. Although the moment magnitude of the afterslip is similar to the foreshock, the energy released by the foreshock is significantly larger than the afterslip because the stress drop of the afterslip is small.