Understanding the factors controlling earthquake rupture areas in subduction zones is a fundamental question in geodynamic research. In the Nankai Trough, Japan, three major controlling factors have been proposed so far: surface roughness, such as seamounts, on the subducting Philippine Sea plate, a locally dense and rigid upper plate, and stable or unstable frictional properties of the plate-boundary megathrust with abnormal pore-fluid pressure. The most prominent rupture segment boundary in the Nankai Trough is located off the Kii Peninsula. When the rupture starts and propagates beyond this boundary, the entire megathrust breaks, as in the 1707 Hoei earthquake. When...