The 1946 Nankai earthquake and segmentation of the Nankai Trough

We perform a simple subevent analysis of seismic P-waveforms from historical records of the 1946 Nankai earthquake. The results of this analysis establish that the earthquake rupture consisted of two main subevents, one lying within the 1-day aftershock area off the Kii Peninsula, while the other lies much farther to the west in the area of pronounced crustal deformation of western Shikoku. These conform to two segments of the Nankai Trough which have long been referred to in studies of historical earthquakes and geology of the Nankai Trough, but to our knowledge have not previously been established as having distinct megathrust rupture behavior. We show that these two subevents are bounded by pronounced features in the geometry of the subducting Philippine Sea Plate (PSP): a tear in the plate to the east beneath the Kii Peninsula, and a subducting seamount farther to the west. We speculate that these features may be associated with low-strength parts of the upper and/or lower plates that may be susceptible to permanent, anelastic deformation which prevents the accumulation of the elastic strain energy necessary to sustain seismic rupture.