Temperature distribution of the upper surface of the subducted Philippine Sea Plate along the Nankai Trough, southwest Japan, from a three‐dimensional subduction model: relation to large interplate and low‐frequency earthquakes

SUMMARY The Nankai subduction fault in southwest Japan is a plate boundary along which large megathrust earthquakes have repeatedly occurred. A previous 2‐D thermal model suggested that the updip and downdip limits of the seismogenic zone are closely related to the temperature of the plate interface. We estimated a new temperature distribution for the plate interface based on a 3‐D thermal convection model that incorporates revised governing equations, temporal changes in relative plate motion, the shape of the subducted slab, temperature variations related to basin evolution and more heat flow data and model regions than have been used in previous models. The margin‐parallel spatial variation in the temperature of the plate interface was caused by spatio‐temporal changes associated with the cooling of the Shikoku Basin. The updip limit of the thermally delineated seismogenic zone (thermal seismogenic zone) was approximately parallel to the trough axis due to a spreading rate that increased with distance from a fossil ridge, although the margin‐parallel age dependency of the subducting plate was taken into account. The downdip limit of the thermal seismogenic zone was determined by temperature, except in eastern Kyushu, where it was determined by a boundary between the continental Moho depth and the upper surface of the subducting plate. The maximum coseismic slip associated with the 1944 Tonankai ( M 7.9) and the 1946 Nankai ( M 8.0) earthquakes, and a strongly coupled region estimated from GPS data inversion obtained by previous studies, were both located in the thermal seismogenic zone on the plate interface. Low‐frequency earthquakes located at the downdip of the thermal seismogenic zone took place at temperatures of about 400–500 °C. This may be related to the phase transformation of hydrous minerals in oceanic crust from lawsonite + blueschist + jadeite to amphibole + eclogite and the associated decreases in water content of approximately 2.4 wt%. The thermal seismogenic zone is narrowest off the Kii Peninsula, where the hypocentres of the Tonankai and Nankai earthquakes were located. The narrow seismogenic zone may have determined the rupture initiation points for these earthquakes, as was previously demonstrated by numerical simulations of the earthquake cycle.