Three‐dimensional numerical modeling of temperature and mantle flow fields associated with subduction of the Philippine Sea plate, southwest Japan

Abstract

AbstractWe investigated temperature and mantle flow distributions associated with subduction of the Philippine Sea (PHS) plate beneath southwest Japan, by constructing a three‐dimensional parallelepiped model incorporating a past clockwise rotation, the bathymetry of the Philippine Sea plate, and distribution of the subducting velocity within its slab. The geometry of the subducting plate was inferred from contemporary seismic studies and was used as a slab guide integrated with historical plate rotation into the 3‐D simulation. Using the model, we estimated a realistic and high‐resolution temperature field on the subduction plate interface, which was constrained by a large number of heat flow data, and attempted to clarify its relationship with occurrences of megathrust earthquakes, long‐term slow slip events (L‐SSEs), and nonvolcanic low‐frequency earthquakes (LFEs). Results showed that the oblique subduction coupled with the 3‐D geometry of subducting PHS plate was a key factor affecting the interplate and intraplate temperature distributions, leading to a cold anomaly in the plate interface beneath western Shikoku, the Bungo Channel, and the Kii Peninsula. Temperatures in the slab core in these regions at a depth near the continental Moho were nearly 200°C lower than that in eastern Shikoku, indicating a high thermal lateral heterogeneity within the subducting plate. The geothermal control of the LFEs beneath western Shikoku was estimated to be within a range from 400 to 700°C, and the interplate temperature for the L‐SSEs with a slip larger than 15 cm beneath the Bungo Channel was estimated to be approximately 350–500°C. A large horizontal temperature gradient of 2.5 ~ °C/km was present where the LFEs occurred repeatedly. The steep temperature change was likely to be related to the metamorphic phase transformation from lawsonite or blueschist to amphibolite of hydrous minerals of the mid‐ocean ridge basalt of the subducting PHS plate.