Two-dimensional thermal modeling of subduction of the Philippine Sea plate beneath southwest Japan

In this study, we constructed a two-dimensional thermal convection model associated with subduction of the Philippine Sea (PHS) plate beneath southwest Japan to estimate the thermal state. To evaluate the validity of the calculated temperature distribution, we compared the calculated heat flows with observed heat flow data such as the High Sensitivity Seismograph Network Japan (Hi-net) borehole, land borehole, marine probe, and bottom-simulating reflectors. Hi-net heat flow data, which have never been used for thermal modeling, have spatially high resolutions on land and enable detailed estimation of the thermal state beneath the Japanese islands. We considered the spatio-temporal change in the age of the PHS plate, change in its plate motion direction at 3. Ma, and the geometry of its upper surface. Calculated heat flows associated solely with subduction of the PHS plate, passing through central and eastern Shikoku and the Kii Peninsula, were not consistent with observations. They tended to be lower on the seaward side of the corner of the mantle wedge and to have much longer spatial wavelengths on the continental side. To explain heat flow data in southwest Japan, frictional heating at the plate interface on the seaward side of the corner of the mantle wedge and temperature changes due to surface erosion and sedimentation on land associated with crustal deformation during the Quaternary must be incorporated into the model. The most suitable pore pressure ratio to explain the heat flow data is estimated to be 0.95. Temperatures on the upper surface of the PHS plate obtained in this study are lower at shallow depths and higher at greater depths than those obtained by Hyndman et al. (1995), indicating that the seismogenic zone inferred from this study is narrower than that of their model. Microearthquakes within the subducting PHS plate occur at temperatures below 700-800 °C.