Three-dimensional Thermal Modeling Associated with Subduction of the Philippine Sea Plate, Southwest Japan

Abstract:

We investigated distributions of temperature and mantle flow associated with subduction of the Philippine Sea (PHS) plate beneath southwest Japan by constructing a parallelepiped three-dimensional thermal convection model. We proposed a new, realistic, and high-resolution thermal model on the plate interface, and attempted to clarify its relationships with occurrences of low-frequency earthquakes (LFEs) and long-term slow slip events (L-SSEs) beneath the Bungo Channel. For this purpose, we newly developed a numerical model to be able to deal with subduction of an oceanic plate with three-dimensional arbitrary geometry and thickness. We modeled subduction of the PHS plate by using the up-to-date three-dimensional slab geometry, referring to high resolution P-wave seismic tomography and seismic reflection studies. We also used large number of heat flow data such as BSRs, land boreholes, marine heat probes, and Hi-net observation wells to constrain calculated temperature field. We took account of complex subduction history in southwest Japan for the last 15 Myr and temperature change due to erosion and sedimentation rates during the Quaternary period. The results showed that a cold anomaly appeared on the plate interface beneath western Shikoku, the Bungo Channel, and the Kii Peninsula due to the effect of large true subduction angle, with a temperature nearly 200°C lower in the slab core near the continental Moho than in eastern Shikoku, indicating a high lateral heterogeneity in the thermal regime of the PHS plate. Temperatures where the LFEs occur were estimated to be within a range from 350 to 550°C. The existence of a large horizontal temperature gradient of approximately 2.5~5.0°C/km from the surface to the inside of the PHS plate was identified where the L-SSEs occurred beneath the Bungo Channel. Such large temperature gradient was likely to yield phase transformations of hydrous minerals within the oceanic crust.