Two‐Dimensional Thermal Modeling of the Philippine Sea Plate Subduction in Central Japan: Implications for Gap of Low‐Frequency Earthquakes and Tectonic Tremors

Abstract

AbstractWe performed 2‐D high‐resolution thermomechanical numerical simulations associated with subduction of the Philippine Sea (PHS) plate in central Japan, where a well‐defined gap of low‐frequency earthquakes (LFEs) and tectonic tremors (TTs) is observed. We investigate the complex relationships among megathrust earthquakes, LFEs, TTs, and interplate temperature distributions and dehydrations caused by phase transformations of hydrous mid‐ocean ridge basalt (MORB) of the oceanic crust and ultramafic rock of the slab mantle. Our models predict a temperature range of 480–600 °C corresponding to both active LFEs and TTs regions and their gap region located on the upper surface of the PHS plate. A moderate amount of fluids (<0.8 wt%) is released due to phase transformations of the hydrous MORB near the thermally estimated seismogenic zone on the upper surface of the PHS plate. Hydrous MORB was found to transform from greenschist to epidote amphibole, releasing a small amount of fluids (>0.3 wt%) near the belt‐like form regions of LFEs and TTs. We found that the total amount of dehydrated water near the active region of LFEs and TTs is larger than that near the gap of LFEs and TTs. Our modeling results show that this difference stems from the presence of the phase transformation from epidote amphibole to eclogite in the former, and it may play a significant role for the occurrence of LFEs and TTs. The difference in the amount of released fluid between the active and gap regions of LFEs and TTs may be related to their generation mechanism.