Low-frequency tremors, intraslab and interplate earthquakes in Southwest Japan - from a viewpoint of slab dehydration

[ 1] Low- frequency tremors ( LFT) recently found in the Southwest Japan fore- arc likely occur due to hydrofracturing from fluids leaving the dehydrating Philippine Sea slab. However, there are some places without such LFT; they are N. Izu- Kanto, E. Shikoku, and S. Kyushu. These are the places where island- arc type crust is subducting. We propose that dehydration of the subducted crust does not occur beneath these regions, because it is composed mainly of tonalite, lacking the quantity of hydrous minerals seen in normal subducting oceanic crust. We show that almost no earthquake occurs within the subducted crust in such regions, consistent with dehydration embrittlement hypothesis for intraslab seismicity. The lack of dehydration from the crust would also affect the mode of occurrence of interplate earthquakes, because pore fluid pressure is more difficult to rise at the thrust zone. This might be reflected in the irregular occurrence of great earthquakes in these regions.