Contrasting subduction structures within the Philippine Sea plate: Hydrous oceanic crust and anhydrous volcanic arc crust

We show contrasting subduction structures within the Philippine Sea plate inferred from active‐source wide‐angle reflection data. Previous studies showed that large‐amplitude reflections from the slab are observed in southwest Japan and indicated that a thin low‐velocity layer with a high fluid content is formed along the top of the subducting oceanic crust. On the contrary, we found that the slab reflections have smaller amplitudes in the Izu collision zone, central Japan, where the Izu‐Bonin volcanic arc has been colliding/subducting, suggesting that such a low‐velocity layer does not exist beneath the collision zone. This structural difference is also supported by P‐wave and S‐wave velocity anomalies by passive‐source tomography and electrical conductivity, and correlates with the regional distribution of deep tremors and intraslab earthquakes, both of which are induced by dehydration processes within the downgoing slab. Based on these comparisons, we suggest that the original structure of the incoming plate controls the contrasting subducting systems: typical oceanic plate absorbs water by hydrothermal circulation at spreading centers and/or seawater infiltration at outer rises, whereas volcanic arc crust consumes a large amount of hydrous minerals for melt production and metamorphoses to more stable, anhydrous forms before subduction.