Revisiting Intraslab Earthquakes Beneath Kyushu, Japan: Effect of Ridge Subduction on Seismogenesis

Abstract

AbstractThis study investigates intraslab seismicity of the Philippine Sea Plate beneath Kyushu, Japan, where the Kyushu‐Palau Ridge (KPR) is being subducted and the age of the incoming plate varies across the ridge. Intraslab earthquakes at depths of 30–270 km are first relocated, and focal mechanism solutions of 181 earthquakes are determined. The relocated hypocenters show marked along‐arc variations in their depth limits and distributions. The geometry of the slab Moho is delineated by considering observed focal mechanisms, precisely relocated hypocenters, and the depth extent of the hydrated slab crust previously estimated by receiver function analysis. Intraslab earthquakes are classified into crustal and mantle earthquakes based on the inferred Moho geometry. The number of crustal earthquakes decreases monotonically with depth, and little seismicity occurs beyond 100‐km depth, but mantle earthquakes become increasingly active with increasing depth and reach a peak at a depth of ~150 km. An area of locally thickened crustal seismicity (up to 12 km thick) in offshore central Kyushu corresponds to the subducted KPR, suggesting that the KPR influences crustal seismicity to a depth of at least ~60 km. The double seismic zone is clearly observed at depths of 80–150 km in southern Kyushu, where the old Philippine Sea Plate has continued to subduct over at least >5 Myr. Our observations suggest that the genesis of intraslab earthquakes is locally affected by the subduction of the KPR and regionally governed by the along‐arc variation in the formation age of the incoming plate.