Shear wave velocity (Vs) estimations of accretionary prisms can pose unique constraints to the physical properties of rocks, which are hard to obtain from compressional velocities (Vp) alone. Thus, it would help better understand the fluid processes of the accretion system. This study investigates the Vs structure of the Hyuga-nada accretionary prism using an array of ocean-bottom seismometers (OBSs) with a 2 km radius. Teleseismic Green’s functions and a surface wave dispersion curve are inverted to one-dimensional Vs structures using transdimensional inversion. The results indicate the presence of a low-velocity zone 3–4 km below the seafloor. The reduced Vs is consistent with a reduced Vp feature obtained in a previous seismic refraction survey. From its high Vp/Vs ratio, we conclude that the low velocities represent high pore fluid pressure. In addition, the resolved lithological boundary exhibits a sharp offset that extends laterally across the OBS array. We attribute this offset to a blind fault below while acknowledging other possibilities, such as due to mud diapirism. The predicted fault is located at the Kyushu–Palau Ridge flank, oriented roughly parallel to the ridge axis, and thus likely caused by ridge subduction. The fracture caused by the ridge subduction may act as a fluid conduit, forming a fluid reservoir beneath the well-compacted sediment layers.

A document by Yihuan Ruan. Click on the document to view its contents.

Subducted reliefs, such as seamounts and ridges, affect fluid processes in accretionary prisms of subduction zones. The Kyushu–Palau Ridge subducts along with the Philippine Sea Plate in Hyuganada, which is one of the regions that are best suited for studying the role of subducting topography. This study investigates the shear wave velocity structure using an array of ocean-bottom seismometers (OBSs) with a 2 km radius. Teleseismic Green’s functions and a surface wave dispersion curve are inverted to one-dimensional shear wave velocity structures using transdimensional inversion. The results indicate the presence of a low-velocity zone 3–4 km below the seafloor. The reduced shear wave velocities are consistent with a compressional velocity structure obtained in a previous seismic refraction survey. We conclude that the low velocities are representative of high pore fluid pressure. In addition, the resolved lithological boundaries exhibit a sharp offset that consistently appears across the OBS array, suggesting the presence of a blind fault beneath it. The predicted fault, which is located at the flank of the Kyushu–Palau Ridge and oriented roughly parallel to the ridge axis, is likely caused by the ridge subduction. The fracture caused by the ridge subduction may act as a fluid conduit, forming a fluid reservoir beneath the well-compacted sediment layers. The compilation of previous refraction surveys implies that the reservoir has a lateral extension of >100 km. Its spatial distribution roughly correlates with the ridge location, highlighting the significant role the ridge plays in the formation of the reservoir.