Shallow Low‐Velocity Layer in the Hyuga‐Nada Accretionary Prism and Its Hydrological Implications: Insights From a Passive Seismic Array.

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 corresponds to a reduced Vp feature obtained in a previous seismic refraction survey, and the high Vp/Vs ratio suggests the presence of 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 or intense fracturing. 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. This fault may act as a fluid conduit, contributing to the formation of a fluid reservoir beneath the compacted sediment layers. Plain Language Summary: Propagation speeds of seismic S‐waves offer unique constraints on physical properties in shallow subduction zones, which is hard to know from only seismic P‐wave velocity. This study investigates the subsurface structure in Hyuga‐nada in the southwestern Japan subduction zone by exploring S‐wave speeds. For this purpose, we use natural seismic and noise data recorded by densely installed ocean‐bottom seismometers. The results reveal a region with a reduced S‐wave velocity at a depth of ∼3–4 km below the seafloor, which may be a water reservoir. The depth of the potential water reservoir changes abruptly across the array. This offset may suggest the presence of a hidden fault below, although we cannot exclude other possibilities. We propose that a fault created by subducting seamounts acts as a conduit that transports water to the reservoir. Key Points: The shear wave velocity structures of the shallow Hyuga‐nada accretionary prism were derived using a passive seismic arrayA low shear‐wave velocity zone exists ∼3–4 km below the seafloor, possibly indicative of a fluid reservoirA Fault induced by the subducting Kyushu–Palau Ridge may act as a fluid pathway, supplying fluids to the reservoir [ABSTRACT FROM AUTHOR]