Correlation between ridge subduction and a fluid reservoir in the
Hyuganada accretionary prism: insights from a passive seismic array
Abstract
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.