[S-SS03] Seismological advances in the ocean

[SSS03-09] Fluid reservoir in the Hyuganada accretionary prism related to the ridge subduction: implication from a passive seismic array

★Invited Papers

*Takeshi Akuhara¹, Yusuke Yamashita², Shukei Ohyanagi², Yasunori Sawaki², Tomoaki Yamada¹, Masanao Shinohara¹ (1.Earthquake Research Institute, The University of Tokyo, 2.Disaster Prevention Research Institute, Kyoto University)

Keywords:Hyuganada, Kyushu–Palau Ridge, Transdimensional inversion, Ocean-bottom seismometer, Fluid process

Subducted reliefs, such as seamounts and ridges, influence fluid processes in accretionary prisms in subduction zones. Hyuganada, where the Kyushu–Palau Ridge subducts along with the Philippine Sea plate, is one such region best suited for studying the role of subducted topography. This study investigates shear wave velocity structures using a dense array of ocean-bottom seismometers (OBSs) with a 2 km radius. Teleseismic Green’s functions and a surface wave dispersion curve are retrieved from seismic records and inverted to 1-D shear wave velocity structures beneath each station through transdimensional Markov-chain Monte Carlo inversion. The results indicate a low-velocity zone underlying marine sediment layers with a total thickness of 3–4 km. The reduced shear wave velocities are consistent with a compressional velocity structure from a previous seismic refraction survey. We interpret that these low-velocities represent high pore fluid pressure. Additionally, resolved lithology boundaries exhibit a sharp offset that appears consistently across the OBS array, suggesting the presence of a blind fault beneath it. This predicted fault is located at the flank of the Kyushu-Palau Ridge, orienting roughly parallel to the ridge axis, likely caused by the ridge subduction. The fracture caused by the ridge subduction may act as a fluid conduit to form a fluid reservoir beneath the well-compacted sediment layers. A compilation of previous refraction surveys conducted in the Hyuganada implies that the reservoir extends laterally to ~100 km wide. Its spatial distribution roughly correlates to the ridge location, highlighting the significant role that the ridge plays in the formation of the reservoir.