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 seafloor-cable seismic and tsunami observation system is ideal for marine geophysical observation because the data can be obtained in real-time. We have developed a new compact seafloor cable seismic and tsunami observation system using Information and Communication Technology (ICT) since 2005. Our new system secures reliability by using Transmission Control Protocol/Internet Protocol technology and provides observational flexibility via observation nodes with a software-based system using up-to-date electronics technology. These features contribute to cost reduction and production sustainability. The system was installed on the Pacific Ocean floor off Sanriku, northeast Japan, in September 2015, in the source area of the 2011 Tohoku-oki earthquake. Our purpose is to better monitor seismic activity and to observe tsunami activity through spatially dense observation. The obtained high quality seismic and pressure data are stored continuously by the new system.

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.

The off-Ibaraki region is a convergent margin at which a seamount subducts. An intensive event location was performed around the subducting seamount to reveal the regional seismotectonics of this region. By applying a migration-based event location to an Ocean Bottom Seismic network record of both P- and S-waves, over 20,000 events were determined in the off-Ibaraki region below ~M4. The seismicity showed clear spatiotemporal patterns enough to identify the seismicity changes and geometry of the interface. At the updip side, the shallow tectonic tremors and earthquakes are shown to be spatially complementary bounded by an updip limit of the seismogenic zone. At the downdip side, a semicircular low-seismicity zone was identified, which is possibly a rupture area of the Mw7.9 event. The event depth profile exhibited a gently sloped planar downdip interface subparallel to the subducting slab. This plane appears to be stably active from 2008 to 2011. Comparison with the active source seismic survey profiles exhibits that this planar downdip interface is several kilometers deeper than the top of the oceanic crust. After the Mw7.9 event, a high-angle downdip seismic interface was activated above the planar interface. Further, below the planar downdip interface, broadly scattered events occurred with a swarm manner. We successfully illuminated the complicated subsurface structures around the subducting seamount. It is suggested that most of the event occur along or below the plate interface as the top of the oceanic crust.

Seismic waves can be significantly amplified by soft sediment layers. Large dynamic strains can trigger a nonlinear response of shallow soils having low strength, which is characterized by a shift of the resonance frequencies, ground motion deamplification, and in some cases, soil liquefaction. We investigate the response of marine sediments during earthquake ground motions recorded along a fiber-optic cable offshore the Tohoku region, Japan, with Distributed Acoustic Sensing (DAS). We compute AutoCorrelation Functions (ACFs) of the ground motions from 103 earthquakes in different frequency bands. We detect time delays in the ACF waveforms that are converted to relative velocity changes (dv/v). dv/v drops, which are characteristic of soil nonlinearity, are observed during the strongest ground motions. Moreover, the dv/v values show a strong variability along the cable. This study demonstrates that DAS can be used to infer the dynamic properties of the shallow Earth with an unprecedented spatial resolution.