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Active‐source seismic surveys have resolved the fine‐scale P‐wave velocity (Vp) of the subsurface structure in subduction forearcs. In contrast, the S‐wave velocity (Vs) structure is poorly resolved despite its usefulness in understanding rock properties. This study estimates Vp and Vs structures of the Nankai Trough forearc, by applying transdimensional inversion to high‐frequency teleseismic waveforms. As a result, a thin (∼1 km) low‐velocity zone (LVZ) is evident at ∼6 km depth beneath the sea level, which is located ~3 km seaward from the outer ridge. Based on its high Vp/Vs ratio (∼2.5) and comparison to an existing seismic reflection profile, we conclude that this LVZ reflects a high pore pressure zone at the upper portion of the underthrust sediment. We infer that this overpressured underthrust sediment hosts slow earthquake activities and that accompanied strain release helps impede coseismic rupture propagation further updip. Plain Language Summary: Many geophysical surveys have investigated the subsurface structures of shallow subduction zones by estimating the propagation speed of compressional waves emitted from artificial explosive sources. Although shear and compressional wave speeds are necessary to understand rock properties (e.g., water pressure) of the subsurface, it has been difficult to constrain the shear wave speed with a high‐resolution. In this study, we estimate both compressional and shear wave speeds by applying an advanced technique to earthquake waveforms recorded at ocean‐bottom seismometers deployed at the Nankai subduction zone, Japan. The results show a sufficiently high spatial resolution to detect a thin (~1 km) layer, which we interpret as water‐rich subducted sediment. This water‐rich zone may promote slow slips on the megathrust fault and work as a barrier against rupture propagations during large earthquakes. Key Points: We applied transdimensional inversion of teleseismic waveforms to seafloor cabled stations at the Nankai subduction zoneThe resultant high‐resolution velocity structures allow a direct comparison to active source surveysA low‐velocity zone beneath the outer ridge is evident, which is interpreted as an overpressured portion of underthrust sediment [ABSTRACT FROM AUTHOR]

The Sanriku ocean‐bottom seismometer system uses an optical fiber cable to guarantee real‐time observations at the seafloor. A dark fiber connected to a Distributed Acoustic Sensing (DAS) interrogator converted the cable in an array of 19,000 seismic sensors. We use these measurements to constrain the velocity structure under a section of the cable. Our analysis relies on 24 hr of ambient seismic field recordings. We obtain a high‐resolution 2‐D shear‐wave velocity profile by inverting multimode dispersion curves extracted from frequency‐wave number analysis. We also produce a reflection image from autocorrelations of ambient seismic field, highlighting strong impedance contrasts at the interface between the sedimentary layers and the basement. In addition, earthquake wavefield analysis and modeling help to further constrain the sediment properties under the cable. Our results show for the first time that ocean‐bottom DAS can produce detailed images of the subsurface, opening new opportunities for cost‐effective ocean‐bottom imaging in the future. Plain Language Summary: Distributed Acoustic Sensing (DAS) is a relatively new measurement method that has the potential to convert existing fiber optic communication infrastructure into arrays of thousands of seismic sensors. In this research, we connected a DAS to a cable that was originally installed at the bottom of the ocean to sustain a seismic and tsunami observatory in the Sanriku Region. We show that this new type of measurement can provide reliable information to image and explore the shallow subsurface under this fiber cable. This is the first time such analysis is performed in an oceanic environment, and our methods could be readily exportable to other fiber‐optic cables that are the backbones of our modern telecommunication. Key Points: Ocean‐bottom Distributed Acoustic Sensing is used to image shallow VS structureRayleigh wave phase velocity dispersion curves are extracted from frequency‐wave number analysisReflection image is obtained from autocorrelations of ambient seismic field [ABSTRACT FROM AUTHOR]

Seismic waves from earthquakes recorded on the seafloor are composed of complex multiple arrivals. Here, distributed acoustic sensing (DAS) observations along a cable located offshore the Sanriku Coast, Japan, show that the local earthquake wavefield is particularly rich in Scholte waves. We introduce a processing pipeline to extract these surface waves from DAS records. We then invert hundreds of dispersion curves along a section of the cable to form a shallow high‐resolution shear‐wave velocity model. Moreover, we focus on the possible generation mechanisms of Scholte waves through a series of 2D and 3D full‐wavefield numerical simulations. We show that water phase reverberations greatly contribute to the generation of Scholte waves on the ocean floor. This study demonstrates the potential of DAS to observe and better understand a poorly known marine wave phenomenon and image the offshore shallow seismic structure with an unprecedented spatial resolution. Plain Language Summary: Distributed acoustic sensing (DAS) is a measurement technique that has recently demonstrated its utility for marine geophysics. DAS offers the possibility to observe the seismic wave at a scale and an extent previously unattainable with traditional passive seismic surveys. Here, we use a linear DAS array located offshore the Sanriku Coast, Japan. We propose a processing pipeline to extract surface waves from local earthquake DAS records and obtain hundreds of measurements along the cable. These measurements are used to infer a high‐resolution model of the near‐shore subsurface. Supported by a series of numerical simulations, we suggest that acoustic reverberations in the water column greatly contribute to the generation of surface waves on the ocean floor. This study further demonstrates that DAS can be used to understand marine wave phenomena better and image offshore seismic structures. Key Points: Passive high‐resolution Vs imaging of shallow sedimentary layers with ocean‐bottom distributed acoustic sensing and low‐magnitude earthquakesGridded slant‐stack method to extract Scholte waves from local earthquake wavefieldsFull‐wavefield simulations suggest that water phase reverberations generate Scholte waves at the ocean floor [ABSTRACT FROM AUTHOR]