Detailed S-Wave Velocity Structure of Sediment and Crust off Sanriku, Japan by a New Analysis Method for Distributed Acoustic Sensing Data Using a Seafloor Cable with Seismic Interferometry

Abstract S-wave velocity (Vs) structures of sediments and the uppermost crust in the landward slope of a subduction zone are essential for elucidating the dynamics of the overriding plate. Although distributed acoustic sensing (DAS) measurements have improved the horizontal resolution of Vs structure in marine areas, the estimation has been limited in the upper sedimental layer. This study imaged the structure from shallow to deep sediments at a depth up to 7 km and the uppermost crust off Sanriku, Japan, with high lateral resolution of 2.5km, by applying seismic interferometry to DAS data of 13 hours. We grouped the DAS data into 10-km-long subarrays with 75% overlaps. We first applied a frequency-wavenumber filter to the DAS data to remove DAS instrumental noise and to effectively extract surface waves from short-time records. We then applied the seismic interferometry method and estimated phase velocities at each subarray. The estimated phase velocities of the fundamental- and first higher-mode Rayleigh waves were then used to determine one-dimensional Vs structures for each subarray. The resultant 2-D Vs structure was interpreted as representing sediments and crust. The upper sediment layer became thick seaward, and a discontinuity between sediment and crust layers at depths of 3–7 km showed strong lateral heterogeneities. By combining the result with the P-wave velocity (Vp) structure along the nearest survey line, we could estimate the Vp/Vs value to be 2.28–4.0 in the lowest sedimental layers. Our developed analysis method applying seismic interferometry to marine DAS data can broaden the horizons for estimating heterogeneous Vs and Vp/Vs structures of sediment layers and the upper crusts in subduction zones. These results show that the application of both frequency-wavenumber filtering and seismic interferometry to marine DAS data can be a new tool for estimating the heterogeneous Vs and Vp/Vs structure of sediment layers and the upper crust in subduction zones.