Abstract

The ratio of P-wave to S-wave velocity (Vp/Vs) is a key indicator of subsurface fluid pressure and rock properties. However, its spatial distribution in offshore regions has been poorly constrained because of difficulties in resolving shallow seismic velocity structures, particularly Vs structures. Recent advances in distributed acoustic sensing (DAS) enable dense seismic observations even on the seafloor, improving imaging of both Vp and Vs. In this study, we estimated the spatial distribution of Vp/Vs from 25 to 85 km offshore and down to 6 km depth off Sanriku, Japan, by combining controlled-source seismic surveys for Vp and surface-wave analysis for Vs. Three distinct units with contrasting Vp and Vs values were identified. The top layer (unit-1, 0.6–2.6 km depth) shows strong spatial variations in Vp/Vs. Within 50 km from the coast (unit-1a), the spatial average is 3.33. Further offshore (50–85 km; unit-1b), a distinct vertical variation is observed, with Vp/Vs exceeding 6.00 in the uppermost layer and decreasing to approximately 3.50 in the lowermost layer. Immediately below unit-1, the second unit exhibits a significantly more uniform Vp/Vs distribution, with a spatial average of 2.84. This increased uniformity likely reflects progressive sediment compaction or a distinct change in lithology. The third unit at the depth of 1.2–6.8 km shows anomalously high Vp/Vs (spatial average of 3.27) despite high Vp (~ 4.4 km/s) at the distance range of 25–65 km from the coast, which numerical modeling attributes to seismic anisotropic structures. These findings demonstrate the utility of DAS for marine seismic imaging, fluid assessment, and other seismological analyses.