3D Traveltime Tomography Using Ocean-bottom DAS Data

Summary The Sanriku-Oki subduction region in northeastern Japan is a tectonically active zone where the Pacific Plate subducts beneath the Okhotsk Plate, generating frequent earthquakes. In this study, we present a three-dimensional unstructured S-wave traveltime tomography model of the off-Sanriku forearc using local earthquake data recorded by ocean-bottom Distributed Acoustic Sensing (DAS). The dense spatial sampling provided by DAS enables cost-effective, high-resolution imaging of the forearc region that is difficult to achieve with conventional ocean-bottom seismometer deployments. Eight local earthquakes recorded near the DAS cable provided 209,193 high-quality S-wave arrival times after applying data-selection criteria. These earthquakes were recorded using two DAS interrogator units: the AP Sensing N5200A (70 km coverage, 5 m channel spacing) and the OptaSense QuantX (100 km coverage, 2 m channel spacing). The initial reference models were constructed on a 3D tetrahedral mesh with target cell sizes of 1.5 and 3.0 km, accurately incorporating the DAS cable geometry and earthquake hypocentres. Synthetic traveltimes were efficiently computed using the Fast Sweeping Method (FSM) and the traveltime models were subsequently refined through traveltime inversion. The resulting S-wave tomography models reveal significant spatial heterogeneity within the off-Sanriku forearc region. Low S-wave velocities (Vs ∼ 0.78–0.85km s−1) indicate shallow, unconsolidated Neogene sediments, underlain by higher velocities in the Cretaceous basement (Vs ∼ 1.2–2.4km s−1). The lower crust and uppermost mantle wedge exhibit clear along-strike segmentation of the forearc. In the landward domain, high S-wave velocities in the lower crust (4.15–4.4km s−1) and uppermost mantle (4.6–4.65km s−1) indicate a mechanically strong overriding plate and a thick, cold mantle wedge, with no evidence of significant partial melting or serpentinisation associated with subduction processes. In contrast, the central offshore region exhibits moderately reduced S-wave velocities (3.9–4.1km s−1), suggesting a mechanically weaker overriding plate. Toward the trench, S-wave velocities decrease markedly within the forearc crust, defining a low velocity zone associated with crust–crust interaction above the subducting Pacific Plate and likely reflecting fluid-rich, deformed forearc material in the shallow subduction environment.