Forearc trench-parallel anisotropy above a subducting plate revealed by the S-net ocean-bottom seismometers

We have estimated the shear-wave splitting parameters (fast direction and delay time) offshore NE Japan based on the S-net cabled ocean-bottom seismometers. The S-net system installed and operated by the National Research Institute for Earth Science and Disaster Resilience, Japan, covers 300 × 1000 km area along the Japan and Kuril trenches by 150 seismic stations. The data which was made publicly available in 2018 enable us to observe anisotropy beneath the offshore stations. The result show trench-parallel fast directions and ~0.1 second delay times for the forearc area. They do not depend on the source depths located along the plate boundary. The previous study based on land forearc stations and our additional result from shallow (depth ≦35km) earthquakes observed by S-net suggest the forearc splitting mostly occur in the crust part of the overlying plate. The fast direction correlate with both of the strike direction of active faults and the maximum horizontal stress estimated from stress tensor inversions, suggesting both structure and stress contributes on the crustal anisotropy. The comparison with previous land data also shows abrupt change in splitting parameters across the volcanic front which is probably related to the spatial extent of corner flow in the backarc mantle wedge.