Fig. 1: The S-net seafloor seismometers and the principle of shear-wave splitting analysis.

(a) Distribution of the S-net stations (squares) and the earthquakes used in this study. Orange and green circles show the interplate earthquakes and upper-plate earthquakes (depth ≤ 35 km), respectively. Red squares are the stations that have one or more splitting parameter estimations. Red triangles are the Quaternary volcanoes and thick pink lines show the volcanic front. Dashed contours show the depth to the subduction interface with an interval of 10 km. (b) Example of the procedure of the splitting parameter estimations. Left panel shows S waves. Bottom two traces show the original EW and NS components, and the top two traces show rotated waveforms to fast and slow directions. The dashed line in the slow direction is the waveform shifted with the amount of delay time. On the top, the station name, cross-correlation coefficient, delay time, and the earthquake source information are shown. The station location is also shown in (a). The right panel shows the particle motion for the original (uncorrected), and rotated and delay-time-corrected (corrected) waveforms. Note that the split shear-waves have similar waveforms in the fast- and slow- directions, and the cross-correlation value between the waveforms in the fast and slow directions becomes maximum for the direction and time shift. (c) Schematic cross-section view of the structure around 39°N [modified from Uchida et al., 2010⁵⁰]. The source earthquakes are located on the subduction interface and the S-net stations are located above them. The S-net covers a large area in the forearc. Orange and green circles represent the offshore interplate and upper-plate earthquakes used in this study while crosses represent intraslab earthquakes used in previous studies.