Study area and observation periods. (a) Study area and station distribution. The yellow circles and red squares show the location of OBSs and on-land stations, respectively. OBS names are also shown. The black dashed line depicts the Nankai Trough. The location of the study area is shown in the inset map with a red rectangle. PA and PH in the inset map represent the Pacific Plate and the Philippine Sea Plate, respectively. (b) Observation periods for each OBS station. The colored horizontal bars show the periods of available data at each station. The locations of OBS stations along the vertical axis are presented in Figure 1a.