Fig. 1

The location of the Tokai district where the slip rate was estimated. a The blue curves denote the depth contours of the Philippine Sea Plates for 20, 30, and 40 km. The green stars denote the epicenters of historical large earthquakes with M > 7.5 (Set C in Tanaka 2014). The light blue circle represents a stationary cold-water mass, which appears when large-scale meandering of the Kuroshio Current occurs. b The thick black curve denotes the anticipated source area of a megathrust earthquake. The black dots and the green circle denote the epicenters of the tremors (Idehara et al. 2014) and the area where a large slip occurred due to a long-term slow slip event from 2000 to 2006, respectively. The solid earth and ocean tides along the red vertical line were compared in Fig. 2a. The slip rate at site H1 was mainly estimated. The ocean bottom pressures at H2–6 are shown in Fig. 3b. c, d The epicenters and magnitudes of small earthquakes with M > 3.5 in the Tokai area, for which the background seismicity from Fig. 4a was inferred. The events took place in an area within 150 km from the Suruga Trough in the direction of subduction (surrounded by the red diamond in (c))