Fig. 1
a Tectonic setting of four converging plates around Japan. b Enlarged map of the rectangular area in (a). The broken contours represent the iso-depth contours of the plate interface between the subducting Philippine Sea Plate and the overriding Amur Plate. The contour interval is 20 km. The red dots indicate the epicenters of low-frequency earthquakes, and the numbered ellipses schematically indicate long-term slow-slip events (l-SSEs) along the Shikoku and Kyushu coasts. (1) Kii channel, (2) Eastern Shikoku, (3) Central Shikoku, (4) Western Shikoku SSE, (5) Bungo SSE, (6) Northern Hyuga-nada SSE, and (7) Southern Hyuga-nada SSE. The stars show the epicenters of the 2019 Hyuga-nada Sea, 2019 Tanegashima, and 2022 Off Ohsumi earthquakes with Mw 6.2, 6.4, and 5.7, respectively. c The 1946 Nankai area showing the source region of the 1946 Nankai earthquake (Sagiya and Thatcher 1999), which is locked strongly and expected to slip in the future Nankai earthquake. The green line indicates the area of the plate interface adopted in the time-dependent inversion. The blue circles show the locations of the GNSS sites used in the time-dependent inversion, whereas the white circles show the locations of the GNSS sites whose position time series are shown in Fig. 2. The T-direction and S-direction represent the directions toward which the parameters of the parametric spline surface increase (Ozawa et al. 2020)