Fig. 1: Tectonic setting of the study region.

Tremor probability is shown by the brownish/cream heatmap (probabilities lower than 0.2 are muted for visibility). Circles and rectangles represent the temporarily deployed ocean-bottom seismometers and DONET stations, respectively, used to locate the tremors. Green contours depict the slip distribution of the 1944 Tonankai earthquake at 0.5-m intervals, reproduced from ref. ⁶⁰ with permission from SNCSC. The red-shaded area delineates the subducted Paleo–Zenisu ridge, reproduced from ref. ⁴⁰ with permission from Elsevier.