Fig. 1

Study area and observed shallow very-low-frequency earthquake (sVLFE) distribution. a Distribution of the centroid moment tensor (CMT) solutions of the sVLFEs and the Dense Oceanfloor Network System for Earthquakes and Tsunamis (DONET) stations on a bathymetry map. The color of the CMT solutions indicates the focal depth. The area outlined by the pink dashed line indicates the location where the paleo-Zenith Ridge subducted (Park et al. 2004). The epicenters of the 1944 Tonankai and 2016 off-Mie earthquakes are indicated by red and yellow stars, respectively. The right-top panel shows the regional map and the surrounding plate boundaries (Bird 2003). The dark brown rectangle indicates the area shown in a. Blue-masked areas correspond to the assumed areas of the Tokai, Tonankai, and Nankai seismogenic zones. The arrow shows the relative motion vector of the Philippine Sea Plate to the Amur Plate (Argus et al. 2011). b Distribution of the Kagan angle of the CMT solutions with respect to the reference mechanism assuming a pure thrust fault along the plate boundary. c Relationship between the average focal depth estimated using the bootstrap method and the plate boundary model (Nakanishi et al. 2018). The color and the vertical bar in the left panel indicate the Kagan angle with the reference mechanism and the standard deviation of the depth estimation, respectively