Fig. 4

Hypocenters determined in this study. a Map view and N–S and E–W vertical cross sections showing the hypocenter distribution (open circles) determined from the data recorded by OBSs (orange triangles) and the two island stations (inverted yellow triangles) from 15 July to 10 October 2015. The diameters of the circles indicating hypocenters vary according to the magnitude of the earthquake. Hypocenters in the upper seismic zone are shown by red circles, and those in the lower seismic zone by blue circles. Hypocenters away from the plate boundary are shown by green circles. SBJ hypocenters determined with high precision from January 2002 to December 2017 around the deployment area of the OBSs are also shown by solid gray circles. The dashed purple line in the map view shows the survey line of Takahashi et al. (2009). The dotted black line in the E–W cross section indicates the upper surface of the Pacific slab along 27.3°N, as determined by Hayes et al. (2012). α and β indicate two clusters of hypocenters at 60–80 km depth. b Magnitude-time diagram. The green line shows the cumulative number of earthquakes with M 2.2 or greater. c Magnitude–frequency distribution. Open circles denote the number of earthquakes in each magnitude bin, and solid circles show the cumulative number of earthquakes. The best-fit line according to the Gutenberg–Richter law is also shown (Gutenberg and Richter 1944)