Fig. 3

Mainshock and aftershock activity of the September 20, 2017 earthquake. a Map showing epicenter distribution of the mainshock and aftershocks. The bathymetry relief is represented in grayscale based on the red relief map in Fig. 10b and c (Chiba et al. 2008). The earthquakes within the dashed rectangles are indicated by circles color-coded with time after the mainshock, as indicated in panel (b). The mainshock is marked with a green star. Other earthquakes before the mainshock and after September 23 are indicated with open circles. The focal mechanism of the mainshock from the GCMT catalog and OBS first-motion polarities are indicated. b Magnitude-time diagram of the earthquakes within the dashed rectangles on panel (a). c Cross sections of the hypocenters along the profile C–C’ on panel (a). Earthquakes within 5 km from the profile (within the dashed rectangle) are projected. The gray rectangle indicates the area of the earthquakes that occurred within 1 h after the mainshock. The seafloor and the approximate depth of the Moho, 7 km below the seafloor, are indicated by solid and dashed lines, respectively. d, e Cross sections of the hypocenters along the profile A–A’ on panel (a). The dashed green lines with black arrows are the projection of the westward-dipping planes along which the aftershocks are aligned. The focal mechanisms from the first-motion polarities observed by the OBSs are also indicated on panel (e). The OBS focal mechanisms are color-coded by the faulting types according to the triangle diagram of Frohlich (1992) as shown in Fig. 2. f–h Space-time plots of the mainshock and aftershocks projected along the profiles B–B’ (f), C–C’ (g), and D–D’ (h). The gray-shaded region is the 1-h aftershock area along the profile C–C’ (g), and its projection is on the other two profiles (f, h). Vertical dashed lines indicate the origin time of the mainshock and 2 h since