Fig. 2.

Distribution of CMTs and Kagan’s angles of earthquakes in periods (a) (c) (e) January 1, 2003—March 11 5:45, 2011 (before the mainshock) and (b) (d) (f) March 11 5:46—May 24, 2011 (after the mainshock). (a) and (b) show horizontal distributions; (c) and (d) show depth distributions along the thick black line AB. The size of the focal mechanism indicates the moment magnitude. The color scale denotes the centroid depth. Larger earthquakes plotted with their dates and times correspond to the 12 large events listed in Table 1. In (a) and (b), the inset shows the distribution of the CMTs of the foreshocks (focal mechanisms with thick outlines) that occurred in the two days prior to the mainshock in the rectangular area. The star indicates the mainshock epicenter. The gray thick and thin lines respectively denote the trench axis and depth contour of the subducting Pacific Plate (Hasegawa et al., 1994). In (c) and (d), red, green, blue, and black focal mechanisms respectively denote interplate type, thrust type, normal-fault type, and others. Thrust and normal-fault type earthquakes were selected from non-interplate earthquakes based on Frohlich (1992). In (e) and (f), the histograms show the angular distributions of Kagan’s angles for the earthquakes. White, gray, and black respectively denote the histograms for all earthquakes, earthquakes that occurred within 20 km from the plate boundary, and earthquakes with rake λ ≥ 0.