Fig. 1.

Distribution of earthquake events between foreshock and main-shock. (a) Filled circles denote the epicenters of the earthquakes and their colors represent elapsed time from the foreshock as shown by the color bar scale. Stars indicate the epicenters of the foreshock and the mainshock. The red dotted rectangle very roughly encloses the fore-shock focal area. Seismicity is sampled for Fig. 2 from two gray dotted rectangle areas; the colored tick marks inside them represent calculated rupture front locations at each elapsed time shown by the colors assuming the diffusion coefficient D = 0.78 × 10⁴ (refer to the text for the detail). (Inset) Mainshock focal area with seismicity from March 9 to 13, 2011 (gray dots). The area enclosed by a red rectangle is magnified. (b, upper) Map view showing relocated hypocenters (red circles) and initial hypocenters determined by JMA (black circles). The blue and black stars are hypocenters of the foreshock and the mainshock, respectively, after and before the relocation. (b, lower) Vertical cross-sectional view of the upper panel along X−X′. Green circles denote hypocenters in a green rectangle in (b, upper). (c) CMT (Centroid Moment Tensor) solutions colored with time after the foreshock.