The distribution of normalized total energy release on the assumed fault plane. The dotted rectangle denotes the assumed fault plane. Gray circles show epicenters of aftershocks (M > 4) in the first three weeks after the occurrence of the main shock whose epicenter is plotted by a white star. The black star indicates the M 7.4 preshock that occurred two days before the main shock. Epicenters of large aftershocks (M > 7) are denoted by blank stars. Distributions of the released energy estimated by the back-projection method for the preshock and the largest aftershock are indicated by thick solid contours. Dotted light blue, green and magenta contours mark estimated asperities for the 1978 Miyagi-oki earthquake, 1968 Tokachi-oki earthquake and 2008 Ibaraki-oki earthquake, respectively. Squares represent epicenters of the 1938 Shioya-oki cluster event for which thrust type and normal type shocks are plotted by solid squares and dotted squares, respectively. Iw., My., Fk., Ib., and Tk. indicate Iwate, Miyagi, Fukushima, Ibaraki Prefectures and Tokyo, respectively.