(a) Normalized strength distribution of the radiated energy obtained by applying the back projection approach to an aftershock which occurred at 17:19 (JST) on March 11, 2011. The white star is the epicenter. A thick dashed line, a dot-dashed line, triangles and black dots are the same as those in Fig. 1(b). (b) Same as (a) except for the 2011 Ibaraki-oki event. Contours by dotted lines are the same as those in Fig. 1(b). The black star indicates the epicenter.