Fig. 1.

(a) Coseismic slip distribution estimated from observed displacements at all GPS stations. Arrows denote slip vectors on the hanging wall of the plate interface. Contours of the interplate slip are also shown with a color scale. The contour interval is 5 m. Purple arrows and the areas surrounded by the purple solid lines indicate that here the estimated values are greater than the estimation errors. White squares and red circles denote GPS stations conducted by the GSI and Tohoku University, respectively. The black dashed line denotes down-dip limit of interplate earthquakes determined by Igarashi et al. (2001). The broken red lines show the depth of the subducting plate interface. The epicenter of the main shock and mechanism solutions of the main shock and an aftershock on 7 April, 2011, (M 7.1) estimated based on W-phase inversion analysis are indicated by a yellow star and the beach-ball symbols (USGS, 2011a, b). The gray contours denote the slip areas for recent major earthquakes at Tokachi-oki in 2003 (Yamanaka and Kikuchi, 2003) and 1968, Miyagi-oki in 1978, 1981 and 1936 (Yamanaka and Kikuchi, 2004), Fukushima-oki in 2003 (Yamanaka, 2003) and 1938 (Murotani, 2003). The thin brown lines denote the prefectural borders. (b) Estimation error distribution. The contour interval is 5 m.