(a) Distribution of observation sites (triangles) and subfaults (circles) used in this study. Contour lines overlapping the circles indicate the depth of the upper surface of the Pacific plate (Nakajima and Hasegawa, 2009). The epicenter of the largest aftershock is shown by a black star together with the F-net CMT solution. The dot-dashed line is the same as that in Fig. 1. (b) Velocity waveforms in a frequency range of 0.1–1.0 Hz observed at the reference site E.MBSM. Horizontal waveforms in the shaded time window are used in the analysis.