Depth sections along lines A–B, C–D, and E–F shown in Fig. 2. The focal mechanisms with magnitudes ≥3.0 are also shown. As in Fig. 2, the colors indicate the depths of the hypocenters. The broken lines indicate the location of the conversion plane estimated by Tsumura et al (1993). The red broken rectangles indicate Regions A and B, which were selected as the reference areas.