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Epicentral area of Matsushiro earthquakes. The epicentral area of Matsushiro earthquake swarm that commenced in August, 1965, was circular in its initial stage but gradually spread out selectively in northeast and southwest directions, along with rise and fall of seismicity. The maximum area attained was an ellipse with a major and minor axes 34 km and 18 km, respectively, [Hagiwara et al., 1968]. In the center of the epicentral area, surface fault appeared that trends parallel to the minor axis of the area [Nakamura et al., 1967]. The gradual spreading seemed to fill up a certain geologic province known as ‘mid-belt of uplift’ among geologists.