Mechanism of a character of Suwa basin to amplify earthquake motion from certain direction

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We made microtremor observation at the Suwa Basin. Site for array observations of vertical motion of the ground are Okaya, Toyota, Ogawa, Suwa minami and Fumide. A spac method was applied to obtain phase velocity of surface wave. We detected depth and s-wave velocity using a genetic algorithm. We also made observation of vertical and horizontal motion of the ground at forty points on Suwa basin. We calculated H/V spectra from the observation and detected s-wave velocity of soil layers at the forty points of Suwa basin. Finally, we made 3-D s-wave velocity model of Suwa basin. Depth of bed rock is from 2.7 km to 4.3 km and its s-wave velocity is about 4 km/s and that for layer which s-wave velocity is 400 m/s are form 20 m to 130 m. The depth of bed rock is deep at central part of the base and it is sallow at the edge. The length of the basin is 15 km and its width is 5 km. This Long and slender shape and the fact that depth of the bed rock is deep at the central part of the basin made amplify earthquake motion from certain direction.