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

我们在苏瓦盆地进行了微震观测。用于地面垂直运动阵列观测的站点有Okaya、Toyota、Ogawa、Suwa Minami和Fumide。用SPAC方法求取了面波的相速度。利用遗传算法进行了深度和S波速的检测。并对苏瓦盆地40个点位的地面垂直和水平运动进行了观测。计算了观测的H/V谱，测定了苏洼盆地40个点土层的S波速。最后，建立了苏洼盆地三维S波速模型。基岩埋深为2.7~4.3公里，S波速约为4公里/S，S波速为400m/S的地层为20~130m，基岩中央较深，边缘较浅。盆地长15公里，宽5公里。这种细长的形状和盆地中部基岩深度较深的事实，放大了来自一定方向的地震运动。

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.