Abstract：Zigong Dense Seismograph Array is in Sichuan province of China, which successfully recorded strong motions during 2008 Wenchuan earthquake (Ms 8.0). Because the separation distances between random two stations are less than 400 meters, these recordings offer some interesting insights into spatial variations of seismic ground motions that suits for engineering scale. In this study, the two horizontal directions and the vertical direction of the ground motion coherency function are calculated by using digital signal processing techniques, and the variation mechanisms of the coherency functions at various directions along with the separation distance and frequency changes were analyzed. The results show that (1) when the circular frequency is less than , the coherency function is more than 0.8. However, the decreasing trend of the coherency function becomes significant with the separation distance elongation when frequency increases gradually; (2) the local site conditions significantly influence the variation of coherency function in the high-frequency range, with changing dramatically of the peaks and valleys , but the local site conditions have no significant effects on its decreasing trend as frequency increases. However, the topography of the station will influence the variation of the coherency function in high-frequency range, which increases gradually along with the increasing frequency. Nonetheless, this effect decreases along with the separation distance elongation; (3) the local site conditions and topography influences can reduce the value of coherency function in the middle-frequency range; (4) it is indicated that, under the conditions of large-earthquake and far-field, the local site conditions and topography have fewer effects on the vertical ground motions, and the spatial variations of coherency function have certain differences in different directions.
2013, Vol. 32 
