基于汶川地震加速度记录的地震动相干函数变化特性

架设于我国四川省的自贡密集地震台阵成功记录到了2008年汶川地震(Ms 8.0)加速度记录。由于各台站间空间相对距离小于400 m,所以有益于分析工程尺度范围内地震动空间变化的特性。应用数字信号处理技术,分别对两个水平方向和垂直方向的地震动相干函数进行了计算,讨论各方向上地震动相干函数随空间距离和频率的变化规律。结果表明:(1)在频率小于3π时,相干函数的取值基本上都大于0.8;随着频率的增加,相干函数随空间距离的增加而递减的趋势明显;(2)局部场地条件影响相干函数在高频部分的变化,出现剧烈变化的峰和谷,但对其随频率增大而减小的变化趋势影响不明显;然而台站所在地形将影响其在高频部分的变化趋势,即在高频部分随频率的增加有逐渐增大的趋势,这种影响随着空间相对距离的增大而减小;(3)局部场地条件和地形的影响,在中频段可降低相干函数值;(4)大震远场条件下,局部场地条件和地形对竖向地震动影响较小,并且不同方向的地震动相干函数的变化有一定的差异。

Spatial Variation of Ground Motions Based on Wenchuan Acceleration Records

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.