基于汶川地震加速度记录的地震动相干函数变化特性

Spatial variation of ground motions based on wenchuan acceleration records

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。

The Zigong Dense Seismograph Array,located in Sichuan province of China,successfully recorded strong motions during 2008 Wenchuan earthquake（Ms 8.0）.Because the distance between any two stations is less than 400 meters,these recordings offer some interesting insights in spatial variations of seismic ground motions that suit to discuss in engineering scale.The coherence functions of the ground motions at two horizontal directions and one vertical direction were calculated by using digital signal processing techniques,and the variations of coherence functions at various directions along with the changes of distance and frequency were analyzed.The results show that： when the circular frequency is less than 3π,the values of coherence function are all more than 0.8.However,the decreasing trend of the coherence function becomes significant with the increase of distance when the frequency increases gradually.The local site conditions significantly influence the variation of coherence function in high-frequency range,with dramatic changes at peaks and valleys,but the local site conditions have no significant effect on its decreasing trend as the frequency increases.However,the topography of the station will influence the variation of coherence function in high-frequency range of which the value increases gradually along with the increase of frequency.Nonetheless,this effect decreases along with the increase of distance.The local site conditions and topography can reduce the value of coherence function in the middle-frequency range.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 coherence function are of certain differences at different directions.