汶川地震近场加速度基本参数的方向性特征

摘要

以2008年5月12日的Ms8.0级汶川地震强震记录为基础,根据场地条件和断层距,选取198组三分量近场加速度记录,按照台站相对于破裂传播方向的位置将其划分为破裂前方和破裂后方两类场点.为了考虑破裂传播的方向性和地震辐射图对地震动不同分量的影响,将每组地震动的两个正交水平分量旋转成为垂直断层走向和平行断层走向的分量,然后分别对破裂前、后方向上的垂直断层、平行断层和竖向分量地震动的三个基本工程参数——峰值、反应谱和持时进行定量对比和分析.研究结果表明,汶川地震中地震动的峰值、反应谱和持时均显示出了明显的方向性特征:(1)在峰值方面,破裂前方场点的峰值明显大于破裂后方的峰值,而且破裂前、后方的峰值差别随着断层距的增大而逐渐减小；(2)在反应谱方面,在反应谱的各个自振周期段,破裂前方的平均反应谱明显高于破裂后方的平均反应谱,并且当周期大于2 s时,破裂前、后方反应谱谱比最高可以达4倍左右;(3)在持时方面,破裂后方场点的持时明显大于破裂前方的持时,差别也可达数倍.因此,虽然汶川地震是一次以逆冲为主兼具右旋走滑的地震,但是由于此次地震以单侧破裂为主,破裂传播的方向性使得近场地震动的峰值、反应谱和持时三个基本参数中均体现出了明显的方向性特征.%We study the directivity characteristics of the acceleration ground motions during the MS8. 0 Sichuan Wenchuan earthquake. Totally 198 sets of three-component ground motions recorded on similar soil site conditions are analyzed in this study. Firstly, the recordings are classified into two groups according to the relative location to the epicenter and the rupture direction, then the original orientated recordings are rotated to fault (strike) normal and fault (strike) parallel directions. Then, the three basic engineering parameters, including the amplitude, response spectra and duration, are analyzed. Results indicate that: (1) the peak ground acceleration is higher in the rupture propagation direction than that of the opposite direction. And the closer the station to the fault plane, the smaller the difference of the amplitude between the two opposite directions. (2) The average response spectra in the forward direction are higher than that of the backward direction. For natural periods smaller that 2 second, the average response spectra in forward direction is about two times that of the backward direction; for natural periods larger than 2 second, the maximum average forward-backward spectra ratio may reach 4 times. (3) The duration in forward direction is especially shorter than that of other areas. In conclusion, although Wenchuan earthquake is mainly a thrust fault event with right-lateral component, there still arises an obvious directivity in the three basic engineering parameters which are distinctly affected by the predominantly unilateral rupture of the fault propagation.