黄土地区场地条件对地震动放大效应影响研究

汶川Ms8.0特大地震在甘肃黄土灾区局部场地的震害和地震动具有明显的放大效应。通过在该区布设的地形调查流动强震动观测台阵,获取了同一山体不同高程的余震加速度记录,结果呈现出山顶记录到的峰值加速度大于山腰,而山腰记录也略大于山脚的规律。为了进一步分析黄土地区地震峰值加速度的放大机理,运用地脉动观测手段调查了其局部场地条件,得到了山体不同高程的卓越频率。并以典型黄土塬为背景,运用二维等价线性时程响应动分析法对不同覆盖层厚度和不同坡度的黄土塬进行了地震荷载作用下的动力响应分析,分析了不同条件下的山顶和山腰的地震峰值加速度放大特征。结果表明,地震峰值加速度放大效应和黄土覆盖层厚度及黄土边坡坡度有直接的关系。数值计算结果与实际震害观测吻合较好,证实了黄土覆盖层厚度和坡度是峰值加速度放大的重要因素。

Study on the Effects of Site Conditions on Earthquake Ground Motion in Loess Sites

Loess is widely distributed in China and in the loess plateau of Gausu province, The seismic amplification effects were discovered during the Wenchuan Ms8.0 earthquake. Based on the acceleration records of aftershocks,it is discovered that the Peak ground acceleration (PGA) is larger at the mountaintop and mountainside than the value at the foot of mountain. In order to analyze the mechanism of the seismic amplification effects in loess area, a microtremor survey was carried out to detect the ground condition and the predominant frequency of the different sites at the same mountain was obtained as well. Moreover, the typical loess plateau mesa is studied applying the dynamic finite element analysis method of two dimensions linearly equivalent for dynamic response numerical calculation, with different loess thickness and different slope grades. The magnifications of PGA at the mountaintop and mountainside are analyzed under different conditions. It is found that the magnification is related to the loess thickness and slope grades, which is in accordance with the earthquake damage observation. The result shows that the loess thickness and slope are important factors influencing earthquake damages.