汶川地震远震区黄土场地地震反应特征分析

在实际场地钻孔资料的基础上,选取和构造45个有代表性的典型场地剖面,利用一维等效线性化波动方法,计算各个场地在三种不同强度地震动输入下的地表加速度反应。结果表明:(1)在远场地震动作用下,黄土场地PGA较大,反应谱平台范围较宽,集中在0.2~0.6s;(2)随着黄土覆盖厚度的增加,PGA减小,但对于自振周期大于一定范围的结构物震害加重;(3)对于无地形影响的厚层黄土地区,当黄土厚度在20~100 m时,随厚度的增加,反应谱下降段谱值增大1.1~1.4倍,特征周期放大1.1~1.25倍。

Earthquake Response Analysis for Loess Sites in Far-Field of Wenchuan Earthquake

In recent years, several studies have been conducted on the effect of far-fault ground motions on deep soft soil sites; however, relevant research works for thick loess sites are still seldom. The Loess Plateau is seated on the upper and middle stream of the Yellow River in North China, covering an area of 440 000 km², and the thickness of loess is up to above 500 meters. More than 1.4 million people have been killed by the earthquakes in this region, among which is the Wenchuan M_S8.0 earthquake of 2008, which led to collapse of infrastructures, tremendous casualties, and economic losses. However, according to field investigations, observations, and analyses, the collapse and damage of buildings were not only caused by poor seismic performance or landslides, but also by amplified effects of site conditions, topography, and thickness of loess deposit on ground motion. In this paper, to explore the relationship between amplification factors and site conditions in terms of thickness of loess deposit, we chose a typical loess site for numerical analysis. Based on the drilling data of actual engineering sites, this paper selects 45 typical site profiles. Using the one-dimensional equivalent linear fluctuation method, the ground surface acceleration responses of each site were calculated under the input of ground motions with three different intensities. The results indicate the following: (1) Under the far-field ground motion, the PGA of loess site is large, and the platform of response spectrum is wide, mostly in 0.2~0.6 s; (2) the PGA decreases with increase in the loess thickness, but the earthquake damage on structures with natural periods above a certain range is aggravated; (3) for thick loess areas without topography, with increase of the loess thickness (20~100 m), the value of the descending segment of response spectrum amplifies 1.1~1.4 times, and the characteristic period amplifies 1.1~1.25 times. The numerical result is basically consistent with the field investigation; therefore, it is of great scientific and practical importance in earthquake engineering for seismic fortification in loess regions.