芦山地震仁家村斜坡地震动监测

Monitoring Result Analysis of Renjia Village Slope Ground Shock Response of Lushan Earthquake in Sichuan,China

芦山地震次生地质灾害一个显著特点是地形放大效应。震后第二天,课题组在极震区仁家村斜坡局部孤突地带谷底基岩及斜坡中部坡折部位各放置一台地震监测仪,捕捉到一系列余震数据。数据显示,坡折（2#监测点）相对谷底监测点（1#监测点,二级阶地高程）的最大峰值加速度一般放大最大可达3.4,最小为1,说明坡折部位的地震动能量大于谷底部位。阿里亚斯强度计算显示,坡折部位阿里亚斯强度最大值为0.004 855 m/s,谷底部位阿里亚斯强度值最大为0.003 145 m/s,前者约为后者的1.5倍,阿里亚斯强度放大系数最大可达6.9。傅里叶频谱分析可知,1#监测点主频范围为4.81-22.81 Hz,2#监测点主频范围为3.31-20.94 Hz,说明局部孤突地形并不影响坡体接收到地震波的丰富程度。通过与桅杆梁监测成果对比,说明了软弱覆盖层地震波的低频部分有放大作用,而对高频部分存在滤波作用;三面临空山体表现更显著的放大效应。研究认为,地震条件下局部孤突地形对地震动加速度有明显的放大效应,因而在这些地区较易达到岩土体的强度极限,从而增加震裂、崩塌、滑坡等次生地质灾害发生的数量和规模,在进行山区工程选址和城镇规划时应充分考虑局部地形的放大作用和影响。

A illustrious phenomenon of the disaster induced by Lushan earthquake is the topography amplification effect. The research group,just in the next day after ＂4 · 20＂ Lushan earthquake, assigned two seismic instruments at the bottom of the valley（ 1# monitoring point） and the middle of the slope（2# monitoring point） respectively in the Renjia village meizoseismal area and collected a amount of data. The paper seeks to research the seismic response characteristics and topographic amplification effect through the analysis of the monitoring data. The analysis of the data indicates that the amplification coefficient of 2# monitoring point ＇ s PGA is from 1 to 3.4, illustrating that the seismic energy of the 2# monitoring point is much more than #1 monitoring point. The computation of the Arias intensity shows that the maximum of 2# monitoring point＇ s Arias intensity is 0.004 855 m/s and the counterpart of 1# monitoring point is 0. 003145 m/s,the former is as 1.5 times as the latter. The maximum of the Arias intensity amplification coefficient is 6.9. According to the fouriers spectrum analysis, it represents that the dominant frequency of 1 # monitoring point is 4.81 - 22.81 Hz and that of 2# monitoring point is 3.31 - 20.94 Hz, which indicates that the protrusion terrain has not influence on the number of earthquake waves. By comparison with Qingchuan monitoring achievement, it has been verified that the soft coveting layer amplifies the low frequency part of seismic wave and filters the high frequency part;three-side hanging mountain shows much more fierce amplification effect. Due to the monitoring data＇ s computation and analysis, it is concluded that, under earthquake condition, the protrusion terrain has obvious amplification effect on the ground acceleration. Therefore, the rock mass in this area is much more easier to reach the strength limitation, increasing the number and the scale of the collapse, the landslide and the crack. It is suggested that we should take full consideration of the topography amplification effect, when we choose the field and make the urban plan in the mountain areas.