Identifying anomalous change of short line leveling on Gengda site preand afterMS8.0 Wenchuan earthquake
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摘要: 耿达短水准AB测线观测资料2006——2007年出现了累计4.59mm的张性变化，但同时段该场地A点附近进行建筑施工，这引起了该变化是否属于汶川地震前兆异常的争论．通过对耿达短水准场地现场地质和观测环境考察，在综合分析观测资料和场地内建筑施工有关信息的基础上，使用岩土力学分析程序计算施工对A点点位的影响的数值解．结果显示，建筑施工导致了约3.6mm的竖向沉降，这与同时段短水准观测的犃点点位的变化幅度相似，发展趋势一致．认为耿达短水准AB测线测值从2006——2007 年出现的变化是由于A点附近建筑施工影响所致．本文还对耿达短水准观测的2008年5月12日汶川MS8.0地震同震变化进行了分析，认为A点位于很厚的洪积扇上，当汶川地震发生时，在强烈地震动的作用下，洪积层内部空隙结构产生崩溃性破坏，从而形成地表的下沉，其沉降幅度与类似烈度条件下黄土场地的沉降幅度相当．Abstract: A total of 4.59 mm tensional change appeared in Gengda site faultcross shortline leveling route ABobservational data from 2006 to 2007, but a construction project surrounding leveling pointAwas carried out in the same period. This led an argument on whether the change is a precursory anomaly, or not, of the May 12,2008WenchuanMS8.0 earthquake. This paper introduced the field geological and environmental investigations at Gengda leveling site, and then, by using a rock and soil mechanics computation program and analyzing comprehensive observation data and construction information, we numerically calculated pointArsquo;s vertical displacement possibly induced by the construction. The calculated result shows that a subsidence of about 3.6 mm is caused by the construction progress, and its variation is similar to the leveling data change in range and trend. Therefore, we draw the conclusion that the change of Gengda leveling routeABobservational data from 2006 to 2007 is caused by the construction works near pointA .We also analyzed the coseismic change of Wenchuan earthquake observed by Gengda leveling. Because pointA is located on a very thick proluvial fan, and when the Wenchuan earthquake occurred, the internal gap structure of torrential deposit damaged by strong ground motion, as a result, the ground surface subsided, and its subsidence range is similar to the loess field under the same seismic intensity condition.
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