Identifying anomalous change of short line leveling on Gengda site preand afterMS8.0 Wenchuan earthquake

A total of 4.59 mm tensional change appeared in Gengda site faultcross shortline 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.