Study of ground surface tilts from strong motionrecords of the Wenchuan earthquake

Based on the difference of dynamic response to tilts between horizontal pendulums and vertical pendulum, spectral ratio method was applied to estimate the coseismic surface tilts deduced from strong motion records of the Wenchuan earthquake. The result shows that the ground surface tilts are generally less than 1 degree and mainly appears within30 kmto fault rupture surface traces, while rarely discovered in the area outside100 kmor root-mean-squared horizontal peak accelerations are less than200 cm/s2. The frequency band influenced by tilts is less than 0.1 Hz. In general, tilts in hanging wall is greater than that in footwall in near fault areas, and tilts in normal direction of fault is less than those in parallel direction. The spectral ratio of Qingping station located between central fault and front mount fault is relatively low and stable, which may imply ae special movement feature of the area. The tilts of Wolong station may be controlled by deformation transition from the hanging wall to laterally stationary area. Comparison of tilts between the stations at reverse and strike slip fault segments showed that the tilts may be influenced by local site condition.