A case study of site effects on seismic ground motions at Xishan Park ridge in Zigong, Sichuan, China

The city of Zigong faces severe seismic hazards because of the amplifying effects of its topography and geology. To estimate the local site effects, an array of 8 accelerographs has been in operation along the Xishan Park ridge since 2007. In this study, site amplifications associated with the ground motion produced by the 2008 Wenchuan earthquake is evaluated using the Standard Spectral Ratio (SSR) technique and root-mean-square acceleration (a(rms)) method. 2D numerical analysis using finite difference method is then performed and results are compared with monitoring data. The results show that topography and local presence of soft materials significantly increase the ground motion in both field observations and the numerical model. Along the ridge slope, the maximum ground motion amplification was obtained at the hilltop for a frequency range 1-10 Hz for both NS and EW components. Significant amplifications occurred at higher frequencies (> 10 Hz) and are considered to result from local specific conditions such as rock fracturing and ridge steps; thus they do not necessarily occur at the hilltop. A slight difference between the two horizontal components (NS and EW) was observed and this is possibly due to the azimuthal dependence of site effect. The computed amplification ratios give amplitudes that are comparable to those indicated by field observations. However, the calculated spectral amplifications are underestimated at frequencies larger than 3 Hz. This can be due to the fact that a signal damping ratio used for the whole model does not adequately represent the energy dissipation at high frequency.