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

Zhang, Zezhong, Fleurisson, Jean-Alain, Pellet, Frederic L.
Engineering geology 2018
basins, case studies, disasters, earthquakes, engineering, geophysics, landslides, mathematical models, tectonics, topography, China
The city of Zigong, which is characterized by hilly landforms and weathered deposits, lies in the southern part of the Sichuan Basin, where active faults are very well developed. The city faces severe seismic hazards because of the amplifying effects of its topography and geology, as well as its complex tectonic conditions. To estimate the influence of surface topography on ground motion, an array of 8 accelerographs has been in operation along the ridge since 2007. These accelerographs recorded the ground motion associated with the 2008 Wenchuan earthquake, which caused considerable damage and loss of life and triggered a huge number of geological disasters, such as landslides and quake lakes.In this study, the topographic amplification is estimated using the standard spectral ratio (SSR) technique. Numerical analysis is also performed using finite difference methods for comparison with instrumental observations. Ground motion amplification is also evaluated using the spectral ratio and root-mean-square acceleration (arms) methods. The results show that topography and the presence of weathered materials significantly aggravate the ground motion in both field observations and the numerical model. A slight difference between the two horizontal components (NS and EW) is also observed that results from directional resonance. The computed amplification ratios give amplitudes that are comparable to those indicated by field observations. However, the computed peak amplifications are lower than those indicated by field observations. An underestimation of spectral amplification at high frequencies is also observed. This could be linked to mismatches between the numerical models and the actual situation regarding damping values and model size.