Theory Study On The Generation Of Spatially Variable Seismic Underground Motions In Multi-layer Circular-arc Canyons And Program Development

Among all kinds of different terrain in China,mountains,plateau and hills take up almost 2/3 of total land areas.The terrain especially in the southwest region of China is more complex and diverse.Many seismic records show that the topographic factors have a great influence on the seismic response of structures,especially the long-span structures,such as long-span bridges and underground pipelines,which need to consider the spatial variation of earthquake ground motion.Many researchers paid great attention to the influence of the Earth's surface topography on the dynamic response under seismic excitation,however,most of researches assumed that the site is homogeneous.The geological structures usually have the characteristics of layered distribution in practice,and seismic waves will reflect and refract at the layer-interface compared to homogeneous site,which lead to different dynamic response under seismic excitation in layered-site compared to homogeneous site,especially in topography such as canyon with scattering.Therefore,it is of great theoretical and practical significance to study the dynamic response of layered-site under earthquake excitation.This paper focuses on underground seismic motions in circular-arc canyons with multi-layer inhomogeneous medium.The main works are as follows:(1)Firstly,the frequency-domain solution for the scattering of SH waves by multi-layered circular-arc arc canyons is obtained,which breaks through traditional theoretical limitations of homogeneous medium and canyons do not pass through multi-layer soil.And then,a two-step transferring method combining the transferring from flat-ground terrain to the canyon surface(step one)with the transferring from the canyon surface to underground location(step two)is adopted to derive the underground target PSD and response spectrum.At last,the simulation of spatially variable seismic underground motions in muiti-layered circular-arc canyons is given,furthermore,a numerical example is given to verify the rationality and reliability of the results.(2)The frequency-domain solution for the scattering of SH waves by a circular-arc arc considering multi-layer effect is obtained,and it breaks through the theory of canyons pass through double layer soil above,so it is of great significance for theoretical and practical application.And then,a two-step transferring method combining the transferring from flat-ground terrain to the canyon surface(step one)with the transferring from the canyon surface to underground location(step two)is adopted to derive the underground target PSD and response spectrum.Furthermore,the coherence function taking account of the flat-concave correlation of circular-arc field and its derivation is described in detail.Finally,a program based on the theory for simulating spatially variable underground seismic motions in circular-arc canyons taking the multi-layer effect into account is developed,the comparison of fitting contrast between simulated and target result including PSD,response spectrum and coherence coefficient is given by an numerical example,which further verified the rationality and reliability of the result.