Numerical Investigation of Topographical Site Effects: Parametric Study on Simplified Geometries and Impact of the Inner Geological Structure

Abstract eng:
Amplification of the seismic ground motion is often observed in presence of strong topography or in sedimentary basins in which the seismic waves are trapped. The study of site effects can be achieved by the analysis of seismological recordings. Measurement campaigns at specific sites allowed highlighting high amplification for some frequencies but it is often difficult to deduce from the field data what are the main parameters responsible for local amplification. Numerical simulation is then an interesting tool for understanding such phenomena. In particular, it allows series of simulations in the frame of parametric studies. In few decades, many numerical methods have been developed, improving their accuracy by including new numerical schemes and more complex physical models. They have been widely applied to lithological site effects, in sedimentary basins, but much less to study the topography influence. Many past studies which aimed at understanding these effects have been limited to homogeneous models and concluded that the numerical simulations underestimate the amplification levels of field data. Topography alone cannot produce the measured amplification levels; the medium in-depth must also be accurately described and heterogeneous models must be considered. This helps explaining the strong amplifications measured on very smooth hills. The objective of this study is to use numerical simulations to improve the understanding of topographical site effects and, in particular, the influence of some parameters on the ground motion amplification at the surface. These simulations are done using a discontinuous Galerkin finite element method which is high-order accurate, flexible and allows an accurate description of the topography and the in-depth medium properties. This method is applied to the solution of the velocitystress system in time domain to study the surface response to series of plane waves of various frequencies. First, a very detailed parametric study is proposed for a canonical simplified geometry in order to exhibit the main parameters responsible for amplification. Then we consider a more realistic topography of the Rognes area, place where occurred the Provence earthquake (1909), the most significant French earthquake of the 20th century and compare the results of the simulations to the field measurements.

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