Numerical simulations of ground motion amplifications due to elevated topography and surface geology

Abstract : Seismic recordings of numerous earthquakes worldwide have shown that elevated topographic features cause highly amplified ground motions, especially at the top of slopes. These topographic site effects give rise to many engineering problems dealing with structure design in seismic mountainous regions. Topographic site effects are indeed considered in most of the Seismic Design regulations but using very empirical approaches. In order to improve the scientific knowledge in this field, 2D numerical simulations were performed using the FLAC software with the objective at analysing the sensitivity of ground motions along slope to geometrical, geological and seismic parameters. Modelled surface amplifications were compared to the amplification factor t used in the French seismic design code. The results show that the calculated amplification factor is much lower than the value of factor t recommended in the regulations. In addition, the part of the slope subjected to high amplifications is generally larger than recommended in the regulations. Moreover, the simulations mainly highlight the effects of subsurface geology which are added to the topographic effects and can therefore contribute to higher values of amplification which are not considered in the regulations.