Numerical simulation of site effects in the sedimentary basin of Paine (Chile) comparing 3D, 2D and 1D approaches – First Results

Abstract:

During the 2010 Mw=8.8 Maule Earthquake, five bridges located in the Paine sedimentary basin (south of Santiago, Chile) suffered great damage or collapsed in part due to local site amplification. We studied the seismic response of the basin using geophysical methods and numerical simulations. Ddigital seismographs were used to record microtremors and compute H/V spectral ratios (HVSR) and dispersion curves (DC). Shear wave velocity (Vs) profiles were obtained using the joint inversion of DC and HVSR. The shear wave velocities vary from 200 to 1400 m/s and the predominant vibration periods vary from 0.6 to 2.8 s in the study area. These results are consistent with the characteristics of the basin, whose maximum depth is 450 m and is filled with sands, gravels and fines soils transported by the Maipo and the Paine rivers. A kriging technique was used to interpolate the Vs profiles and assist the generation of a 16·20·20 km³ (width·long·depth) 3D Vs model. The numerical model of the basin was implemented in the finite-difference code FDSim3D that allows solving the seismic wave propagation in 3D. The model was discretized with uniform soil layers varying every 100 m/s and the surface topography was neglected. In addition, we choose two representative cross-sections across the basin, in the north-south and east-west directions, to compute their 2D and 1D responses using the finite difference code 2DFD_DVS. The results of the numerical simulations were used to compare amplification factors (AF) for different earthquake ground motions and provide aggravation factors (AGF) for 2D/1D and 3D/2D effects. The first results show that the main differences in amplification and duration of the strong ground shaking are between 2D and 1D models, rather than those between 3D and 2D. Moreover, the area where the bridges collapsed shows reverberation of surface waves that may contribute to the observed damage. These results highlight the importance to account for local basin effects in the seismic design of critical infrastructure located in sedimentary basins.