Advances in the 2D seismic modelling of the Santiago Basin, Chile

Abstract:

The Santiago City is located on a sedimentary basin in the central depression between the Andes and the Coastal Cordilleras in a highly active seismotectonic environment. In this study, we built three representative two-dimensional (2D) cross-sections across the Santiago Basin in order to examine their seismic response. The wave velocities of the shallower quaternary sediments were determined from the joint inversion of H/V spectral ratios and dispersion curves obtained from ambient seismic noise records throughout the city. The cross-sections span from north to south (NS cross-section), east to west (EW cross-section), and north-east to south-west (CD cross-section) from soft sediments of average shear wave velocity of the upper 30 m Vs₃₀= 250 m/s in the north area to stiff gravels with Vs₃₀= 800 m/s in the south. The depth of the sedimentary cover was constrained with available gravimetric models and the average thickness of the sediments resulted in nearly 200 m, with few deeper depocenters. The wave velocities of the bedrock were obtained from an available ambient seismic noise tomography from 0.6 to 7 km depth. The representative cross-sections were used to perform dynamic numerical simulations using the finite-difference code 2DFD_DVS. The input ground motions are impulse signals polarized in the longitudinal, transverse, and vertical directions with respect to the cross-sections imposed inside the model domain at 5 km depth. In addition, the one-dimensional (1D) response of the cross-sections were calculated to compare with the 2D seismic response. The numerical results show that the differences between the 1D and 2D modelling are more pronounced in the softer sediments, where surface waves are generated at the contact between the sediments and the rock outcrops. The spectral amplification factors and ground motion duration are also larger in these areas compared to the stiffer soils in the south of the basin.