DYNAMIC NUMERICAL MODELLING OF THE SEISMIC RESPONSE OF THE SANTIAGO CITY, CHILE

The Santiago City is located in Central Chile on a sedimentary basin, whose s eismicrespons e is controlled by the properties and geomorphology of the s hallower quaternary depos its, as well as the impedance contrast with the underlying geological formations, among other factors . In this s tudy, we built three representative two-dimens ional (2D) cross-sections across the Santiago Basin in order to examine their s eismic response. The wave velocities of the shallower quaternary s ediments were determined from local geophys ical measurements and the depth of the sedimentary cover was cons trained with available gravimetric models and the H/V spectral ratio (HVSR) method. The cross-s ections span from north to south (NS cross-section), east to west (EW cross-section), and north-eas t to south-wes t (CD cross-s ection) from soft sediments of average shear wave velocity of the upper 30 m (V_s30) of 400 m/s in the north to s tiff gravels of average Vs30 of 880 m/s in the south. The average thickness of the sediments is nearly 200 m, but it can reach 600 m at s everal depocenters in the city. The representative cross-sections were used to perform dynamic numericals imulations using the finite-difference code 2DFD_DVS. The input ground motions are impulse signals polarized in the longitudinal, trans verse, and vertical directions with res pect to the cross-s ections imposed inside the model domain at a depth of 5 km. In addition, the one-dimens ional (1D) res ponse of the cross sections were calculated to compare with the 2D seis mic response. The numerical results show that the more pronounced differences between the 1D and 2D modelling are found in the softer sediments, which also tend to amplify the ground motions more than s tiffer s ediments, in terms of spectral amplification factors and ground motion duration.