Effects of Topography and Basins on Seismic Wave Amplification: The Northern Chile Coastal Cliff and Intra-Mountainous Basins

Abstract

During earthquakes, structural damage is often related to soil conditions. Following the 01 April 2014 Mw 8.1 Iquique earthquake in Northern Chile, damage to infrastructure was reported in the cities of Iquique and Alto Hospicio and clear evidence of site amplification was recorded in various seismic stations located in the area. In this study, I investigate the causes of site amplification in the Iquique region by numerically analyzing the effects of topography and basins on synthetics waveforms in the frequency range 0.1 – 3.5 Hz using the spectral element method. The results show that topography produces changes in the amplitude of the seismic waves recorded by stations located in steep areas such as the ca. 1 km-high coastal cliff, a remarkable geomorphological feature that runs north–south, that is, parallel to the coast and the trench. The modeling also shows that secondary waves—probably related to reflected waves produced in the coastal cliff—propagate inland and offshore, augmenting the duration of the ground motion and the energy of the waveforms by up to a factor of three. Additionally, the results show that, as expected, sedimentary basins have a considerable effect on ground motion amplification at stations located within basins and in the surrounding areas. This can be attributed to the generation of multiple reflected waves in the basins, which increase both the amplitude and the duration of the ground motion, with an amplification factor of up to 3.9 for frequencies between 2 and 3.5 Hz. Comparisons show a good agreement in the frequency range between 0.1 and 0.5 Hz. However, for frequencies higher than 1 Hz, the fit between real and synthetic seismic waveforms progressively deteriorates, especially for stations located in or near to steep topography and basin areas. The poor data misfit at high frequencies is most likely due to the effects of small-scale 3D velocity heterogeneities, which is not yet resolved in seismic images of our study region. Finally, the main conclusion of this work is that the interaction between the local basins and topography produces important amplification of seismic ground motion and may be the cause of the site effects observed in the cities of Alto Hospicio and Iquique during the 01 April 2014 Mw 8.1 earthquake.