Evaluation of Proxies for Seismic Site Conditions in Large Urban Areas: The Example of Santiago de Chile

Presenter: Siti Fatimah

Date: 2016/03/03

Abstract

Identification of the main controlling parameters of site effects is an important part of the seismic hazard assessment. Microtremor measurements are used to derive 3D velocity model in the Santiago de Chile, the country’s capital city. The study area is a basin mainly covered by alluvial sediments with thickness varies up to more than 550 m. The horizontal-to-vertical (H/V) spectral ratios are calculated for all measurement sites and then inverted using modified genetic algorithm method. The total thickness of sediments derived from the gravimetric data is used as a constraint in the inversion of H/V ratios. Once the velocity profiles are obtained from the inversion process, 2-D S-wave velocity models are then generated using ordinary kriging interpolation technique which is tested using cross validation. A comparison between geological cross-sections and 2-D velocity models shows that both of them have a good agreement. Other cross-sections of the velocity model show that the S-wave velocity in the eastern part of the basin is generally higher than in the west. We derive  (S-wave velocity in the uppermost 30 m) values from our inversion results and evaluate its correlation with the topographic slope.  However, no correlation can be seen from those two parameters. The basin has low topographic gradient, but the surface geology is heterogeneous which indicate that the deposition might not play a major role in the material differentiation. The  is also compared with macroseismic intensity of two events, 1985 Valparaiso and 2010 Maule earthquake. The comparison shows there seems to be a tendency that the intensity distribution for those events are partially influenced by local variations of . For further work, model resolution can be improved by adding more measurement sites.

Reference

Pilz, M., Parolai, S., Picozzi, M., Wang, R., Leyton, F., Campos, J., Zschau, J., 2010. “Shear wave velocity model of the Santiago de Chile basin derived from ambient noise measurements: a comparison of proxies for seismic site conditions and amplification.” Geophys. J. Int. 182, 355–367

Pilz, M., Parolai, S., Picozzi, M., Zschau, J., 2011. “Evaluation of proxies for seismic site conditions in large urban areas: the example of Santiago de Chile.” Physics and Chemistry of the Earth. 36, 1259-1266