Site Response Taxonomy for Assessing Complexity using H/V Ratios for Mexico City

Date: 4/26/2019

Time: 06:00 PM

Room: Grand Ballroom

Sedimentary basins with high impedance contrasts pose a significant risk to infrastructure and local populations when strong shaking from local site response during an earthquake occurs. In standard site response analyses, we generate a theoretical transfer function between the base of the soil layer and the surface by assuming vertically propagating shear waves through horizontal, laterally homogenous soil systems, a set of assumptions collectively referred to as “SH1D”. In real soil systems, however, these assumptions tend to collapse due to wave scattering through heterogenous materials, significant attenuation, non-vertical incidence, and other complexities in the subsurface. In many cases, therefore, it is essential to perform more robust site response analyses. In work by Thompson et al. (2012), the authors developed a taxonomy using surface-downhole spectral ratios from weak ground motions for classifying a site’s resonant behavior referenced to the SH1D condition. In this work, we apply the taxonomy to single station recordings by using Nakamura’s H/V ratio (Nakamura, 1989) as a first estimate of the site empirical transfer function (Lermo and Chávez-Garcia, 1994) using ground motion data from 224 earthquakes at 68 stations from the Mexico RACM network. The H/V ratio method is non-intrusive, requires one three component seismometer station and can use microtremor data or earthquake records, thus allowing for rapid, cheap field collection (Yilar et al. 2017). Mexico City is an ideal test case for this method, as the region consists of high impedance contrast between the lakebed sediments and underlying materials. Nakamura’s technique is known to perform well at identifying the site response resonant behavior in this type of environment (Bonnefoy-Claudet et al. 2006; Parolai, 2002). We show that the H/V ratio shape, when averaged over many earthquakes is similar to the traditional spectral ratio which means that the H/V ratio is an appropriate estimate for the empirical transfer function within the Thompson et al. taxonomy in Mexico City.

Presenting Author: Marshall Pontrelli

Authors