A Virtual Reference Site for the Valley of Mexico

Abstract

We propose the use of the average spectra of northern strong-motion stations located at the hill zone (e.g., MD, TX, ES, 64) as the reference for Mexico City's ground motion. This virtual-site proposal is based on the analysis of recent data from the Mexico City Acelerometric Network. The northern stations show amplitudes, both in time and frequency, that are consistently smaller than those of hillzone stations located south and west of the city (e.g., CU). It is well known that CU, the historical reference site in Mexico City, and other sites to the southwest, present amplifications, whereas the northern ones appear to be free of such effects. The spectral ratio of the averages of the stations located in the south and west with respect to the northern stations shows a relatively constant amplification of up to 3 times in the 0.7-10.0-Hz-frequency band. This amplification is a very unusual feature that should be explained. The geologic conditions at the hill zone show that older, Miocene-age deposits are located north of the city. Considering that northern sites represent the basement, we assume that the configuration along the hill zone in the N-S direction can be approximated by a simple, dipping, homogeneous layer. We computed the antiplane seismic response for this model and averaged and compared it with the spectral ratio obtained from strong ground motion data. The agreement is good and suggests how a smooth, large-scale feature could amplify seismic ground motion in a broad frequency band.

High-Resolution Early Earthquake Damage Assessment System for Mexico City Based on a Single-Station

“…Since the early nineties, this station has also a broadband seismometer. It is located at a hill zone site over basaltic lava flows, and it has become the reference site to study the dynamic amplification at the lakebed zone of Mexico City (Singh et al, 1988a,b;Ordaz et al, 1989;Montalvo-Arrieta et al, 2002).…”

A Robust Method for Assessing 3-D Topographic Site Effects: A Case Study at the LSBB Underground Laboratory, France

“…They thus suggest that the more suitable statistical procedure to estimate the amplification factor in these sites is not the average but the median of the ground motion (i.e., the MRM, yellow dashed lines in Figure 2b). Another statistical approach was proposed by Montalvo-Arrieta et al (2002). They used the average spectra to define a virtual reference site for Mexico City.…”

Seismic response of the Mexico City Basin: A review of twenty years of research

“…For spectral ratios relative to an averaged Hill Zone effect, the variations are less than in the case when only one station is used (Reinoso and Ordaz 1999). It might be best to use records from the North Hill Zone (which has the oldest lithology), as it also has the lowest amplitudes within the basin (Montalvo-Arrieta et al 2002). One problem in the North Hill Zone is the poor data, both as to quantity and quality-which is less of a problem at CU.…”