The Mexico Earthquake of September 19, 1985—A Study of Amplification of Seismic Waves in the Valley of Mexico with Respect to a Hill Zone Site

Abstract

Since the installation of an extensive digital strong motion array by Fundación Javier Barros Sierra in 1987 three moderate earthquakes have been recorded by the array and by the accelerographs operated by Instituto de Ingeniería, UNAM. Using this new data and results from the analysis of previous accelerograms we present spectral ratios at 40 sites in the valley of Mexico with respect to a hill zone site in Ciudad Universitaria (CU). Clear evidence for nonlinear behaviour of the clay is found at Central de Abastos Oficina (CDAO) site during the great Michoacán earthquake (Ms=8. 1). At four other lake bed sites this behaviour is not seen either because none occurred or because of poorer quality of data. The spectral ratio at a given site appears to be roughly independent of magnitude (except, perhaps, during great earthquakes when lake bed sites may behave nonlinearily), azimuth, and depth of earthquakes with epicenters ≥ 200 km from the city. On the lake bed sites of the valley the relative amplification (RA) varies between 8 and 56 and the natural period lies between 1.4 to 4.8 sec. Relative amplification maps at periods centered at 3, 2.5, 2, 1.5, and 1 seconds are presented. The area where severe damage and collapse of buildings in the city was concentrated during the Michoacán earthquake correlates well with the area with RA≥14 in the period range of 1.75 to 2.75 sec.

Preliminary Analysis for Evaluation of Local Site Effects from Strong Motion Spectra by an Inversion Method.

“…In the meantime, there have been some destructive earthquakes such as the 1983 Nihonkai-chubu event (M=7.7), the 1985 Michoacan event 8.1), the 1989 Loma Prieta event (M=7.1) and others. After these events, it is reconfirmed that the ground conditions of the sites are closely connected with the damage of the structures (e.g., Singh et al, 1988). Many numerical techniques for estimating the wave propagation in an irregular ground structure such as a sedimentary basin have been developed to explain the local amplification of seismic waves (e.g., Aki and Lamer, 1970;Sanchez-Sesma et al, 1988;Trifunac, 1971).…”

Site Amplification of S-Waves from Strong Motion Records in Special Relation to Surface Geology.

“…These events have clearly shown that local site effects have a greater influence on structural damage than was previously thought (e.g., Kobayashi et al, 1986;Kudo and Sakaue, 1984;Singh et al, 1988).…”

Two‐dimensional Scattering of P, Sv and Rayleigh Waves: Preliminary Results for the Valley of Mexico

“…At some frequencies, this ampliÿcation could be as large as 500 times with respect to the registered at near-source stations. 27 The dramatic way in which site e ects manifest in Mexico City is mainly due to the high contrast between the dynamic characteristics of the alluvial strata and the bed rock. In the frequency domain, the form of the ampliÿcation is controlled by the elastic impedances, damping of the soil, characteristics of the incident ÿeld and the geometry of the valley.…”