Is there truly a “hard” rock site in the Valley of Mexico?

Abstract

To understand the cause of observed amplification of seismic waves even at hill‐zone sites in the Valley of Mexico, digital accelerographs have been installed at three especially chosen sites. Two of these sites, MADI and TEXC, located on hard Pleistocene lava (a few meters in thickness) overlying Oligocene andesites, were expected to be free of site effects. Analysis of the data recorded by these and other accelerographs during three moderate, shallow subduction zone events, however, shows significant amplification at MADI and TEXC between 0.2 and 0.6 Hz with respect to established attenuation relations. The cause of the amplification at hill‐zone sites in the Valley of Mexico, including MADI and TEXC, may be pervasive low S‐wave velocity in, and complex structure of, the upper layers of the volcanic rocks. If so, then there may not be a truly “hard” rock site in the valley.

Lateral variation of Lg wave propagation in southern Mexico

“…This result agrees with Singh et al [1995], who reported a maximum amplification of 10 at frequencies between 0.2 and 0.4 Hz for CUIG. For TUIG, high amplification is obtained at high frequencies, which is expected since the station is located on sediments.…”

Evidence of low Q below Popocatépetl Volcano, and its implication to seismic hazard in Mexico City

“…It may be argued that the ground motion at frequencies above I Hz is of no consequence to seismic risk in Mexico City since the damage results from extraordinary amplification of seismic waves in the soft subsoil of the lake-bed zone of city at the dominant period of the site which is around 2 sec [Singh et at., 1988[Singh et at., , 1995. There is, however, reason to believe that large intraplate earthquakes, which occur in the subducted Cocos plate, can cause damage to low-rise structures which are sensitive to high-frequency ground motion.…”

Estimación Del Peligro Sísmico Debido a Sismos Interplaca E Inslab Y Sus Implicaciones en El Diseño Sísmico

“…La figura muestra que la forma del EPU para la estación de CU es significativamente diferente que para los sitios cercanos a la región de la costa del Pacífico Mexicano, y que las magnitudes del EPU son comparables a aquéllas para la zona costera solamente para estructuras con periodos de vibración largos. Cabe mencionar que Singh et al (1995) señalaron las diferencias de los efectos sísmicos en el valle de México y en la zona de la costa del Pacífico para algunos sismos históricos individuales.…”