IS THERE TRULY A HARD-ROCK SITE IN THE VALLEY OF MEXICO

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.