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

To understand the cause of observed amplification of seismic waves eve n at hill-zone sites in the Valley of Mexico, digital accelerographs h ave been installed at three especially chosen sites. Two of these site s, MADI and TEXC, located on hard Pleistocene lava (a few meters in th ickness) overlying Oligocene andesites, were expected to be free of si te effects. Analysis of the data recorded by these and other accelerog raphs during three moderate, shallow subduction zone events, however, shows significant amplification at MADI and TEXC between 0.2 and 0.6 H z with respect to established attenuation relations. The cause of the amplification at hill-zone sites in the Valley of Mexico, including MA DI and TEXC, may be pervasive low S-wave velocity in, and complex stru cture of, the upper layers of the volcanic rocks. If so, then there ma y not be a truly ''hard'' rock site in the valley.