Site response of the dam el Infiernillo, Guerrero-Michoacan, Mexico

We estimated site effects of the earth dam Fl Infiernillo, located between the states of Guerrero and Michoacan, Mexico. We analyzed 125 strong-motion records from stations located at three different levels of the dam (80, 12 0, and 180 m) and in the free field. The selected earthquakes are located i n the Cocos-North America subduction zone, with magnitudes between 3.8 and 7.9 and epicentral distances that range between Is and 433 km. We used thre e methods to estimate the characteristic frequency of vibration (f(n)) of t he recording sites. First, we calculated spectral ratios between the horizo ntal and the vertical components of motion (UN ratios). Second, we estimate d f(n) calculating standard spectral ratios using a free-field station as a reference site. Finally, we made a third estimate using cross-spectral rat ios. We found that although the three methods give consistent estimates of f(n), on average the H/V ratio gives more stable estimates. The H/V ratios, however, subestimate the site amplification at low frequencies (f < 8 Hz) at the top and middle sites. We suggest that this result is due to the late ral heterogeneity of the structure that favors wave-field propagation in th e vertical direction.