S-WAVE SITE-RESPONSE ESTIMATES USING HORIZONTAL-TO-VERTICAL SPECTRAL RATIOS

We tested the hypothesis involved in determining the dynamic response of soils using spectral ratios between horizontal and vertical compone nts of ground motion. This hypothesis proposes that the vertical compo nent of motion contains mainly the source effect and is approximately insensitive to site amplifications. We used the vertical component of strong-motion records from earthquakes of the region of Friuli, Italy, to evaluate the site response for different geologic conditions. S-wa ve source functions of the events used were first retrieved from the a cceleration records using a generalized inversion in the frequency dom ain; then, an average site response was calculated for each site analy zed. The resulting site functions show small vertical amplifications f or most of the sites (within a factor of 2). This result confirms the validity of the method to estimate S-wave site responses. However, whe n the S-wave arrivals are contaminated by surface waves, the vertical amplification may exceed the uncertainty expected by the spectral rati o technique.