SITE AMPLIFICATION AT 5 LOCATIONS IN SAN-FRANCISCO, CALIFORNIA - A COMPARISON OF S-WAVE, CODAS, AND MICROTREMORS

We compare microtremor data to weak-motion S-wave and coda recordings at sites in San Francisco in order to clarify the range of applicabili ty of microtremor data to ground-motion prediction. We also compare S- wave results to coda results. For each type of data, we compute spectr al ratios of motions from two soil/rock station pairs and from an upho le/downhole pair in the Marina district. We compute horizontal/vertica l ratios (Nakamura's method) at a soil site, a rock site, and the surf ace and borehole instruments. In the station-pair analyses, microtremo r data show amplifications at the same fundamental frequency as S wave s, but the frequencies of other peaks do not agree. The amplification at frequencies higher than 2 Hz is greater in the microtremor data. St ation-pair ratios of coda data generally show spectral peaks occurring at the same frequencies, but with levels varying from one to four tim es the amplification from S-wave ratios. Nakamura's method of analyzin g microtremors agrees better with S-wave station-pair results than the microtremor station-pair method over a limited frequency band that va ries from station to station.