THE ACCURACY OF SOIL RESPONSE ESTIMATES USING SOIL-TO-ROCK SPECTRAL RATIOS

Spectral ratios between soft soil and reference rock sites are often u sed to predict the sedimentary site response to earthquakes. However, their relationship with the genuine site-specific amplification functi on is often unclear. We compare the soil-to-rock spectral ratios betwe en the stations that are 3.3 km apart with the ''genuine'' response gi ven by the ratios between the surface and 17 and 47 m downhole. Data f rom the SMART1 array in Taiwan are used. The ''weak'' and ''strong'' m otion records are addressed separately to allow for nonlinear soil res ponse. The soil-to-rock spectral ratios are nearly identical to the '' true'' amplification at the frequencies from 1 to 10 Hz, if the finite depth of the borehole is taken into account. They correctly capture t he strong-motion deamplification effect. However, the soil-to-rock spe ctral ratios are roughly 1.4 times more uncertain than surface-to-47-m ratios. In summary, the soil-to-rock spectral ratios can be considere d as the reliable estimates of the real site response.