Methodology for Site Classification Estimation Using Strong Ground Motion Data from the Chi-Chi, Taiwan, Earthquake

The ground motions of the Chi-Chi, Taiwan, earthquake ( Mw=7.6) were recorded at 420 strong-motion stations, including 69 near-fault sites. However, the site conditions of many stations are not available. Among 420 strong-motion stations, the site conditions are known for only 87 stations, which were classified into four groups ( S1, S2, S3, and S4) by using borehole data and some surface geology. This paper presents a methodology to estimate the missing site condition information at strong-motion stations in Taiwan. The method is based on the shape of the 5% damped pseudo-acceleration spectrum of the horizontal ground motion component normalized with respect to average PGA, where the classification scheme is developed using the data from the 87 stations for which the site conditions are known. Possible effects of soil nonlinearity, and distance to the fault on the classification are investigated. The results obtained from the proposed methodology are well correlated with the available known site classification information data. The methodology is then applied to estimate the site condition for the other 333 stations without known site classification. Our results are compared to previous results obtained based on interpretation of geologic maps and geomorphologic data. We find that the two approaches agree in 71% of the cases. We also tested the horizontal-to-vertical spectral ratio technique to estimate the site classification of other 333 strong-motion stations. However, this technique resulted in lower accuracy than does the proposed technique based on the spectral shape of normalized response spectra.