Comparisons of ground motions from the 1999 Chi-Chi earthquake with empirical predictions largely based on data from California

This article has the modest goal of comparing the ground motions recorded d uring the 1999 Chi-Chi, Taiwan, mainshock with predictions from four empiri cal-based equations commonly used for western North America; these empirica l predictions are largely based on data from California. Comparisons are ma de for peak acceleration and 5%-damped response spectra at periods between 0.1 and 4 sec. The general finding is that the Chi-Chi ground motions axe s maller than those predicted from the empirically based equations for period s less than about 1 sec by factors averaging about 0.4 but as small as 0.26 (depending on period, on which equation is used, and on whether the sites are assumed to be rock or soil). There is a trend for the observed motions to approach or even exceed the predicted motions for longer periods. Motion s at similar distances (30-60 km) to the east and to the west of the fault differ dramatically at periods between about 2 and 20 sec: long-duration wa ve trains are present on the motions to the west, and when normalized to si milar amplitudes at short periods, the response spectra of the motions at t he western stations are as much as five times larger than those of motions from eastern stations. The explanation for the difference is probably relat ed to site and propagation effects; the western stations are on the Coastal Plain, whereas the eastern stations are at the foot of young and steep mou ntains, either in the relatively narrow Longitudinal Valley or along the ea stern coast-the sediments underlying the eastern stations are probably shal lower and have higher velocity than those under the western stations.