CALIBRATION OF TIME HISTORY SIMULATION METHODS

Ground-motion time histories for use in engineering analyses of struct ures in eastern North America are often simulated from seismological m odels, owing to the paucity of real recordings in the magnitude and di stance ranges of interest. Two simulation methods have been widely use d in recent years: the stochastic method and the ray-theory method. In the stochastic method, as implemented in this study, ground motion is treated as filtered Gaussian noise whose underlying spectrum is deter mined from an empirical region-specific seismological model of the sou rce and propagation processes. In the ray-theory method, as implemente d in this study, the ground motions are simulated by convolving an emp irical source function with theoretical Green's functions for a specif ied crustal structure model. This article compares results of the two simulation methods for four well-recorded ''calibration'' events and a ssesses the applicability of the methods. The assessment is based on c omparisons of ground-motion parameters from the simulated data with th ose of the actual recordings. Ground-motion parameters in the frequenc y range from 1 to 10 Hz are satisfactorily predicted by both methods. Averaged over the four events studied, the stochastic method underpred icts 1-Hz response spectra by 20 to 40% but accurately predicts respon se spectra for frequencies of greater than 2 Hz; it also accurately pr edicts peak ground acceleration and velocity. The wave-propagation met hod underpredicts 1-Hz response spectra by 10 to 40% but accurately pr edicts response spectra for higher frequencies; it overpredicts peak g round acceleration and velocity by 10 to 40%. Both methods are impreci se: the standard error of an estimate is a factor of about 2.2. The bi as and standard error of an estimate for the wave-propagation method a re generally slightly lower than for the stochastic method, if the foc al depth of the event can be specified (i.e., as for a past earthquake ). If the focal depth of the event is not known (i.e., as for a future earthquake) then the accuracy and precision of the two methods are ab out the same. The chief advantage of the wave-propagation method is it s predictive power; since its attenuation function is derived from the focal depth and crustal structure it does not require knowledge of th e empirical attenuation function. The chief advantage of the stochasti c model is its economy and simplicity.