Stochastic finite-fault modeling of ground motions from the 1994 Northridge, California, earthquake. I. Validation on rock sites

The stochastic method of simulating ground motions from finite faults is va lidated against strong-motion data from the M 6.7 1994 Northridge, Californ ia, earthquake. The finite-fault plane is subdivided into elements, each el ement is assigned a stochastic omega(2) spectrum, and the delayed contribut ions from all subfaults are summed in the time domain. Simulated horizontal acceleration time histories and Fourier spectra at 28 rock sites are compa red with observations. We first perform simulations using the slip distribu tion on the causative fault derived from strong-motion, teleseismic, GPS, a nd leveling data (Wald et nl., 1996). We then test the performance of the m ethod using quasi-random distributions of slip and alternative hypocenter l ocations; this is important because the rupture initiation point and slip d istribution are in general not known for future earthquakes. The model bias is calculated as the ratio of the simulated to the observed spectrum in the frequency band of 0.1 to 12.5 Hz, averaged over a suite of rock sites. The mean bias is within the 95% confidence limits of unity, sho wing that the model provides an accurate prediction of the spectral content of ground motions on average. The maximum excursion of the model bias from the unity value, when averaged over all 28 rock stations, is a factor of a pproximately 1.6; at most frequencies, it is below a factor of 1.4. Interestingly, the spectral bias and the standard deviation of the stochast ic simulations do not depend on whether the fault slip distribution and hyp ocenter location are based on data or are randomly generated. This suggests that these parameters do not affect the accuracy of predicting the average characteristics of ground motion, or they may have their predominant effec t in the frequency range below about 0.1 Hz (below the range of this study) . The implication is that deterministic slip models are not necessary to pr oduce reasonably accurate simulations of the spectral content of strong gro und motions. This is fortunate, because such models are not available for f orecasting motions from future earthquakes. However, the directivity effect s controlled by the hypocenter location are important in determining peak g round acceleration at individual sites. Although the method is unbiased when averaged over all rock sites, the simu lations at individual sites can have significant errors (generally a factor of 2 to 3), which are also frequency dependent. Factors such as local geol ogy, site topography, or basin-propagation effects can profoundly affect th e recordings at individual stations. To generate more accurate site-specifi c predictions, empirical responses at each site could be established.