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Validation of a procedure for calculating broadband strong-motion time histories with empirical Green's functions

Lawrence Livermore National Lab, P.O. Box 808 L-205, Livermore, California 94551

Abstract

We quantitatively evaluate the validity of an approach to calculating strong ground motion time histories that involves the kinematic modeling of earthquake rupture by using recorded empirical Green's functions to constrain the propagation path and site contribution in the period range 0.05 to 2.0 sec, leaving only the contribution of the source to be specified. In addition, we use numerical Green's functions for long periods (> 10 sec) where the average crustal structure is assumed to be well known. In the intermediate band (2 to 10 sec) where site and path effects are still important but empirical Green's functions have small signal-to-noise ratios, we use an arbitrary smoothing method to connect the numerical and empirical Green's function synthetics. The evaluation presented here consists of comparing actual recordings of the Loma Prieta earthquake at 26 sites to syntheses from a simple source model similar to what is thought to have occurred during the Loma Prieta earthquake. We show that the standard error between observed and predicted response spectra is less than or equal to errors from other methods for periods between 0.05 and 0.4 sec and is significantly less than regression methods based on pre-Loma Prieta empirical strong-motion data at periods between 0.5 and 5.0 sec.

The strong ground motion prediction methodology that we are validating provides an accurate, defendable means to characterize site and path effects and, therefore, allows the uncertainties in the predicted hazard to be due to unresolved issues about the earthquake source such as the geological constraints of a particular fault and details about the physics of earthquakes. This methodology produces accurate and realistic acceleration and displacement time histories, which are becoming increasingly important for modeling the nonlinear response of large, distributed structures and soft soils. The implication for strong ground motion prediction is that a range of representations of an earthquake source based on physically realistic parameters can be used to generate a suite of possible time histories that represent the range of ground motion for that specific site and earthquake. In addition, in the future, as more information about a particular fault and earthquakes in general becomes available, the uncertainty in ground-motion prediction should decrease.

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