Synthetic Earthquake Ground Motions for the Design of Long Structures

The SYNACC method for synthesizing earthquake ground motion time histories is reviewed, with emphasis on the recent extension to generating motions on an array of points in space. This method is a combination of empirical and physical model-based approach, and involves unfolding in time a site-specific Fourier amplitude spectrum of ground acceleration, obtained for a scenario earthquake by an empirical scaling model, or for an ensemble of earthquakes by probabilistic seismic hazard analysis. The unfolding consists of representing the ground motion as a superposition of traveling surface Love and Rayleigh waves and of body P and S waves, which propagate with phase and group velocities consistent with the dispersion characteristic of the site geology, approximated by parallel layers. The synthesized motions are site-specific and consistent statistically with observations within the recording range of typical accelerographs (0.02-25 Hz). The output consists of synthetic accelerations, velocities and displacements, and also of point strains, rotations and curvatures, at a point or at an array of points. Such time histories are useful for the design of spatially extended structures, like pipelines, bridges and tunnels.