Theoretical Synthesis and Analysis of Strong Motion Spectra of Earthquakes

Abstract

A comparison of theoretical Fourier amplitude spectra of strong ground motion (FS) with the corresponding spectra of real earthquake accelerograms is useful in elucidating the physical processes that contribute significantly to the ground motion signal. At low freqencies the contribution from surface waves tends to predominate. At intermediate frequencies the joint contributions from the direct shear wave and from complex crustal reverberations are important. At high frequencies the attenuation factor and/or source mechanism effects tend to dominate, depending on type of material and distance traversed.For a specified ground motion spectra and a particular acceleration response level, there are many theoretically predicted smoothed response spectra for velocity and displacement, depending on the particular combination of earthquake magnitude and source-to-receiver distance selected. In contrast the Newmark–Hall method (for nuclear reactor facilities) predicts a single value for the velocity and displacement response level when the acceleration response level is specified; the Newmark–Hall response values for velocity and displacement are somewhat greater than the corresponding maximum theoretically predicted values. Much of this difference is likely due to the fact that, in the theoretical spectra used for comparison purposes, contributions from complex crustal reverberations and from surface waves were not included.