RMS accelerations and spectral amplitudes of strong ground motion during the San Fernando, California earthquake

abstract An estimate of the Fourier amplitude spectrum of horizontal shear-wave motion is obtained using the Brune seismic source model in the presence of anelastic attenuation, from which the root-mean-square acceleration (arms) can be calculated. Predicted spectral amplitudes and values of arms are compared to observations for 160 free-field or basement-level components of horizontal acceleration of the San Fernando earthquake. For a stress drop of 50 bars and a faulting duration of 10 sec, observed and predicted spectral amplitudes and values of arms are in reasonable agreement: the observations conform to the predicted attenuation with distance (which is frequency-dependent for spectral amplitudes and proportional to R−3/2 for arms), but exceed predicted values by an average factor of about 3. Some of this difference is attributable to azimuthal variation in source excitation and the highly nonuniform distribution of stations with azimuth; the remainder results from the high-stress drop associated with the initial rupture for this earthquake. Predictions made by the model are nonetheless in agreement, both in terms of attenuation and amplitude, with spectral amplitudes estimated by empirical models calibrated with records from a wide variety of earthquakes.