Empirical Attenuation of Ground-Motion Spectral Amplitudes in Southwestern Western Australia

Abstract

A dataset comprising some 389 strong- and weak-motion records for 69 events from the Burakin 2001–2002 earthquake swarm, and additional recent events, is compiled to develop a regional ground-motion model for the Yilgarn Craton, southwestern Western Australia. Events range in size from moment magnitude 2.2 ≤ M ≤ 4.6. The decay of horizontal-component spectral amplitudes can be approximated by a geometrical attenuation coefficient of R −1.0 within 80 km of the source. The associated regional seismic quality factor can be expressed as Q ( f ) = 457 f 0.37 for frequencies 1.07 ≤ f ≤ 25.0 Hz. Average corner frequencies for events with magnitude M >3.0 do not vary significantly with seismic moment M 0 , indicating a steep distribution of M 0 versus corner frequency. This causes anomalously low estimates of stress drop for smaller magnitude events ( M Fourier spectral amplitudes, corrected for geometric and anelastic attenuation, were regressed with M to obtain quadratic attenuation coefficients. Modeled horizontal-component displacement spectra fit the observed data well. Amplitude residuals (predicted–observed amplitudes) are, on average, relatively small and do not vary significantly with hypocentral distance. Source spectra (i.e., at R = 1 km) predicted from the regression parameters give self-consistent amplitudes at low frequency ( f less than approximately 2 Hz), equivalent to predictive models from eastern North America (ena) for the same moment magnitude. However, our model predicts lower spectral amplitudes with increasing frequency as a consequence of the low- stress-drop events. This is particularly apparent for the smaller magnitudes. Western Australian source spectra begin to converge with ena models at increasing magnitudes. If hypocentral distance is increased (i.e., R ≫ 1 km), the models begin to diverge at low frequencies owing to differences in geometrical attenuation coefficients. The bulk of these data were recorded from an earthquake swarm with very shallow focal depths ( h