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Attenuation of Ground-Motion Spectral Amplitudes in Southeastern Australia

¹ Risk Research Group
Geoscience Australia
GPO Box 378
Canberra, ACT 2601, Australia

^* Present address: Geologic Hazards Team, U.S. Geological Survey, Golden, Colorado 80401; tallen{at}usgs.gov.

A dataset comprising some 1200 weak- and strong-motion records from 84 earthquakes is compiled to develop a regional ground-motion model for southeastern Australia (SEA). Events were recorded from 1993 to 2004 and range in size from moment magnitude 2.0 M 4.7. The decay of vertical-component Fourier spectral amplitudes is modeled by trilinear geometrical spreading. The decay of low- frequency spectral amplitudes can be approximated by the coefficient of R^–1.3 (where R is hypocentral distance) within 90 km of the seismic source. From approximately 90 to 160 km, we observe a transition zone in which the seismic coda are affected by postcritical reflections from midcrustal and Moho discontinuities. In this hypocentral distance range, geometrical spreading is approximately R^+0.1. Beyond 160 km, low-frequency seismic energy attenuates rapidly with source–receiver distance, having a geometrical spreading coefficient of R^–1.6. The associated regional seismic-quality factor can be expressed by the polynomial: log Q(f) = 3.66 – 1.44 log f + 0.768 (log f)² + 0.058 (log f)³ for frequencies 0.78 f 19.9 Hz.

Fourier spectral amplitudes, corrected for geometrical spreading and anelastic attenuation, are regressed with M to obtain quadratic source scaling coefficients. Modeled vertical-component displacement spectra fit the observed data well. Amplitude residuals are, on average, relatively small and do not vary with hypocentral distance. Predicted source spectra (i.e., at R = 1 km) are consistent with eastern North American (ENA) models at low frequencies (f less than approximately 2 Hz) indicating that moment magnitudes calculated for SEA earthquakes are consistent with moment magnitude scales used in ENA over the observed magnitude range.

The models presented represent the first spectral ground-motion prediction equations developed for the southeastern Australian region. This work provides a useful framework for the development of regional ground-motion relations for earthquake hazard and risk assessment in SEA.