Stochastic finite-fault ground-motion prediction equations (GMPEs) are developed for the stable continental region of southeastern Australia (SEA). The models are based on reinterpreted source and attenuation parameters for small-to-moderate magnitude local earthquakes and a dataset augmented with ground-motion records from recent well-recorded moderate-magnitude earthquakes relative to those used in prior studies (Allen et al., 2007). The models are applicable for median horizontal-component ground-motions for earthquakes 4.0 <= MW <= 7.5 and at rupture distances less than 400 km.
Careful analysis of well-constrained Brune stress drops indicates a dependence on hypocentral depth. It is speculated that this is the effect of an increasing crustal stress profile with depth. However, rather than a continuous increase, the change in stress drop appears to indicate a discrete step near 10 km depth. Average Brune stress drops for SEA earthquakes shallower and deeper than 10 km are estimated to be 23 MPa and 50 MPa, respectively. These stress parameters are subsequently input into the stochastic ground-motion simulations for the development of two discrete GMPEs for shallow and deep events.
The GMPEs developed estimate response spectral accelerations similar to the Atkinson and Boore (2006) GMPE for eastern North America (ENA) at short rupture distances (less than approximately 100 km). However, owing to higher attenuation observed in the SEA crust (Allen and Atkinson, 2007), the SEA GMPEs estimate lower ground-motions than ENA models at larger distances. These differences become most obvious at distances greater than 200 km.
A correlation between measured near-surface shear-wave velocity (VS30) and the site-dependent diminution term (K0) was developed from the limited data available to determine the average site condition to which the GMPEs are applicable. Assuming the correlation holds, a VS30 of approximately 820 m/s is obtained assuming an average path-independent diminution term K0 of 0.006 s from SEA seismic stations. Consequently, the GMPE presented herein can be assumed to be appropriate for rock sites of B to BC site class in the modified National Earthquake Hazards Reduction Program (Wills et al., 2000; Building Seismic Safety Council, 2003) site classification scheme.
The response spectral models are compared against moderate-magnitude (4.0 <= MW <= 5.3) earthquakes from eastern Australia. Overall the SEA GMPEs show low median residuals across the full range of spectral period and distance. In contrast, ENA models tend to overestimate response spectra at larger distances. Because of these differences, the present analysis justifies the need to develop Australian-specific GMPEs where ground-motion hazard from a distant seismic source may become important.
