Trevor Dhu

Attenuation of Ground-Motion Spectral Amplitudes in Southeastern Australia

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.

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

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

SUA: A computer program to compute regolith site-response and estimate uncertainty for probabilistic seismic hazard analyses

The presence of soils, geological sediments and weathered rock (collectively known as regolith) can amplify the level of ground shaking experienced during an earthquake. Consequently, including the affect of regolith on earthquake ground shaking is an important component of any seismic hazard analysis. This manuscript provides a detailed and comprehensive description of equivalent linear site-response analysis, a technique for modelling the amplification of seismic waves due to propagation through regolith. The description includes a theoretical solution of the wave equation, derivation of a transfer function relating bedrock acceleration to surface acceleration, calculation of a response spectral acceleration and computation of an amplification factor. This paper also presents a simple approach for estimating the level of uncertainty in the modelled amplification factors due to variations in regolith thickness and velocity structure. A suite of MATLAB routines referred to as SUA are provided to implement an equivalent linear site-response analysis with the option of including an assessment of uncertainty. An example from Sydney, Australia demonstrates the techniques ability to successfully estimate site-response and associated uncertainties.

The use of fractal dimension estimators for enhancing airborne magnetic data

Airborne magnetic data is routinely enhanced by amplitude based filters such as horizontal and vertical derivatives. Texture is defined as the spatial distribution of amplitudes over a region. Textural analysis provides a possible alternative method of data enhancement. This paper investigates the potential of using fractal dimension for quantifying texture and highlighting textural contrasts in airborne magnetic data. Profiles have been created by combining theoretical data with fractal dimensions (FD) of 1.1, 1.3 and 1.5. Estimates of FD using the semi-variogram and variation methods clearly distinguish between sections of the profiles with different theoretical FD. Fractal dimension estimates made on a real airborne magnetic profile, using the variation method, clearly define two regions of visible textural contrast. A series of other variations in the estimated FD suggests that the method is able to resolve subtle contrasts that are not easily detected visually. The semi-variogram method of FD estimation is not able to resolve the obvious textural contrasts in real data, a result that is perhaps due to the stochastic nature of this methodology. The variation method has been used to estimate FD on a series of airborne magnetic profiles. This profile data was then gridded to generate an image of FD that moderately improves structural resolution. Whilst more work needs to be carried out, it is obvious that estimates of FD do detect textural contrasts in both theoretical and real data, and that this information can be used to enhance aeromagnetic data.

Digital earth Australia – unlocking new value from earth observation data

Abstract Petascale archives of Earth observations from space (EOS) have the potential to characterise water resources at continental scales. For this data to be useful, it needs to be organised, converted from individual scenes as acquired by multiple sensors, converted into “analysis ready data”, and made available through high performance computing platforms. Moreover, converting this data into insights requires integration of non-EOS data-sets that can provide biophysical and climatic context for EOS. Digital Earth Australia has demonstrated its ability to link EOS to rainfall and stream gauge data to provide insight into surface water dynamics during the hydrological extremes of flood and drought. This information is supporting the characterisation of groundwater resources across Australia’s north and could potentially be used to gain an understanding of the vulnerability of transport infrastructure to floods in remote, sparsely gauged regions of northern and central Australia.