One of the important inputs to a probabilistic seismic hazard assessment is the expected rate at which earthquakes within the study region. The rate of earthquakes is a function of the rate at which the crust is being deformed, mostly by tectonic stresses. This paper will present two contrasting methods of estimating the strain rate at the scale of the Australian continent. The first method is based on statistically analysing the recently updated national earthquake catalogue, while the second uses a geodynamic model of the Australian plate and the forces that act upon it. For the first method, we show a couple of examples of the strain rates predicted across Australia using different statistical techniques. However no matter what method is used, the measurable seismic strain rates are typically in the range of 10-16s-1 to around 10-18s-1 depending on location. By contrast, the geodynamic model predicts a much more uniform strain rate of around 10-17s-1 across the continent. The level of uniformity of the true distribution of long term strain rate in Australia is likely to be somewhere between these two extremes. Neither estimate is consistent with the Australian plate being completely rigid and free from internal deformation (i.e. a strain rate of exactly zero). This paper will also give an overview of how this kind of work affects the national earthquake hazard map and how future high precision geodetic estimates of strain rate should help to reduce the uncertainty in this important parameter for probabilistic seismic hazard assessments.

Geoscience Australia has recently released the 2012 version of the National Earthquake Hazard Map of Australia. Among other applications, the map is a key component of Australia's earthquake loading code AS1170.4. In this presentation we will provide an overview of the new maps and how they were put together. The new maps take advantage of the significant improvements in both the data sets and models used for earthquake hazard assessment in Australia since the current map in AS1170.4 was produced. These include: - An additional 20+ years of earthquake observations - Improved methods of declustering earthquake catalogues and calculating earthquake recurrence - Ground motion prediction equations (i.e. attenuation equations) based on observed strong motions instead of intensity - Revised earthquake source zones - Improved maximum magnitude earthquake estimates based on palaeoseismology - The use of open source software for undertaking probabilistic seismic hazard assessment which promotes testability and repeatability Hazard maps will be presented for a range of response spectral acceleration (RSA) periods between 0.0 and 1.0s and for multiple return periods between a few hundred to a few thousand years. These maps will be compared with the current earthquake hazard map in AS1170.4. For a return period of 500 years, the hazard values in the 0.0s RSA period map were generally lower than the hazard values in the current AS1170.4 map. By contrast the 0.2s RSA period hazard values were generally higher.

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The recently released ISC-GEM catalogue was a joint product of the International Seismological Center (ISC) and the Global Earthquake Model (GEM). In a major undertaking it collated, from a very wide range of sources, the surface and body wave amplitude-period pairs from the pre digital era; digital MS, mb and Mw; collated Mw values for 970 earthquakes not included in the Global CMT catalogue; used these values to determine new non-linear regression relationship between MS and Mw and mb and Mw. They also collated arrival picks, from a very wide range of sources, and used these to recompute the location, initially using the EHB location algorithm then revised using the ISC location algorithm (which primarily refined the depth). The resulting catalogues consists of 18871 events that have been relocated and assigned a direct or indirect estimate of Mw. Its completeness periods are, Ms - 7.5 since 1900, Ms - 6.25 1918 and Ms - 5.5 1960. This catalogue assigns, for the first time, an Mw estimate for several Australian earthquakes. For example the 1968 Meckering earthquake the original ML, mb and MS were 6.9, 6.1 and 6.8, with empirical estimates of Mw being 6.7 or 6.8. The ISC-GEM catalogue assigns an Mw of 6.5. We will present a poster of the Australian events in this ISC_GEM catalogue showing, where available, the original ML, mb, Ms, the recalculated mb and Ms, and the assigned Mw. We will discuss the implications of this work for significant Australian earthquakes.

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This paper describes the methods used to define earthquake source zones and calculate their recurrence parameters (a, b, Mmax). These values, along with the ground motion relations, effectively define the final hazard map. Definition of source zones is a highly subjective process, relying on seismology and geology to provide some quantitative guidance. Similarly the determination of Mmax is often subjective. Whilst the calculation of a and b is quantitative, the assumptions inherent in the available methods need to be considered when choosing the most appropriate one. For the new map we have maximised quantitative input into the definition of zones and their parameters. The temporal and spatial Poisson statistical properties of Australia's seismicity, along with models of intra-plate seismicity based on results from neotectonic, geodetic and computer modelling studies of stable continental crust, suggest a multi-layer source zonation model is required to account for the seismicity. Accordingly we propose a three layer model consisting of three large background seismicity zones covering 100% of the continent, 25 regional scale source zones covering ~50% of the continent, and 44 hotspot zones covering 2% of the continent. A new algorithm was developed to calculate a and b. This algorithm was designed to minimise the problems with both the maximum likelihood method (which is sensitive to the effects of varying magnitude completeness at small magnitudes) and the least squares regression method (which is sensitive to the presence of outlier large magnitude earthquakes). This enabled fully automated calculation of a and b parameters for all sources zones. The assignment of Mmax for the zones was based on the results of a statistical analysis of neotectonic fault scarps.

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The effect of ground motion models, site response and recurrence parameters (a, b, Mmax) on the uncertainty in estimating earthquake hazard have been widely discussed. There has been less discussion on the effect of the choice of source zones and the implied seismicity model. In the current Australian national seismic hazard map we have adopted a 3 layer source zone model. This attempts to capture the variability of the spatial distribution of the seismicity in the stable continental crust of Australia. PSHA has an implied assumption that the spatial distribution of earthquakes within a source zone is either uniform or random - with the random distribution approaching uniformity as it becomes sufficiently dense. At almost any scale in no area of Australia does the seismicity conform to either a random (single Poisson model) or a uniform distribution - it is more clustered. Generally, at least three Poisson models are required to match the observed spatial statistical distribution; typically zones of low, moderate and very high seismicity. Using the full (not declustered catalogue) at least 4 Poisson models are required. In all cases examined there are more bins than expected with <1 and >3 earthquakes and a deficit of bins with 1 or 2 earthquakes. This observaion is consistent with emerging models of earthquakes in stable continents being a non-stationary or episodic, rather than a steady state, process. In order to account for this observation, we use a three layer source zone model, consisting of: a Background layer, with three zones covering 100% of the continent, based on the geological and geophysical properties; a Regional layer, of 25 zones covering ~50% of the continent, based on the pattern of earthquake density; and a Hotspot layer, of 44 zones covering 2% of the continent, based on the areas of sustained intense seismicity. In the final hazard model the maximum of the three hazard values is used, not a weighted average of the three layers.

On the 16 April 2011 (05:31:18 UTC) Geoscience Australia (GA) recorded a ML 5.3 earthquake 50 km west of Bowen in central Queensland. This event was widely felt on the north Queensland coast and was followed by a number of aftershocks, which resulted in GA receiving more than 400 felt reports. Fifty earthquakes of magnitude <5.0 have been recorded in the region between Charters Towers and Mackay since 1900. However, during this same period only one M >5.0 earthquake is recorded; a ML 5.7 event located north of Ravenswood in December 1913. The April 2011 Bowen main shock was quickly followed by five smaller aftershocks ranging in magnitude from ML 3.2 to 4.1. Aftershocks were recorded by permanent seismic stations (e.g. Charters Towers, Eidsvold, Quilpie and Roma), however, the location, magnitude and depth of the smaller events is improved by four temporary stations which were established within four days of the main shock. The temporary sites were deployed between 10 and 48 km from the epicentre of the main shock to maximise the azimuthal coverage. Three-component seismometer and accelerometer data were recorded for a total of six weeks. With this dataset, existing information about several aftershocks was improved.

The wide-angle reflection seismic survey coincident with regional transect through Northern Yilgarn focused on the Leonora-Laverton Tectonic Zone, Western Australia was carried out to supplement deep seismic reflection studies. The major objectives were to collect high-density refraction information for offsets up to 60 km, to carry out a comparative study of near-vertical and wide-angle recordings of vibroseis energy at various offsets within the Leonora-Laverton tectonic zone and to obtain velocity information for upper crust. The survey deployed 120 short period recorders with spacing of 500 m. Acquisition parameters used for wide-angle reflection experiment were selected to fit into conventional reflection survey. The same vibrations were recorded in both surveys simultaneously. The major challenge in processing the Vibroseis data is to manage the huge volume of data. The processing of data includes several steps: sorting into receiver and source gathers, cross-correlation with reference sweeps and summing original seismic traces to form single source point traces, producing seismograms from individual traces and finally creating seismic record section from separate seismograms. The major step in processing and interpretation of data is to analyse recorded wave fields on the basis of seismological criteria prior to seismic velocity modelling. Seismic velocity models developed by using forward and inverse travel modelling software will supplement geological interpretations for this complex region and allow an estimation of its crustal composition.