Development of the Australian National Regolith Site Classification Map

A key recommendation of the Council of Australian Governments review into natural disaster management arrangements in Australia is that a five-year national program of systematic and rigorous disaster risk assessments be developed and implemented. This process requires the construction of national databases and standardised methods and models that allow objective comparison of risks between regions and across hazards. A significant component of this process is the completion and delivery of a series of national earthquake risk assessments. The need for an improved understanding of earthquake ground shaking in Australia was recognised following the 1989 Newcastle earthquake, which resulted in 13 fatalities and A$4.5 billion in estimated losses. An enhanced capability to anticipate the impacts of such events will facilitate improved earthquake disaster mitigation and planning for Australian communities, and influence the development of relevant engineering codes and standards. To achieve this it is necessary to model earthquake events, the mechanisms by which earthquake energy dissipates, and the potential influence of variation in geological materials on the ground shaking. At present, national scale earthquake hazard products for Australia do not included the effect of regolith site response on ground shaking, and as such may provide inconsistent or inaccurate estimates of ground shaking in some areas. The development of the National Regolith Site Classification Map represents a significant advance in our ability to model the potential influence of near-surface geological materials on earthquake ground shaking, and therefore to assess earthquake hazard and risk in Australia. The National Regolith Site Classification Map presented here has been developed through the application of a pre-existing methodology which has been modified to suit Australian conditions. The changes include the development and application of an innovative method to account for weathering in bedrock geological units. To the extent that the data permits, site classification takes into account the age and physical properties of the geological materials and relates them to key geophysical parameters that most accurately represent the behaviour of these materials under the influence of earthquake ground motion. The compilation of data at both national and regional scales has led to the development of a multi-resolution tool that provides more detailed information in and around the major population centres. This same variation in spatial resolution does, however, make map sheet edge mismatches unavoidable.
The National Regolith Site Classification Map provides a tool for estimating the regolith site response to ground shaking at any location in Australia. This product has potential implications for revision of earthquake-related Standards and Building Codes in Australia, particularly regarding the criteria used to classify sites according to ground shaking potential. When implemented within Geoscience Australia's National Earthquake Risk Model (EQRM) the National Regolith Site Classification Map and associated amplification factors represent fundamental components of the most rigorous available method for assessing earthquake risk in Australia.