Extending Australia's Earthquake Record Beyond the Instrumental Era
Australia's large intraplate earthquakes
An earthquake of magnitude 6.8 struck the small town of Meckering located 130km east of Perth on 14 October 1968. It injured 20 people and destroyed 65 per cent of buildings in the town (Figure 1). The earthquake is the second largest onshore earthquake recorded in Australia. Such events are rare in the historical record of seismicity, which spans the two centuries since European settlement. This is because large earthquake recurrence on a given 'active' fault in Australia is measured in tens to hundreds of thousands of years or more.
Limitations of the historic record
The historic seismic record of 100 - 200 years is poorly suited for assessing maximum magnitude earthquake and identifying earthquake regions prone to damaging earthquakes because of the long recurrence cycles of large earthquakes. In order to avoid this problem, Geoscience Australia uses palaeo-seismology techniques to improve and extend the historic records of earthquakes out to tens of thousands of years.
How do we get around limited data?
The standard way of obtaining data on the locations and recurrence intervals of large, destructive earthquakes is to find active faults. Once found, soil and rock layers displaced across the fault can be mapped and dated to obtain single event displacements (a proxy for earthquake magnitude) and the ages of large earthquakes. Undisturbed layers draping the fault can be dated to give the time since the last event. The length of the scarp is also a proxy for palaeo-earthquake magnitude.
However, fault scarps can be only subtly defined and are often difficult to recognise in the landscape. The vastness of the Australian continent also limits the effectiveness of traditional methods to identify these features, such as aerial photograph reconnaissance. High-resolution digital elevation models (DEMs) have recently emerged as an important tool for finding fault scarps (Figure 2). DEMs are well suited to exploration over large or remote areas, and so are useful for defining and mapping areas that are likely to have an elevated earthquake hazard.
Mapping large earthquake distribution
Examination of a 10m resolution DEM supplied by the Western Australian Department of Land Information and selected Shuttle Radar Topography Mission 3 arc-second DEM tiles enabled the identification of 33 previously unrecognised fault scarps of probable Quaternary age in the southwest and central west of Western Australia (Figure 3). This finding more than doubles (to 60) the number of Quaternary scarps known in this area. Twenty one of the scarps were validated for reliability by ground-truthing, and four have been trenched to obtain recurrence and magnitude data.
The new features are between 15km - 45km long and between 1.5m - 20m high. As the 1968 magnitude 6.8 Meckering scarp is 37km long and up to 2m high, some of the newly discovered features may have been associated with significantly larger earthquakes, and/or multiple earthquakes.
Many of the newly discovered scarps are not associated with historic earthquakes suggesting that the focus for earthquake activity (i.e. crustal deformation) moves over time. The roughly even distribution of scarps suggests that seismicity is evenly distributed at a timescale of tens to a hundred thousand years, which provides a framework for crustal deformation models.
Further information about the recurrence rates of large earthquakes associated with individual scarps is needed to improve the certainty of seismic hazard assessments for short return periods. However, the fault scarps presented here identify 'earthquake-prone' regions that could be used in further investigations.
What is the impact of this information?
The new scarp data allows for robust estimates of maximum magnitude earthquake to be made in this region, which is a fundamental parameter underpinning the hazard map used in the national building code. Furthermore, the identification of earthquake prone regions allows emergency managers and planners to educate the local community and develop earthquake effect mitigation and response strategies. The data also provides constraint for crustal strain models.
Neotectonic Features Database
The Neotectonic Features database contains information on faults, folds and other features within Australia that are believed to relate to large earthquakes during the Neotectonic Era (e.g. the past 5–10 million years).