What have we learned in the 50 years since the 1968 Meckering earthquake?

Geoscience Australia records and locates earthquakes across Australia on a national network of seismographs. Earthquake locations and magnitudes are used to forecast the rates at which damaging ground shaking is likely to occur in a given region (http://www.ga.gov.au/scientific-topics/hazards/earthquake). This national-scale seismic hazard assessment informs building codes and relies on earthquake records from the last hundred years or so. Because large and damaging earthquakes are very rare in Australia, the short historic record might not reveal the full pattern of potentially damaging earthquakes.
If an earthquake is large enough, the rupture may intersect the surface of the Earth, and produce a scarp that is recognisable in the landscape for thousands of years. The 1968 Meckering earthquake was the first of nine historic earthquakes in Australia documented to have produced a surface scarp (Figure 1, Table 1). Scarps relating to pre-historic earthquakes might be discovered and mapped using satellite images and air photos. Over 300 have been found within the continent so far (Figures 1 & 2). This long record of large earthquakes can be combined with the historic record of earthquakes to improve the accuracy of national-scale seismic hazard forecasts.
Trenches across the 1968 Meckering earthquake scarp
In 2005, a team from the University of Western Australia and Geoscience Australia excavated two trenches across the 1968 earthquake rupture to find out how often large earthquakes occur on the fault (Figures 3, 4 & 5). The northern-most of the two trenches straddles the scarp between the Great Eastern Highway and the Perth – Kalgoorlie Railway, where it is almost 1.5 m high (Figure 4). At this location, an easterly flowing tributary of the Mortlock River has been dammed, forming a marsh in what was once a wheat field.
The trench wall showed sandy stream sediments had been warped into a broad fold, and faulted by ~1.3 m across a shallowly east-dipping fault plane (Figure 5). The sediments in the bottom of the trench are thought to be several tens of thousands of years old, and only show the displacement from the 1968 earthquake. In contrast to faults in plate boundary locations like California and Japan, this means that earthquakes do not occur regularly on the faults that produced the 1968 Meckering earthquake.
One-off large earthquakes in central and western Australia
Across Australia, eight earthquakes are known to have broken the Earth’s surface in historic times, producing a scarp (Figure 1, Table 1). All of these have occurred in the ancient geology of central and western Australia. Detailed investigations of these ruptures did not find any evidence that a large earthquake had occurred on that fault in the previous tens of thousands of years. They are essentially ‘one-off’ earthquakes, and can occur anywhere. These findings suggest that it is unlikely that Meckering will experience another large earthquake in the coming thousands of years. However, the surrounding Southwest Seismic Zone remains a region of high seismic activity, and may experience moderate to large earthquakes in the future. So, how can communities build resilience to earthquakes?
Earthquake damage mitigation case study : York
While the Meckering earthquake and the damage it caused prompted the production of Australia’s first earthquake building code, earthquake hazard was only fully recognised for Australian building design in the early 1990’s following the Newcastle Earthquake in 1989. This has resulted in a significant legacy of buildings that are inherently vulnerable to even low to moderate earthquake ground motion. The Shire of York is partnering with the WA Department of Fire and Emergency Services (DFES), the University of Adelaide and Geoscience Australia in a collaborative project that will examine the opportunities for reducing the vulnerability of the township of York to a major earthquake. The project forms part of the Bushfire and Natural Hazards Collaborative Research Centre project “Cost-effective Mitigation Strategy Development for Building related Earthquake Risk”.

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