Edition 1 - The Long and Winding Murray River
Paleoseismology is the study of prehistoric earthquakes, especially their timing, location and size. For people living around the Murray River, such information is being gathered to understand the history of earthquakes in their region.
For a paleoseismologist, Australia is a great place to work. Scarps relating to large, surface-rupturing earthquakes, often more than tens of thousands of years old, are preserved in the landscape thanks to low erosion rates and negligible glacial influences.
The Cadell Fault, located within the Murray-Darling Basin in south-eastern Australia, is a perfect example of where paleoseismology can be used to study the long-term earthquake history of a fault.
The fault scarp is an 80 km long north-south oriented step in the landscape, which sits over a 50 degree west-dipping reverse fault. Following the pioneering work of Jim Bowler, more recent investigations have acquired data through seismic reflection profiling, analysis of high-resolution digital elevation model (DEM) data, field surveys, and optically stimulated luminescence (OSL) dating of geomorphic surfaces and fault-related sediments. Combining this information, earthquake scientists have reconstructed the displacement history of the fault over the last 100,000 years.
A study of the data also gives information about changes in the course of the Murray River over time.
Eventually, the river changed direction again to turn south, but whether this was the result of tectonic movement or a change in climate is still a mystery.
A common characteristic of the earthquake behaviour of Australian faults is that they rupture episodically. This means that periods of time during which large earthquakes occur frequently are separated by much longer periods of time where there are far fewer large earthquakes.
A closer look at the rupture behaviour of the Cadell Fault tells a dramatic story. In the Neotectonic era (the past 5-10 Ma) the fault has undergone two periods of activity, each involving approximately 15 - 20 metres of uplift. The first is suggested to have occurred around 5 million years ago, based on seismic reflection and borehole data which demonstrate offset in the basement rocks. The second, more recent period of activity, occurred between 70,000 and 25,000 years ago, where 15-20 metres of uplift occurred, leading to a shift in the course of the Murray River. The geological record shows that this happened in at least six large earthquakes, each producing ~2-3 metres of uplift. In order for this huge shift to occur, the entire 80 km length of the fault (between Deniliquin and Rochester) would have had to rupture in each event. The earthquakes produced would have been in the order of magnitude 7.3-7.5.
Modelling the action on the Cadell Fault shows that earthquakes of magnitude 7.3-7.5 (which require the entire fault to rupture) have happened, on average, about every 8,000 years during the most recent active period. This is important information for the locals living in the region of the fault, as large earthquakes represent a major potential hazard to communities and infrastructure. Fortunately, the recent research suggests that this fault may have expended its stored strain for now, and has entered another quiet period. Good to know!
Bowler, J. M. and Harford, L. B. 1966. Quaternary tectonics and the evolution of the Riverine Plain near Echuca, Victoria. Journal of the Geological Society of Australia, 13: 339-354.
Bowler, J. M. 1978. "Quaternary climate and tectonics in the evolution of the Riverine Plain, southeastern Australia". In Landform evolution in Australasia, Edited by: Davies, J. L. and Williams, M. A. J. 70-111. Canberra: Australian National University Press.
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