An average earthquake hits southeastern Australia. Where is the mistake?

The 5.9-magnitude earthquake reminded residents that Victoria is geologically active, but identifying the fault responsible is complex.

Written by De Nenes, Ph.D. (EeDeeNinis) and Elodie Burles, B.Sc. (borleis), Earthquake Research Center and Tamarah King, Ph.D. (amatamarahking), University of Oxford

Citation: Ninis, D., Borleis, E., King, T., 2021, moderate earthquake strikes southeastern Australia. Where’s the Wrong?, Temblor, http://doi.org/10.32858/temblor.207

On September 22, a 5.9-magnitude earthquake struck near Woods Point, about 80 miles (130 kilometers) east of Melbourne, southeast of Australia. Vibrations have been reported from as far away as Launceston to the south (280 miles or 450 km), Sydney to the northeast (370 miles or 600 km) and Adelaide to the west (465 miles or 750 km). Reported damages include the collapse of unsupported construction outside buildings in downtown Melbourne. Fortunately, no lives were lost.

September 22, with a magnitude of 5.9 on the Richter scale. Woods Point earthquake intensity map. Credit: Geoscience Australia.

This was the largest land earthquake to hit Victoria in at least 150 years – since written records have been kept in Australia. More recently, a magnitude 5.6 earthquake occurred near Mo in 2012, and was widely felt throughout Victoria.

In neighboring New South Wales, a 5.6-magnitude earthquake in 1989 struck 7.5 miles (12 kilometres) below the surface near Newcastle. The earthquake killed 13 people, and it is still the only known fatal earthquake in Australia. The widespread devastation caused A$4 billion in losses.

Moderate tremors are not unusual

Contrary to popular belief, earthquakes of magnitude 5.9 and larger earthquakes are not unusual for Australia. The country experiences an earthquake of magnitude 6.0 or greater approximately once every 6 years.

The largest Australian earthquakes were those in the 1989 Tennant Creek sequence, which included three events of magnitude 6.2, magnitude 6.3, and magnitude 6.6. The three earthquakes occurred within a 12-hour period. Other large events include the 6.6 Meakring earthquake in 1968 and the 6.3 magnitude earthquake near Mayberry in 1941.

Some studies have identified evidence of ancient Earth ruptures of magnitude 7.0 or larger earthquakes (eg, Clark et al., 2007; Ninis & Clark, 2019).

Earthquakes of magnitude 6.0 and greater since 1850. Data from Geoscience Australia.

Although Australia does not lie along plate boundaries, earthquakes occur for the same reason that occur elsewhere: the buildup of elastic stress energy in the crust. When the Earth’s crust is stressed, the stress energy increases. In Australia, the surrounding plate boundaries transmit their pressures inland to the continent. As stress builds up, it must eventually be released. This release occurs by movement along weak areas in shallow levels of the crust – ie, fissures. But tectonic processes in stable continental regions such as Australia occur at much slower rates compared to active plate boundaries, resulting in less frequent earthquakes.

Hundreds of aftershocks shook Victoria

Data recorded by seismometers indicate that the Woods Point earthquake occurred at a depth of approximately 7.5 miles (12 km), 9 miles (15 km) northeast of Woods Point. In the ensuing 24 hours, more than 300 aftershocks struck the area — nearly 100 within the first two hours. Most of these small earthquakes were not noticeable. And local residents felt the greatest aftershocks – their strength is 4.2 degrees and 3.3 degrees on the Richter scale. Both hit within an hour of the main shock.

Woods Point earthquake and aftershocks within two hours of the main earthquake, as detected by the Seismology Research Center. Time displays are UTC 2021-09-21 2315 to 2021-09-22 0015.

Locate the responsible error

Uncertainties associated with earthquake locations and nearby fault routing make determining the fault responsible for any given earthquake a challenge. Sometimes a fault will erupt at the surface, which simplifies the identification process (for example, the 1968 Australian Meeking earthquake and the 2016 Petermann earthquake). We don’t know yet if the Woods Point event ripped the surface. Given the magnitude and depth of a moderate earthquake, it may not be.

Focal mechanisms, a visual representation of fault slip during an earthquake, computed by multiple seismic agencies, indicates that the rupture contained slip motion on the NW-SE direction, and a very steep fault. The main and aftershocks are located approximately along the NNW-SSE direction, which is in agreement with the axial mechanisms and previous estimates of maximum horizontal stress in the region (eg, Rajabi et al. 2017).

There is no glitch in the current database of Australian active geology in the area torn apart by this week’s earthquake. This is not necessarily unexpected – moderate to strong earthquakes on a single fault in Australia may only repeat every 10,000 to 100,000 years, or even more than a million years in some areas (King et al., 2019). Erosion in the mountainous region around Woods Point could have easily erased any evidence of compensating for local faults.

However, the current trend of aftershocks is roughly in line with the governor’s fault zone – a thrust belt that was first active hundreds of millions of years ago. In Victoria, this fault extends more than 186 miles (300 km) from near Echuca in northern Victoria, near the Gippsland lakes on the southeast coast of Victoria (Vandenberg, 1997; Vandenberg et al., 2000; Kelly et al., 2002). Results from the recent (not yet published) deep seismic inversion survey show that this structure consists of a complex system of eastern immersion faults, including the Fiddlers Green, Fullarton, Queen Bee, and Barkly Thrust faults, that merge at depth (Ross Kelly, Telecom.).

Given the information we currently have, it is possible that this earthquake occurred in this fault zone. Additional data to come may highlight more evidence of active structures in the area.

Woods Point earthquake and aftershocks were detected within two hours of the main earthquake, showing local structural geology from Vandenberg, 1997.

New data coming

Scientists from Geoscience Australia, the Earthquake Research Center and the University of Melbourne are building a network of seismometers to monitor aftershocks. The group is exploring the area near the epicenter to look for ground deformation that may have been caused by the main shock. We hope that these observations and soon-to-be available satellite imagery across the region will reveal more about the source of this earthquake and, in doing so, may identify a new active fault in Australia.

Wayne Peake of the Seismology Research Center installs a temporary aftershock network station near Woods Point. Credit: Jake Wilcox.

Acknowledgments: For their insights into the structural geology and crustal pressure of the region, we thank Ross Kelly from the Geological Survey of Victoria, Dan Clark from Earth Sciences Australia, and Mujtaba Rajabi from the University of Queensland’s School of Earth and Environmental Sciences.

references

Cayley, R. A., Taylor, D. H., Vandenberg, A. H. M. and Moore, D. H., 2002. Proterozoic – Early Paleozoic and Tinian Mountains in Central Victoria: the Selwyn Massif and its Tectonic Reflections. Australian Journal of Geosciences, 49(2), pp. 225–254.

Clark, D, van Desen, R, Cooper, M, Collins, C, and Prendergast, A, 2007, Nov. Temporal assemblies of surface ruptures on stable continental zone faults: a case study from the Cadell Fault, southeastern Australia. In: Proceedings of the Australian Seismic Engineering Society Conference, 2007.

King, TR, Quigley, M. and Clark, D., 2019. 20 May 2016 Mw 6.1 Petermann surface rupture earthquake, Australia.

Ninis, D. and Clark, D., 2019. Fault investigation practice in Australia and New Zealand: why Australian dam owners and operators should care about active fault data collection. In: Proceedings of the ANCOLD Conference, 2019.

Rajabi, M., Tingay, M., Heidbach, O., Hillis, R. and Reynolds, S., 2017. The current stress field in Australia. Earth Science Reviews, 168.

VandenBerg, AHM, 1997. Australia 1: 250,000 Geological Map Series, Warburton Paper SJ 55-6 Edition 2. Geological Survey of Victoria.

VandenBerg, A. H. M., Willman, CE, Maher, S., Simon, B. A., Cayley, R. A., Taylor, D. H., Morand, V. J., Moore, D. H. and Radojkovic, A., 2000. The Tasman Fold Belt System in Victoria. Geological Survey of Victoria. special post.

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