Australia was surprised by an earthquake of medium size, but the gurgling is not unusual

Like a tree falling in the forest, if no one feels it, the earthquake goes unnoticed. This does not mean that it did not happen.

Written by Caroline M. Aiken, Ph.D. (SeismoCaro) and Meghan S. Miller, Ph.D. (MeghanSMiller), School of Earth Sciences Research, Australian National University

Citation: Eakin, C. and Miller, M., 2021, Australia surprised by moderate earthquake, but rumbling is not unusual, Temblor http://doi.org/10.32858/temblor.208

To the surprise of many Australians, Australia does indeed have earthquakes. On the morning of Wednesday, September 22, a 5.9-magnitude earthquake struck approximately 80 miles (130 kilometers) northeast of Melbourne, in the state of Victoria. The seismic waves were felt across several major cities including Melbourne, Canberra, Sydney, Adelaide, and as far south as Tasmania.

More than 40,000 people have submitted accounts of their experiences during the earthquake to Geoscience Australia. This is the largest number of “feeling reports” the site has ever received. However, Wednesday’s tremor, known as the Woods Point earthquake, did not surprise seismologists.

Map of a network reporting a magnitude 5.9 earthquake near Mansfield, Victoria on September 22, 2021. Credit: Geoscience Australia

Seismic sensors record about 100 earthquakes of magnitude 3 and above on the continent each year. Earthquakes of this size can be felt near the epicenter if the earthquake does not occur far from the surface. However, due to the low population density of the vast Australian continent, very few people feel it. Fortunately, most of the largest earthquakes to date have occurred in remote locations, so the public is largely oblivious to their occurrence. Only when an earthquake strikes near a populated area do people notice.

Earthquake patterns in Australia

Earthquakes in Australia are not unusual events. In the past 50 years or so, Australia has experienced seven land-based earthquakes of magnitude 5.9 or greater.

Medium to large events (size 5 to 6), although rare, tend to occur in the same locations as the more common smaller size events (size 2 to 3). Small earthquakes indicate pre-existing faults and weak rupturable areas. Southeast Australia, which includes Melbourne, Sydney and Canberra, is an area where these events of relatively smaller size are common. Thus, it is not surprising that a devastating earthquake occurred in this part of Victoria.

Across Australia, the distribution of seismic activity is not uniform, but rather tends to be concentrated within specific areas of increased activity. These areas include southeastern and southwestern Australia, as well as the Flinders mountain range that extends north from Adelaide. Earthquakes occur across Tasmania, including near Hobart, as well as along the coast near Brisbane. A devastating earthquake can strike near any of these capitals.

Map of all recorded earthquakes (yellow dots), notable events (red pins), and capital cities (black squares) across Australia. Data from Geoscience Australia

For the past 50 years, the most seismically active region of Australia has been located in the southwestern corner of Western Australia. Two of the 1968 earthquakes of magnitude 6.5 Microring and the Cadox earthquakes of magnitude 6.1 of 1979 were the two largest earthquakes to hit the region.

More recently, a 5.4-magnitude earthquake occurred near Lake Muir in September 2018. A 5.2-magnitude event followed two months later. The 5.4-magnitude event rips a four-mile (seven kilometer) cliff to the surface and a weak tremor was felt in Perth, 155 miles (250 kilometers) away.

Additional monitoring in Western Australia and South Australia

The Australian continent has been pieced together from different building blocks over its 3.8 billion year geological history. The resulting cratons are relatively strong, but the remnants of the old plate boundaries and associated faults tend to be weaker. As the continent responds to the stresses imposed by the surrounding active plate boundary, deformation occurs and is concentrated in those areas of weakness. The identification of weak areas and their associated seismic patterns is one of the goals of current and future seismic propagation processes.

For example, in 2020, we and colleagues installed the Southwest Australian Temporary Network (SWAN), which consists of 25 broadband seismometers spread across 320 by 270 miles (510 by 430 km) in Western Australia. The array will remain until 2022. The goal of the project is to produce improved 3D images of faults beneath the surface. With a denser set of seismic sensors, we can also more accurately locate future earthquakes, which will help scientists identify hazards in the area (Miller et al., 2020).

In the first eight months of operation of the network, the network found more than 1,100 small earthquakes. The vast majority of these would not have been discovered by the Permanent National Grid alone.

Seismic arrays (SWAN in pink and Lake Eyre Basin in cyan) designed to capture intra-plate earthquakes are currently being recorded in two seismically active regions in Australia. These temporary networks are also supported by hardware in the long-term operation of the Australian Schools Seismography (AuSiS) software (in orange) which provides real-time data and educational outreach within both cities and rural communities (Australian National University, 2011). Data from AusPass.

Another seismic network is currently in operation around the Kati Thandi-Lake Eyre in South Australia called the Lake Eyre Basin Seismic Array (Eakin, 2018). Data from this provisional publication has so far revealed more than 70 local earthquakes that would otherwise have been detected by the national network. These events appear to be related to the edge of the Gawler Craton, one of the powerful building blocks that formed part of the early Australian continent. The Lake Eyre Basin Array is also designed to record events in the Simpson Desert, where a 6.4-magnitude earthquake occurred in 1941. Scientists know very little about this remote region, as there are very few people around to feel the earthquake.

Acknowledgments: The SWAN and Lake Eyre Basin seismic arrays are supported by AuScope – Earth Imaging software, as well as ARC grants LP180101118 and DE190100062.

references

Australian National University (ANU, Australia). (2011). Australian seismometers in schools [Data set]. International Consortium of Digital Seismological Networks. https://doi.org/10.7914/SN/S1

Eiken, C (2018). Lake Aire Basin [Data set]. International Consortium of Digital Seismological Networks. https://doi.org/10.7914/SN/5G_2018

Earthquakes @ GA. Geoscience Australia, Canberra. http://pid.geoscience.gov.au/dataset/ga/121774

https://www.abc.net.au/news/2021-09-22/here-what-we-know-so-far-about-the-victorian-earthquake/100481948

https://earthquakes.ga.gov.au/

https://www.ga.gov.au/news-events/news/latest-news/victorias-biggest-earthquake-on-record

Miller, M, Allen, T, Mordy, R, Jessner, K, Yuan, H, and Kennett, B (2020). southwest australia network [Data set]. International Consortium of Digital Seismological Networks. https://doi.org/10.7914/SN/2P_2020

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