Seismic ears on the ground

Australia’s National Seismic Hazard Assessment identifies the southeastern corner of Australia as an earthquake hotspot – with an earthquake of magnitude 5.0 or greater occurring approximately every seven years on average in Victoria .

The 1989 Newcastle earthquake of magnitude 5.6 in New South Wales illustrates the damage expected from such an earthquake near an urban centre.

The southeast corner of Australia is a seismic hotspot monitored by an extensive network of seismometers. Photo: Abraham Jones

The most recent reminder of this activity in Victoria was the 5.9 magnitude earthquake in the remote Woods Point area on September 22, 2021, making it the largest recorded earthquake in Victoria since European settlement .

It also occurred on a previously unmapped fault line.

Woods Point reminds us of the importance of detecting faults and mapping them to determine if they are active and are potential sites of earthquakes.

That’s why we have developed and expanded the seismic monitoring at the University of Melbourne in Victoria to make it possibly the most capable regional seismic network in Australia.

Earthquakes in southeast Victoria likely occur on pre-existing seismic faults, a subset of which are identified in the Australian Neotectonic Features Database based on indirect geological evidence. Neotectonic features are seismic faults that have been active for the past eight million years.

The origin of some of these faults dates back around 160 million years, when Australia began to separate from Antarctica during the breakup of the supercontinent Gondwana. Since then, these faults appear to have been reactivated by several plate tectonic events at different times.

Although still incomplete – as recently demonstrated by the Woods Point earthquake – the database of neotectonic features reveals a maze of faults beneath Victoria.

This includes major faults near urban centers such as the Selwyn Fault which bisects the Mornington Peninsula and the Muckleford Fault located 20 kilometers east of Ballarat.

Considering size alone, these two faults are theoretically capable of hosting magnitude seven earthquakes, and are stark reminders of the importance of better understanding fault lines and seismic activity across the state.

Hundreds of earthquakes can now be detected in Victoria each year. Image: Provided

The key to improved monitoring is the ability to accurately detect and locate small earthquakes below the threshold that humans can sense, as they occur much more frequently than larger events, and to indicate which faults are active.

The University of Melbourne began monitoring earthquakes in Victoria in 2012 following the magnitude 4.9 Thorpdale earthquake which we studied in detail.

Since then, we have been developing a powerful seismic monitoring network covering the Gippsland region to detect and locate earthquakes with unprecedented resolution.

This network uses seismometers – sensors capable of recording the slightest vibrations of the ground resulting from the energy passing through the layers of the earth following an earthquake (seismic waves).

Various types of seismometers are integrated into our network – sensors that sit on or near the surface in boreholes from 10 meters to 1000 meters deep and those that sit on the ocean floor called Ocean Bottom Seismometers.

These instruments are located on land, at island sites like scenic Deal Island and under Bass Strait, with most sensors transmitting live data to our servers at the Parkville campus.

When designing an advanced seismic network like ours, there are several considerations.

For example, having seismometers as close to an earthquake as possible helps determine its depth accurately. Additionally, having 360˚ seismometric coverage around this event helps to accurately determine its epicenter – the point on the Earth’s surface directly above where the earthquake is occurring underground. .

Seismometers have been installed on scenic Deal Island in Bass Strait. Photo: Abraham Jones

Additionally, we prefer to install seismometers in hard bedrock sites, as these transmit seismic waves more efficiently and with little energy loss compared to seismometers in “softer” ground sites that weaken seismic signals and sometimes generate unwanted levels of extraneous noise.

Having cell phone coverage is also a consideration, as the ability to transmit live data allows near real-time analysis of earthquakes or other events that generate seismic waves.

For example, our instruments recorded ground vibrations during the recent demolition of smokestacks at Hazelwood Power Station with an equivalent earthquake magnitude of 1.6.

Over the past four years, with the support of our funders, we have developed new capabilities, including the deployment of high-frequency, shallow seafloor seismometers, and established operational benchmarks to optimize seismic monitoring at the both on land and in shallow marine environments.

As a result of this work, earthquake detections in eastern Victoria have increased to over 400 per year from around 150 per year before 2017, as we now have more stations closer to smaller events recording signals that were previously undetected.

We also detect some events of magnitude as low as -0.5, which are tiny centimetre-scale seismic breaks in the crust – the magnitude of an earthquake is measured on a scale extending below from zero.

These low-energy events correspond to tiny centimetric seismic ruptures in the crust.

We have also reduced the threshold at which all network earthquakes are detected from magnitude 1.2 before 2017 to 0.5 today. We can more accurately locate earthquakes within one kilometer, compared to previous location uncertainties of up to 10 kilometers.

Earthquake monitoring in Victoria has been extended offshore by the deployment of ocean floor seismometers. Photo: Abraham Jones

We believe that these statistics place our seismic network ahead of any other region-wide network in Australia.

We recently launched an open-access, cloud-based web application on the AuScope Virtual Research Environment store to inform seismic network design decisions, developed through a collaboration between the University of Melbourne and CSIRO.

Additionally, our preliminary work on earthquake physics suggests that earthquakes below about magnitude 2.5 in Gippsland emit about 10% more seismic energy than typical intraplate earthquakes, which naturally results in a slightly higher seismic hazard.

However, it is unknown whether larger earthquakes that can cause significant damage will radiate energy in the same way as these smaller events.

Interestingly, the 5.9 magnitude Woods Point earthquake appears to have caused less damage than expected, although this may be partly explained by its depth of approximately 12 kilometers.

This potential for small earthquakes to radiate slightly more seismic energy underscores the need to better understand earthquake behavior at all magnitude ranges in Gippsland so that we can keep rapidly growing urban centers safe. and critical infrastructure.

For example, the new information on seismicity that we have gained through the continued accumulation of volumes of high-quality seismic data is now being used to inform future planning for the CarbonNet project, a shallow marine carbon dioxide sequestration site. world class under consideration for commercial purposes. scale operations.

There is now an extensive network of seismometers spread throughout South East Victoria. Image: Provided

The new technology and research capabilities we are developing to monitor seismicity will lead to a better understanding of seismic processes, benefit both society and industry, and help ensure safer living spaces for Victorians. .

Financial assistance to carry out the research described in this article is provided by the Australian National Low Emissions Coal Research and Development (ANLEC R&D) which is supported by Low Emission Technology Australia (LETA) and the Australian Government through of the Ministry of Industry, Science, Energy and Industry. Resources.

Funding for the development of seismic monitoring infrastructure is provided by the Australian Government through the Education Infrastructure Fund and administered and coordinated by the CO2CRC. AuScope provides partial funding to support operational activities.

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