Earthquake ears

The National Australia Earthquake Risk Assessment identifies the southeastern corner of Australia as a seismic hotspot – with a magnitude 5.0 or greater earthquake occurring roughly every seven years on average in Victoria.

The 5.6 New South Wales earthquake in Newcastle exemplifies the damage expected from such an earthquake near an urban

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

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

It also occurred on a fault line that was not previously set.

Woods Point reminds us of the importance of fault detection and mapping to determine if they are active and if they are potential earthquake sites.

This is why we have developed and expanded the University of Melbourne’s Seismic Monitoring System in Victoria to become the best continuously operating seismic network in Australia.

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

The origin of some of these faults can be traced back to about 160 million years ago when Australia began to separate from Antarctica during the disintegration 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 was recently demonstrated by the Woods Point earthquake – the nectonic features database reveals a labyrinth of faults under Victoria.

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

Given only their dimensions, these two faults are theoretically capable of hosting earthquakes of magnitude seven, and are a stark reminder 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. Photo: supplied

The key to improved monitoring is the ability to accurately detect and locate small sub-threshold earthquakes that humans can feel, because these occur more frequently than large events, and are indicative of active faults.

The University of Melbourne began monitoring earthquakes in Victoria in 2012 after the 4.9 on the Richter scale Thorbdale earthquake that we investigated in detail.

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Since then, we have developed a high-performance seismic monitoring network covering the Gippsland region to detect and locate earthquakes with unprecedented accuracy.

This network uses seismometers – sensors that can record the slightest ground vibrations caused by the passage of energy through the Earth’s layers from an earthquake (seismic waves).

In our network there are different types of seismographs – sensors that are placed on or near the surface in wells 10 to 1,000 meters deep and those on the ocean floor are called ocean floor seismographs.

These instruments are located on land, in island locations such as picturesque Dale Island and under Bass Strait, where most sensors transmit live data to our servers on our Parkville Campus.

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

For example, having seismographs as close as possible to an earthquake helps determine its exact depth. Additionally, having a 360-degree coverage of the seismometer around this event helps pinpoint the epicenter – the point on the Earth’s surface just above where the earthquake occurred in the Earth’s interior.

Seismographs have been installed on the picturesque Dale Island in Bass Strait. Photo: Abraham Jones

In addition, we prefer to install seismographs on solid rocky sites, as they transmit these seismic waves more efficiently and with negligible energy loss compared to seismometers on “softer” soil sites which attenuate the seismic signals and sometimes generate unwanted levels of interfering noise.

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

For example, our instruments recorded ground vibrations from the recent chimney destruction at Hazelwood Power Station with a 1.6-magnitude equivalent earthquake.

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In the past four years, with the support of our funders, we have developed new capabilities including the deployment of high-frequency marine surface seismographs, and the development of operational standards to improve seismic monitoring on land and in shallow marine environments.

As a result of this work, earthquake detections in eastern Victoria have increased to more than 400 per year from about 150 prior to 2017, because we now have more stations closer to smaller events that are recording signals that had not been detected before.

We also detect some events with magnitudes as small as -0.5, which is a small centimeter earthquake rupture in the crust – the magnitude of the earthquake is measured on a scale that extends below zero.

These low-energy events correspond to a small centimeter-scale earthquake rupture in the crust.

We have also lowered the threshold at which all earthquakes in the network are detected from 1.2 on the Richter scale before 2017 to 0.5 today. We can more accurately locate earthquakes at a range of less than one kilometer, compared to previous uncertainties of up to 10 kilometres.

Victorian earthquake monitoring has been extended overseas by deploying seismographs to the ocean floor. Photo: Abraham Jones

We believe these statistics put our seismic network ahead of any other regional network in Australia.

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

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Additionally, our preliminary work in earthquake physics indicates that earthquakes of less than magnitude 2.5 in Gippsland radiate 10 percent more seismic energy than regular intra-plate earthquakes, which naturally translates to a slightly greater seismic hazard.

However, it is not known whether large earthquakes that could cause significant damage would radiate energy in a manner similar to those small events.

Interestingly, the 5.9-magnitude Woods Point earthquake appears to have caused less damage than expected, although this may partly be explained by a depth of about 12 kilometres.

The potential for smaller earthquakes to radiate slightly more seismic energy highlights the need to further understand earthquake behavior across all Gippsland intensity ranges so that we can ensure the safety of rapidly growing urban centers and critical infrastructure.

For example, new insights into seismic activity that we gained from the continuous accumulation of high-quality seismic data volumes are now being used to inform the future planning of Project CarbonNet, a proposed world-class shallow marine carbon dioxide sequestration site under investigation for commercial-scale operations.

There is now a large network of seismographs scattered throughout southeast Victoria. Photo: supplied

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

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

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

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