Tracking earthquakes deep underground

In September last year, Victoria was rocked by a magnitude 5.9 earthquake that shook buildings in the state and was felt (if subtly) as far away as Tasmania; it was the largest recorded earthquake in Victoria since European settlement.

Seven quakes struck in total, which generated less damage than might be expected – just one building, a Betty’s Burgers in a Melbourne suburb, was reported significantly affected.

The quake was a mild if sobering reminder that Australians live on a geologically active continent, even though we aren’t proximate to the edge of our tectonic plate like more high-risk places in Japan or New Zealand, for example.

The Victorian earthquake occurred on a previously unmapped fault-line; a fissure in the Earth’s crust sitting idly but menacingly beneath the state.

In fact, south-eastern Australia has a lot of these fault-lines; according to Januka Attanayake, a research fellow in earthquake seismology at the University of Melbourne, these cracks formed when the supercontinent Gondwana cleft apart 180 million years ago, pushing the Australian continent northwards.

Earthquakes along those fault-lines are triggered by the crashing of the Indo-Australian continental plate into the Eurasian plate to the north and the Pacific plate to the east and south-east; all that pushing and pulling creates stresses on the plates, which can create earthquakes at these fault-lines, even if they’re hundreds of kilometres away.

Thankfully, Victoria has an incredibly sophisticated seismic network run by the University of Melbourne. In the past four years, it has more than doubled in size.

But how does a seismic network work, and why, exactly, do we need it?

Measuring subtle shakes with seismic instruments

“When an earthquake occurs, it sets out mechanical disturbances in the crust,” explains Attanayake, who is one of the principal researchers helping to map Victoria’s Gippsland region, a particularly geologically active area. “Our goal is to record these disturbances radiating away from an earthquake.”

But recording these disturbances is not always simple – most of the quakes that occur are so subtle you’re unlikely to notice them.

“We need to put in instruments that can operate with these fairly large amplitudes of disturbances, from those that you don’t feel all the way to those that can shake buildings fairly violently,” Attanayake says.

“So, these are very sensitive instruments that can record these tiny little vibrations in the ground all the way up to several centimetres of vibration.”

The Victorian earthquake occurred on a previously unmapped fault-line; a fissure in the Earth’s crust sitting idly but menacingly beneath the state.

These instruments are called seismometers, and there’s roughly 43 functioning in the network Attanayake oversees on any given day around the Gippsland region. The network has different types of seismometers that operate at different levels and types of location – from those that sit in or near the surface in boreholes between 10 and 1000 metres deep, to those that sit on the ocean floor in the Bass Strait.

The instruments sit in their locations and transmit data automatically to the researchers, but a human analyst is needed to review the signals.

“We record about two to three terabytes of data every year, and we’ve detected about 400 earthquakes a year since the end of 2017,” Attanayake says. That’s a lot of earthquakes; before 2017, when the team installed more stations closer to smaller events that were previously undetectable, they were recording just 150 a year.

Why do we need a seismic network?

University of Melbourne seismologists first set a seismic network up in Gippsland after the 2012 Thorpdale earthquake, when they placed six aftershock-monitoring stations in the region. A 2019 study by Attanayake and colleagues found that the series of quakes may have been triggered by one earthquake stimulating another quake on a different fault – the first evidence of this kind of interaction in the state.

If most of the earthquakes recorded in Victoria are so subtle you’d barely notice them if you were sitting on top of their epicentre, why do we need such a network?

“We know that there are these faults that formed since the Gondwana breakup, and some of these faults are being recorded in the Neotectonic Database by Geoscience Australia,” Attanayake says. “But there could be many other faults that have not been mapped yet.”

The worst earthquake in Victoria’s post-settlement history didn’t do much damage, it’s true, but Attanayake says that while serious events might be rare, their implications could be massive – and thus far, we’ve been lucky.

“Just look at 1989 in Newcastle,” he says.

The 1989 Newcastle earthquake registered 5.6 on the Richter scale. The devastating quake killed 13 people and injured more than 160; the total economic damage was accounted at AU$4 billion (AU$8.5 billion today, adjusted for inflation), though Deloitte has estimated the true cost of such an earthquake today would exceed AU$18 billion.

“Because Australia is a very large continent but the population is concentrated in very small regions, we don’t get earthquakes occurring at urban centres all the time,” Attanayake says. “But we know that these earthquakes happen. A magnitude five event occurs in Victoria every seven years, and we’ve been lucky so far. We know that it’s very difficult to predict the location of these, and we know that they are occurring.

“So, at some point in the future, one of these magnitude five earthquakes will hit an urban centre. We know that it’s going to happen, it’s just a matter of time.”

Preparing for an event like this is important: buildings need to be engineered appropriately, especially critical infrastructure such as hospitals and power supply.

Indeed, in 2016, a magnitude 6.0 earthquake struck in the Petermann Ranges in Central Australia, leaving a 21-kilometre-long scar in the desert. The quake was remote, but a similar event near an urban centre could have major consequences.  

But to determine the kinds of building standards we need to live up to, we need to know how much motion is going to be produced, and where – which is where the seismic network comes in.

The good news is that as the network becomes ever better at detecting subtler quakes, we’re able to understand Victoria’s quakes more wholly, because there’s a relationship between smaller and larger quakes.

“If you know the number of smaller events, you can actually forecast the number of larger events that you can expect from a segment in a given period,” Attanayake says. “We call this the Gutenberg-Richter relationship [or law].”

“If you take a place like Victoria, what we see is that for every one earthquake of a certain magnitude, there are about five to seven quakes of one magnitude unit smaller,” he says. “So, for every five to seven magnitude-four earthquakes, there will be a magnitude-five sometime in the future.” What’s more, seismologists can establish a rough time period in which this larger quake will occur using statistical techniques.

And it’s not just about forecasting the next disaster – smaller earthquakes that do little damage to us above ground can still cause havoc for infrastructure that we depend upon.

In fact, the University of Melbourne research team’s data is being used by Victoria’s CarbonNet project, a commercial-scale carbon capture and storage (CCS) network being built in Gippsland.

Carbon capture and storage is a climate change mitigation strategy identified by the IPCCC as a necessary component of a net-zero future if we’re to keep the world under 1.5–2°C of warming – though the technologies for doing so are imperfect, and not without controversy.

CCS involves sequestering the CO₂ produced from, for instance, natural gas extraction, and you can do this in all sorts of ways – but CarbonNet will sequester its carbon in cavities deep underground beneath the Bass Strait.

All that stored CO₂ needs to be protected, including from natural disasters like earthquakes: “Protecting these assets that are going to directly affect the wellbeing of the planet is of paramount importance,” Attanayake says.