Lake George hides a seismic fault zone, and scientists use road noise to see it

The vast expanse of Lake George, spread out over the mountainsides, is a familiar sight to those traveling between Canberra and Sydney.

But beneath its depths lies an ancient geological story.

Using vibrations from vehicles driving along a federal highway, scientists are piecing together the lake's story.

A team of seismologists from the Australian National University (ANU) has harnessed the energy generated by traffic to create 3D maps of the Lake George fault zone.

By capturing some of the clearest images yet below the surface, the work of these scientists could help improve what we know about seismic activity beyond Canberra's borders.

Imaging revealed an area of ​​broken and fractured rock up to 800 meters deep beneath Lake George. (Supplied: Australian Geosciences)

An innovative method for fault mapping

A fault is basically a break in the rock resulting from seismic activity such as an earthquake.

“There are bugs everywhere in the world, including here near Canberra,” said study co-author Dr Megan Miller.

The Lake George fault, which is 80 kilometers long, is the largest of the three faults in the region. It extends along the western shore of Lake George and partly under the lake.

“Clearly, as seismologists, we are keen to learn about the seismic potential close to our region,” said study leader Dr Chengxin Jiang.

“We know that Lake George has a fault zone, but we don't really know how active it is and what the seismic characteristics are.”

But mapping the fault zone is no easy task.

Traditionally, it requires researchers to measure active earthquakes or establish ground motion themselves.

As far as the Australian National University team was concerned, the nearby federal highway had untapped potential.

Scientists are still trying to understand the causes of seismic activity at faults within plates. (Supplier: Ano)

“When a truck passes, you feel the ground shaking,” Dr. Jiang said.

“This wave actually travels a large distance, say a kilometer. Then sensors can pick up this kind of ground motion.”

In 2020, the team planted 97 small, beer-can-sized sensors along the western edge of the lake.

“They are designed to record earthquakes, but they also record any other types of ground movements such as traffic, people walking by, cows, wind… they are very sensitive instruments,” Dr Miller said.

Using traffic to draw fault lines opens up a lot of opportunities.

Traffic is steady and free and often in urban areas where causing vibrations can be dangerous.

The team's photography used only a small portion of the potential ground motion recordings available in the area.

“In order to fully understand what traffic signal noise is, you basically need to fully understand the ground motion response,” Dr. Jiang said.

“This really takes a lot of time.”

Dr. Megan Miller and Dr. Chengxin Jiang harness the energy generated by traffic to create 3D maps of the Lake George fault zone. (Supplier: Ano)

A snapshot of geological activity

The sensors recorded ambient activity for 30 days, a cutting-edge technology and one of the first few studies globally to successfully use traffic noise in this way.

And the result? Take a look at what lies up to 800 meters below one of the region's most iconic landscapes.

Imaging revealed an area of ​​broken and cracked rocks, an indication of strong seismic activity in the distant past.

“It's a snapshot of geological activity,” Dr. Jiang said.

“This branch of faults we observed in connection with Lake George may be the result of a magnitude 7+ earthquake in the past few hundred or thousands of years.”

These geological events may seem distant in terms of human time scales, but they shaped what we see on the surface today.

What causes earthquakes in Australia?

Dr. Megan Miller and Dr. Chengxin Jiang use highway traffic vibrations to map Lake George. (ABC News: Yasmine Wright Gittens)

Australia is located relatively in the middle of the Australian tectonic plates.

While most people are familiar with earthquakes that occur at the edges of these plates, such as in the Pacific “Ring of Fire,” scientists are still trying to understand what causes seismic activity at faults within the plates.

“Right now, the continent is moving northeast at a rate of seven centimeters a year. And we're kind of colliding [Papua New Guinea] Dr Jonathan Griffin, senior seismologist at the Australian Geosciences Authority, said: “The earthquakes detected in Australia were on the verge of collapse.

“Our continent is under pressure.”

This “stress” is thought to be the reason why earthquakes occur along some faults, Dr. Griffin explains.

Since 1960, the Australian Capital Territory has recorded 50 earthquakes, the largest of which were around magnitude 3.

But in the nearby Gunning-Dalton area, larger earthquakes have been recorded, including a 5.6 magnitude quake at Gunning in 1934.

“South-eastern Australia, including the Canberra region, is one of the relatively higher risk areas in Australia,” Dr Griffin said.

“But it is relatively much less [to New Zealand]”.

Being able to map errors in detail is really useful, says Dr. Jonathan Griffin. (Supplied: Australian Geosciences)

Based on research by Geoscience Australia and ANU, the most recent period of activity on the Lake George fault began about 4.2 million years ago.

The mountains that rise above Lake George today are permanent evidence of this activity.

They form a solid wall, rising sharply toward the sky, and individual earthquakes have pushed them upward meter after meter over the past 4.2 million years.

“This appears to have occurred during pulses of activity,” Dr. Griffin said.

“There was a paleo-river…and when those hills started going up, they dammed the river, and you had the lake formation.”

But at some point, about 800,000 years ago, the rift fell silent.

“This represents a challenge for us and then we think about what danger this mistake poses to us now in Canberra,” he said.

The last period of activity on the Lake George fault began about 4.2 million years ago. (Supplier: Ano)

What to do in case of an earthquake

The work being done by the team at the Australian National University and the Australian Geosciences Foundation cannot predict when earthquakes will occur.

But they can help inform planning and risk assessments.

“If that bug goes across the highway, for example, you'll then end up a few steps across your highway,” Dr. Griffin said.

“Being able to pinpoint where these errors are in detail can be really useful.”

If you experience an earthquake, Dr. Griffin has some clear advice.

“Drop, dive, and catch,” he said.

“We ask people to stay where they are, get down, and hide under something like a table that can protect you from falling debris.

“It's not a good idea to go out the front door of a building… A lot of the injuries and deaths we've seen from recent earthquakes are actually caused by parts of the building falling from the outside.”

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