The vast, brooding expanse of Lake George, nestled against the foothills of the mountains, is a familiar sight to those traveling between Canberra and Sydney. However, beneath its serene waters lies an ancient geological story that is now being unraveled by scientists using an unconventional method.
A team of seismologists from the Australian National University (ANU) has harnessed the vibrations from vehicles driving along the Federal Highway to generate detailed 3D maps of the Lake George fault zone. This innovative approach is capturing some of the clearest imagery yet of the sub-surface, potentially refining our understanding of seismic activity just outside Canberra’s borders.
An Innovative Method for Mapping Faults
A fault is essentially a break in the rock caused by seismic activity such as earthquakes. The Lake George fault, stretching 80 kilometers, is the largest of three faults in the region, running along the western shoreline of Lake George and partially beneath the lake.
“Obviously as seismologists, we are eager to learn about what the seismic potential is near our region,” said study lead 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.”
Traditionally, mapping a fault zone requires researchers to measure active earthquakes or create ground motion themselves. However, the ANU team saw untapped potential in the nearby Federal Highway. “When a truck passes by, you feel the ground shaking,” Dr. Jiang explained.
In 2020, the team planted 97 small, “beer-can-sized” sensors along the lake’s western edge. “They are designed to record earthquakes, but they also record any other sorts of ground motions such as traffic, people walking by, cows, and the wind … they’re very sensitive instruments,” added Dr. Miller.
Using traffic to map fault lines opens up significant opportunities. Traffic is constant, free, and often in urban locations where creating vibrations can otherwise be hazardous. The imaging captured by the team used only a fraction of the potential ground movement recordings available in the area.
“To fully understand what the traffic signal noises are … you basically need to understand the whole ground motion response,” Dr. Jiang said. “That really takes quite a lot of time.”
A Snapshot of Geological Activity
The sensors recorded surrounding activity for 30 days, utilizing cutting-edge technology in one of the first studies globally to successfully use traffic noise in this way. The result? A glimpse at what lies up to 800 meters beneath one of the region’s most iconic landscapes.
The imaging revealed a zone of broken and fractured rock, indicating powerful seismic activity in the distant past. “This branch of faults that we observed relating to Lake George may be the result of a magnitude 7+ earthquake in the past few hundred or thousands of years,” Dr. Jiang noted.
These geological events may seem distant in terms of human timescales, but they have shaped what we see on the surface today. The mountains towering above Lake George are enduring evidence of this activity, forming a rugged wall that rises sharply skyward, pushed upward meter by meter by individual earthquakes over the past 4.2 million years.
Understanding Earthquakes in Australia
Australia sits relatively centered within the Australian tectonic plate. 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 inside plates.
“Presently, the continent is moving to the north-east at about seven centimeters a year. We are kind of crashing into [Papua New Guinea] and Indonesia,” explained Dr. Jonathan Griffin, Geoscience Australia Senior Seismologist.
Dr. Griffin elaborates that this ‘squeeze’ is thought to trigger earthquakes along some faults. Since 1960, the ACT has recorded 50 earthquakes, with the largest only around a magnitude 3. However, in the nearby Gunning-Dalton region, larger earthquakes have been recorded, including a magnitude 5.6 earthquake in Gunning in 1934.
“The south-east of Australia, including the Canberra region, is one of the relatively higher hazard parts of Australia,” Dr. Griffin said. “But it is much lower comparatively [to New Zealand].”
Based on research by Geoscience Australia and ANU, the most recent period of activity on the Lake George fault began around 4.2 million years ago. However, the fault has been silent for the past 800,000 years, posing challenges for assessing current risks.
Preparing for Potential Earthquakes
The work of the ANU team and Geoscience Australia cannot predict when earthquakes will occur, but it can help inform planning and hazard assessments. “If that fault runs through the highway, for example, you’re then going to end up with a couple of steps running through your highway,” Dr. Griffin said.
If you do experience an earthquake, Dr. Griffin offers clear advice: “Drop, cover, and hold. We tell people to stay where they are, get down, get under something like a table that can protect you from falling debris. It’s not a good idea to run out the front door of the building … a lot of the injuries and fatalities that we have seen from recent earthquakes are actually caused by bits of the building falling off the outside.”
As research continues, the findings from Lake George may not only reshape our understanding of local seismic activity but also contribute to broader insights into intraplate earthquakes worldwide.
