Authorities inspect dams after 3.6 magnitude earthquake in Sydney’s west

Thousands of residents felt the ground shift under their feet when a 3.6 magnitude earthquake reverberated through Sydney’s west, as nearby Warragamba Dam escaped damage, on Friday night.

Geoscience Australia senior seismologist Hadi Ghasemi said the agency received more than 5200 “felt” reports from people who detected the small, shallow tremor 20 kilometres south of Penrith.

“Given the time and location of the event, it was widely felt in the region,” he said.

Police covering the Blue Mountains and Nepean areas had reports of “a loud bang” shortly before 9pm and linked the noise to the earthquake. There have been no reports of injuries or damage.

WaterNSW said an initial assessment of critical dams in the catchment area, including Warragamba, under its safety protocols “confirmed no safety risk resulting from the seismic event” on Saturday.

Warragamba Dam supplies most of Sydney’s drinking water and is at 97 per cent capacity.

“Precautionary inspections continue this morning, but the seismic activity was well below the threshold that would pose any threat to WaterNSW dam infrastructure,” the agency said.

“Dams such as Warragamba are designed to withstand major seismic events, and the most extreme weather conditions.”

Ghasemi said a 2.0 magnitude aftershock was recorded about one kilometre north roughly two hours after the initial earthquake, which occurred at 8.53pm and was logged at a depth of eight kilometres.

“Aftershocks are always a possibility, but generally speaking, the size of the aftershocks becomes smaller, and they become less frequent, over time.”

Residents in the Blue Mountains suburbs of Lawson, Leura and Katoomba were among those who felt the tremor. Courtney Hollis, of Lawson, likened the racket to a passing freight train, and recalled she was in her kitchen when she “heard a bit of rumbling and then the whole house shook”.

Ghasemi said about 5280 people had lodged online reports – which he said the agency used to make its maps more accurate – indicating the intensity of shaking they felt on Friday night.

“It’s amazing … personally, I was really surprised [by the number of reports].”

Most had reported weak or light ground movement.

Ghasemi said the agency had logged nearly 100 earthquakes, or an average of five per year, within a 100-kilometre radius of Friday’s earthquake in the past two decades. A 3.8 magnitude earthquake at Appin, on Sydney’s south-west outskirts, was also widely felt by residents in 2017.

“Many of these events were smaller earthquakes that went unnoticed by the public,” he said.

There were many factors which contributed to whether people felt an earthquake: “Magnitude is certainly one of them, the depth of the event is another factor, and also the location and whether it’s closer to the built environment or it’s in the middle of nowhere,” Ghasemi said.

Ghasemi said it was impossible to predict when and where earthquakes would strike, and seismologists instead looked at the chance of an area exceeding a certain level of ground motion.

He said earthquakes were triggered by the slow build up of stress in the interior of the continent. “The stress is produced by the Australian tectonic plates moving towards the north-east every year.”

The 10 most recent earthquakes above 3.5 magnitude in NSW

1. 3.6 magnitude, Penrith, March 8, 2024
2. 3.9, near Goodooga, October 24, 2023
3. 3.5, Appin, September 8, 2023
4. 4.0, Boorowa, November 4, 2022
5. 3.6, near Broken Hill, June 24, 2021
6. 3.5, Muswellbrook, February 7, 2021
7. 3.6, near Manilla, September 25, 2020
8. 3.9, Muswellbrook, May 2, 2020
9. 4.4, Muswellbrook, December 16, 2019
10. 3.8, Muswellbrook, December 16, 2019

with Billie Eder