How ancient geological fault may be behind Townsville’s largest recorded earthquake

Believed to be triggered by a million-year-old geological fault, Townsville experienced its largest earthquake in recorded history on Saturday. We talked to experts on why it happened and whether it may happen again.

Believed to be triggered by a million-year-old geological fault, Townsville experienced its largest earthquake in recorded history on Saturday night — a magnitude 4.5 tremor that rattled the city.

Geoscience Australia Senior Seismologist Jonathan Bathgate confirmed it as the strongest earthquake ever instrumentally recorded in Townsville since monitoring began in Australia.

After further analysis, the research agency upgraded the quake from a 4.4 to a 4.5 magnitude on Monday.

Previously, the largest recorded quake in Townsville itself was a 2.2-magnitude event in 1900. However, looking at the broader North Queensland region, about six similar-sized earthquakes have occurred before, including a 5.7-magnitude quake near Ravenswood in the Charters Towers Region in 1913.

Residents in North Queensland have had a shaky Friday after a 4.45 magnitude earthquake hit. Map: Geoscience Australia

Mr Bathgate explained that although Australia is not as seismically active as some neighbouring regions, its ancient landscape has old faults and weak spots which may have been at the epicentre of the recent earthquake.

“Because Australia is a very old continent, a lot of the rocks have been faulted in the past, and so you might have old faults and sort of weaknesses in the earth’s crust that aren’t necessarily doing a lot, but our entire tectonic plate is kind of getting squeezed from the plate boundary,” he said.

“So as it kind of pushes up into Papua New Guinea and Indonesia to the north, and slides past New Zealand and Tonga to the east, that kind of squeezes, and so what can happen is those stresses can build up over time and get released in earthquakes along old faults, or old structures that may not have done anything for a very long period of time.”

Tectonic plates map. Earthquakes are common along the red lines that are the plate boundaries. Picture: Geoscience Australia

“The rocks underpinning Townsville are kind of carbonaceous, Carboniferous Permian in age, so several hundred 100 million years old.

“And so they’ve got old structures in them that can continue to pop off in earthquakes from time to time.”

The quake was widely felt throughout Townsville, with Geoscience Australia receiving over 5,000 reports from residents.

The city’s location on river flood plains likely amplified the shaking, making the event feel stronger in some areas.

Despite the rarity of such a large quake in Townsville, Mr Bathgate noted that it is not entirely unexpected.

“When you get an earthquake, you can always get aftershocks after, so it’s possible we may see aftershocks following this event, but we haven’t seen any yet,” he said.

He also stated that while another quake of this magnitude is possible, it remains a “low probability” to happen again in the near future.

“We obviously can’t predict when an earthquake will occur,” he explained.

View over Townsville to Mount Stuart where Saturday night’s earthquake originated.

“On average, we see fewer earthquakes in the Townsville region than many other parts of Australia. So we would expect that is most likely what will keep happening.”

A James Cook University seismologist who moved to Townsville to study earthquakes and teach physics said even she was “shocked” by the earthquake.

“This was a random and very rare occurrence,” Dr Lauren Waszek said.

“I’m a seismologist and when it happened even I thought ‘was that an army chopper?’, ‘did something just crash’?”

Dr Waszek said what North Queenslanders felt at 9.30pm was an ‘interplate earthquake’, which was stress from the continental plate itself being briefly released through a weak spot.

“We are on the Australian plate, and it’s moving the fastest of all the continental plates at about the same rate as your fingernails grow,” Dr Waszek said.

“That creates a lot of stress on this big slab of rock under us.”

Occasionally, the rock releases small bits of pressure through ancient, inactive weak spots.

Dr Waszek said what North Queenslanders felt was an ‘interpolate earthquake’, which was only 10km below the surface, which helps explain why people experienced the rare phenomenon of being able to ‘hear’ the quake.

“That bang people are reporting occurred because we were close enough to hear it,” Dr Waszek said.

“How you felt it also depended on what kind of house you were in and what kind of soil and foundations you had.”

The last significant earthquake to shake up the region was Bowen’s 5.8 tremor in 2016

Dr Waszek also addressed a social media theory that there was a fault line running from Townsville to Newcastle, saying this did not exist.

“There was a small earthquake in Newcastle on the same day, but that is a coincidence,” she said.

“There are thousands of earthquakes happening each day.”

Geoscience Australia is encouraging residents to report any felt effects, as this data helps seismologists better understand and monitor earthquake activity across the country.

Originally published as How ancient geological fault may be behind Townsville’s largest recorded earthquake