This article was published on abc.net.au: https://www.abc.net.au/news/2024-08-08/1918-earthquake-epicentre-moved-250km-inland-monto/104193366

More than 100 years on from Queensland's largest earthquake, researchers are re-writing history having discovered the epicentre was ore than 250 kilometres away from where they thought it was.

The 1918 earthquake, commonly called the Bundaberg Earthquake, was believed to have hit off Lady Elliot Island, north east of Bundaberg.

The magnitude-6.0 earthquake was felt across more than 3 million square kilometres, from northern New South Wales to Mackay and inland to Roma.

Stacey Martin, a PhD student at the Australian National University in Canberra, has been researching historical earthquakes.

"In Australia, we only have good instrumentation from about the 1960s, so prior to that, we don't really know much about where these earthquakes might have been centred and how big they were," Mr Martin said.

In a study published in July by the Bulletin of the Seismological Society of America, Mr Martin reviewed newspaper articles, recorded accounts and macro seismic data from 224 geographic locations to conclude the epicentre was 250km inland.

He said it was near the Coominglah State Forest around Monto in the North Burnett, 470km north of Brisbane.

"There's been a lot of work done to study historical earthquakes in the past, but with modern techniques it's always important to go back and double check them and make sure they're correct," he said.

From sea to land

Mr Martin was surprised early on in his research to find evidence from newspaper reports that suggested shaking could have been particularly strong at inland sites like Camboon in the Banana shire, 300km inland from Bundaberg.

The personal papers of Francis Bell, the then-owner of Camboon Station, described not being able to stand up during the earthquake.

"He also talks about aftershocks," Mr Martin said.

"This is quite unusual for an earthquake that's very strongly felt at an inland location, with aftershocks, where the actually instrumented epicentre is offshore."

Keen to delve deeper, Mr Martin looked at the seismogram, the record produced by the seismograph, from Riverview College Observatory in Sydney, the only station to record the earthquake in Australia.

He found the results were "misinterpreted".

"They assumed the earthquake was at a distance of about 1,200km from Sydney, and for the longest time no-one questioned that," he said.

Correspondence from one of Queensland's pre-eminent geologists, Walter Bryan, showed while he was studying a 1935 earthquake, Mr Bryan suggested the 1918 earthquake be re-evaluated.

"We had another look at them and found the earthquake wasn't 1,200km away, but now it was 900km away from Sydney.

"That was really a big 'a-ha' moment, because that distance not only coincided with the distance to Camboon, but it also coincided with the region that experienced a lot of aftershocks, which now coincided with where the epicentre really should be."

Seismic hazard risk needs to be accurate

Coominglah is only 185km north west of Gayndah, the site of Queensland's second largest earthquake in 1935, which measured a magnitude of 5.5.

It's also only 106km north west of Eidsvold, which recorded a 5.2-magnitude earthquake in 2015.

That event was Queensland's most recent notable earthquake, with aftershocks recorded four years later.

Mr Martin said changing the epicentre of the 1918 earthquake helped show the region's seismicity.

"When you plan, and make emergency plans as you would for bushfire, or floods, or cyclones, you might want to consider earthquakes as well."

Seismologist Michael Turnbull has been studying earthquakes for more than two decades, establishing the Central Queensland Seismology Research Group at Central Queensland University in 2002.

He said for a long time seismologists just accepted the ocean location, but agreed an onshore earthquake was far more likely, with "uncountable numbers" of very small earthquake fault lines in the area.

Mr Turnbull said having accurate data on earthquakes, including historical ones, was especially important as Australia's population grew.

"As time progresses, and population gets denser, and particularly as industry develops in an area, it's very important to know what the actual hazard is from earthquakes," Mr Turnbull said.

"For instance, let's just say we're going to build a gas-fired power plant or electrical plant somewhere, we need to know what the actual hazard from earthquakes is going to be.

Australia at risk of large earthquakes

Mr Martin said Australians should not be complacent about earthquake risks.

On average, 100 earthquakes above magnitude 3 are recorded in Australia each year, with destructive earthquakes, like the one that hit Newcastle in 1989, happening every one to two years.

"Most of Australia's recent very large earthquakes have either been in the middle of the desert, like Tennant Creek in 1988, or they've been offshore like the Broome earthquake in 2019," he said.

"There is potential for large earthquakes on land and onshore, and possibly even in regions of the country that are populated.

"I think we've just been through a really good run of luck and I personally hope that run of luck continues."