October 4, 2018 – A Western Australian sheep farmer is amazed to discover large fractures in his paddock left by a magnitude-5.7 earthquake which shook an area from Perth to the state’s far-south coast.
Lake Muir farmer Rob De Campo was unaware the fractures existed until he was alerted by neighbour Mark Muir, who noted irregularities along the surface of Mr De Campo’s paddock while driving.
The force of the earthquake created hundreds of metres of fractures and surface ruptures, some up to half a metre deep, along some sections of his paddock, and displaced sections of dirt road on the boundary.
Additionally, large cracks in the base of his dam, which have since sealed, resulted in the loss of a “substantial” amount of water.
“It’s just incredible to see how much the ground can move,” Mr De Campo told the ABC during an inspection of his property.
“There’s no real damage as such, other than losing a bit of water out of the dam and a few loose wires in the fence posts.
“But these cracks and lifts in the ground are just incredible to see.”
Aftershocks, bangs and cracks continue
The initial earthquake hit shortly after lunchtime on September 16, triggering a wave of social media posts across much of Western Australia’s south.
More than 100 aftershocks had been recorded since and both farmers said they were still hearing loud “bangs and cracks” at various times throughout the day and night.
“They keep you on your toes,” Mr De Campo said of the random noises.
“It sounds like a loud cracking, sonic boom that just comes rattling up the paddock.
“They’re not as bad as that first one though, I certainly won’t be forgetting that in a hurry.”
Region in earthquake hotspot
Geoscience Australia earthquake geologist, Dan Clark, said both fault traces and surface ruptures were visible on Mr De Campo’s paddock.
Dr Clark said Australia was centrally located on the Indo-Australian tectonic plate, thought to have formed about 43 million years ago and located below the continent of Australia, surrounding waters, and beyond.
“The plate is moving roughly northwards relative to the neighboring plates at about 6 to 7cm per year,” Dr Clark said.
“When the plate motion is resisted, this resistance creates stress within the Australia plate, which builds up until the stress exceeds the strength of a fault that then slips to generate an earthquake.”
Dr Clark said the south west of Western Australia “in general” was more seismically active than many other parts of Australia and could be expected to remain a “hotspot” for earthquakes in the years to come.
“Earthquake science is not to the stage where earthquakes can be predicted,” Dr Clark said.
“However, given that there has been an earthquake at Lake Muir we might expect aftershocks to continue for the coming months to years maybe.
“We still record earthquakes in the Meckering region almost 50 years after the magnitude-6.5 earthquake there.”
Exciting viewing for budding geologist
As word of the find spread, University of Western Australia geology masters research student, Sean Standen, made the three-and-a-half-hour journey from Perth to spend a couple of days assessing the impact of the earthquake.
Mr Standen said the ruptures were caused by a build-up of compressive stress fields, reactivating weaknesses in the structure of the basement geology causing a “thrusting event”.
“That refers to one area of land thrusting above the other one. In this case, it’s not overly large, in the vicinity of 30 to 40cms of movement, but it makes for a fairly interesting scarp, especially for geologists.”
Mr Standen said he intended to extract as much information from the ruptures while they were still visible.
“I think I’d definitely be in the minority of people who are getting excited over this,” Mr Standen said.
“This is something that is really quite uncommon but it’s very relevant to my research because I am looking at fault scarps just like this in Western Australia and to be able to come and experience one that has just been formed in the last two weeks wasn’t something I was expecting.
“So, I’m quite keen to have a good look at it.”