Victoria’s monster from the deep: 2021 quake caused by ‘blind fault’ within Earth’s crust

Victoria’s largest earthquake since record-keeping began roughly 200 years ago was caused by the rupture of a previously unknown faultline, according to new research led by experts at The Australian National University (ANU).

The 5.9-magnitude Woods Point earthquake in September 2021 initiated at a depth of roughly 15 kilometres and was caused by an unknown or ‘blind’ fault that likely lies deep within the earth’s crust, without any surface exposure. 

Despite being larger than the devastating 5.6-magnitude Newcastle earthquake in 1989, which killed 13 people and caused approximately $4 billion worth of damage, the Woods Point quake caused relatively minor damage, with its epicentre roughly 130 kilometres from Melbourne.
 
“We’re quite lucky that the earthquake didn’t happen near a more populated area,” lead researcher Dr Sima Mousavi, a seismologist at the ANU Research School of Earth Sciences, said.
 
“One can imagine the scale of the disaster if an earthquake similar in size to the 5.9-magnitude tremor that hit Woods Point was to strike directly beneath Melbourne.”
 
Tremors from blind faults are relatively common around the world and can have devastating impacts. The 7.4-magnitude Tabas earthquake in eastern Iran, which killed in excess of 15,000 people in 1978, was caused by a blind fault, as was the 6.7-magnitude Northridge, California quake in 1994.

The discovery of this new fault will be used in seismic hazard assessments in Victoria, which are important tools for land-use planning, emergency response and the design of building codes.
 
“Blind faults can pose a significant threat as they can generate large earthquakes but they are hidden from us. This can make it difficult to identify areas that are at risk from earthquakes, which can be dangerous for communities living nearby,” Mousavi said.

“While blind faults may be more difficult to identify and study than faults with surface exposure, it is important to understand their location and behaviour and potential hazards in order to properly assess earthquake risk and take appropriate measures to protect communities and infrastructure.”
 
The data behind this discovery was gathered primarily from equipment installed in schools, as part of the ANU Australian Seismometers in Schools (AuSIS) program, which is funded by Auscope.
 
“When the Woods Point earthquake occurred, we were contacted by school teachers telling us that they see large amplitude wiggles on their screen from the seismometer at their schools,” Dr Mousavi said.
 
One of only two permanent seismic networks in Australia, data from the AuSIS network is routinely used by government agencies to locate and monitor earthquakes. This is the first time the location, fault plane and rupture pattern of an Australian earthquake have been identified using only seismic data, without support from additional field or satellite information.
 
The research is published in Seismological Research Letters.