A previously unknown faultline caused Victoria’s largest earthquake since record-keeping began two centuries ago, according to scientists at the Australian National University (ANU).
The 5.9-magnitude Woods Point earthquake in September 2021 started 15 kilometres deep, and was caused when an unknown or ‘blind’ fault ruptured. The fault likely lies deep within the earth’s crust, without any surface exposure.
The Woods Point quake was larger than the devastating 5.6-magnitude Newcastle earthquake in 1989, which killed 13 people and caused approximately $4 billion worth of damage. However, it only caused relatively minor damage, because its epicentre was 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. Blind faults caused the 7.4-magnitude Tabas earthquake in eastern Iran, which killed more than 15,000 people in 1978, and 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,” Dr Mousavi said. “This can make it difficult to identify areas that are at risk from earthquakes, which can be dangerous for communities living nearby.
“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.
Government agencies use data from the AuSIS network, one of only two permanent seismic networks in Australia, to locate and monitor earthquakes. This is the first time the location, fault plane, and rupture pattern of an Australian earthquake were identified using only seismic data, without support from additional field or satellite information.
The research is published in Seismological Research Letters.
