The so-called "blind" fault was behind the 5.9 magnitude earthquake that rocked Victoria in September 2021.
It struck near the small town of Woods Point in the Yarra Ranges, about three and a half hours east of Melbourne, and was felt across south eastern Victoria.
Seismologist Sima Mousavi knew something dramatic happened that day, not because she felt the quake, but because her phone didn't stop ringing.
On the other end of the line were teachers who noticed significant activity on earthquake measuring devices installed in their classrooms by Australian National University.
"They were terrified and also excited," Dr Mousavi told AAP.
That data collected from classrooms in 50 schools was key to discovering the hidden fault line buried under Victoria about 15km below the earth's surface.
It was felt across south eastern Australia but there were no reports of injuries and building damage was limited to in inner Melbourne, which was in lockdown at the time.
Dr Mousavi says the impact could have been far worse considering the infrastructure and population in the Victorian capital.
"We are so lucky that the epicentre of the earthquake was 130km away from Melbourne in a very low populated area," she said.
Hidden fault lines are hard to identify because they cannot be detected by common monitoring methods such as satellite imagery, used to check on those with exposed surfaces.
Earthquake hazard maps will be updated to help safeguard against disasters along the hidden fault line.
"We can help people to find out if areas are safe to live, or they should have better building codes or if insurance policies should be changed," Dr Mousavi said.
Hidden fault lines can be extremely dangerous and were behind an earthquake in eastern Iran that killed 15,000 people and a 1994 quake in California that killed dozens.
The 2021 Melbourne earthquake was Victoria's biggest since record keeping started about 200 years ago and was larger than the 1989 Newcastle earthquake, which killed 13 people.
21°C