At just after 2am on Saturday morning, a magnitude 4.8 earthquake occurred at Arawata, about six kilometres northeast of Korumburra and 100km southeast of Melbourne.
Author
- Dee Ninis
Earthquake Scientist, Monash University
Residents closest to the epicentre described a loud bang - a phenomenon which occurs only in the epicentral region of an earthquake, when seismic energy reaches the surface.
They also described houses and windows rattling, and trees swaying back and forth as a result of the strong shake. The earthquake was also experienced across the state of Victoria, waking many people from their sleep even as far away as Shepparton, Ballarat and Warnambool. Geoscience Australia received more than 21,000 reports within hours .
The earthquake was followed by hundreds of aftershocks, the majority too small to be felt. The largest so far was a magnitude 3.1 less than an hour after the main shock. These aftershocks, which by definition only occur near the epicentre, are still happening several days later.
Although earthquakes can and do happen anywhere in Australia, the Gippsland region of Victoria has heightened seismic activity relative to most other places. But why is this the case?
Gippsland's shaky history
In the last 100 years, there have been about ten earthquakes in Gippsland of about the same size or larger than the one at Arawata last weekend; on average, that's about one every decade, though they haven't always been equally spaced through time.
In 1969, there were two moderate earthquakes of magnitude 5.0 and 5.3 near Boolara, over a period of a few days. This location also produced a magnitude 5.0 in 2000.
In 2012, there was a magnitude 5.4 quake near Moe. Although there were no reported serious injuries resulting from this earthquake, it was the same magnitude as the 1989 Newcastle earthquake , where tragically 13 lives were lost when a building collapsed.
The Korumburra-Arawata region has also experienced a number of earthquakes previous to the ones that occurred on the weekend. In 2009, a magnitude 4.6 event was followed by a similar magnitude earthquake about two weeks later.
Since that time, locals have felt around two earthquakes every year, including a widely felt magnitude 4.4 in 2011 . There have been many more recorded earthquakes during this time, but they have been too small to be felt.
All this is to show that the region is quite prone to shaking - but not for the same reason as earthquake-prone places like Aotearoa New Zealand.
What makes earthquakes happen
Most of the world's earthquakes happen near tectonic plate boundaries - places such as New Zealand, Japan and the western United States. Here, earthquakes are frequent because of the immense forces where tectonic plates meet.
Even though Australia is well within a tectonic plate and far from the edges, the continent is still being stressed by the large forces at our distant plate boundaries - for example through New Zealand to our east, and Indonesia and Papua New Guinea to our north.
These stresses travel through the plate and build up. When the stress becomes too great, it's suddenly released along zones of weakness in the crust, called faults. That release, and rupture of a fault, is what we feel as an earthquake.
The Korumburra-Arawata region and indeed the whole Gippsland region is known for several geological faults. Here, more than 100 million years ago, Earth's crust extended to form the Gippsland Basin and these faults accommodated it.
In more recent times, the last 5-10 million years, these faults were reactivated to accommodate a reverse force: compression of the crust. This has resulted in uplift (Earth's surface rising up) and mountain building.
Earthquakes like the one on Saturday morning are a release of stress associated with all these forces.
Young faults
Faults like some of the ones in Gippsland that have accommodated recent crustal deformation are called "neotectonic", meaning they are geologically young.
We haven't investigated the majority of the neotectonic faults in the Gippsland Basin, so we don't yet know much about them. One fault that has been studied is the Waratah Fault - a northeast oriented fault which is about 37km long, extending from Foster to off the coast near Walkerville.
There is geological evidence this fault has hosted an earthquake which was large enough to rupture the surface in the last 125,000 years. This evidence also suggests that rupture along the full length of this fault could produce earthquakes as large as about magnitude 7.0. Earthquakes of this size are rare in Australia, and smaller earthquakes are much more likely.
Aftershocks associated with Saturday's earthquake so far appear to align in the same orientation as many of the known neotectonic faults in the Gippsland region, however we don't yet know which fault is producing these latest quakes. It may even be that they're from a previously unknown fault; this was the case with the 2021 magnitude 5.9 Woods Point earthquake 120km northeast of Melbourne.
Earthquake geologists are looking to investigate additional Gippsland Basin faults in the coming years to better understand earthquake hazard in the region.
Aftershocks of Saturday morning's earthquake are expected to continue over the next few days to weeks. Generally, these earthquakes will decrease in magnitude and will become less frequent with time.
However, as with all aftershock sequences, there's still a chance that an earthquake of similar magnitude or larger may happen.
Dee Ninis works at the Seismology Research Centre, is president of the Australian Earthquake Engineering Society, and a committee member for the Geological Society of Australia - Victoria Division.
