Nine earthquakes have been recorded in Victoria's Gippsland region in the first nine days of October, Geoscience Australia says.
The sequence began with the magnitude-4.9 earthquake that rocked the region on Saturday. Eight aftershocks have been recorded since, including two overnight tremors of magnitude 2.5 and 1.5.
A magnitude-3.2 earthquake was felt in Leongatha on Wednesday evening, days after the larger quake hit nearby. Geoscience Australia said it received more than 2,000 reports of people feeling Wednesday's tremor, which struck at a depth of 10 kilometres.
The agency produces maps showing where across the state “felt” reports are submitted after an earthquake.
Saturday's quake came one week after a magnitude-3.9 earthquake was recorded at Ensay, in eastern Victoria, which at the time was labelled the most powerful recorded in that area in more than a decade.
Adam Pascale, chief scientist at the Seismology Research Centre, said the recent aftershock activity was expected and that there might be more to come. “We do see this type of activity regularly, just sometimes they're clustered a little bit closer together and other times they're not,” he said.
He expected the aftershocks to continue. “I imagine they'll continue for some days and weeks,” he said. “Whether there's larger events is impossible to tell but again we need to be prepared.”
Dee Ninis, a Monash University seismologist and president of the Australian Earthquake Engineering Society, said Gippsland had a history of earthquakes. “Korumburra is an area that gets quite a few earthquakes; they probably experience an earthquake large enough to be felt a couple of times a year,” Dr Ninis said.
The region had two magnitude-4.6 quakes in 2009 only a few weeks apart, and a magnitude-4.4 quake in 2011. Dr Ninis said that although Australia was not along the edge of a tectonic plate boundary like New Zealand or Japan, it could feel the impact of distant plate movement.
The largest earthquake on record in Victoria was a magnitude-5.9 event near Woods Point in 2021, which was felt 130 kilometres away in Melbourne. Emergency services are encouraging residents to be prepared in the event another quake hits the region.
The count deserves a closer reading. Nine earthquakes have been recorded, but only one of them is an earthquake in the sense of having happened on its own account. The other eight are aftershocks, which is to say consequences.
On that basis Gippsland has experienced a single earthquake accompanied by eight appendices. The appendices were felt by more than 2,000 people, which is a respectable circulation for a document nobody asked to receive.
The rate is unusually tidy. Nine events in nine days is one a day, a cadence more commonly associated with scheduled services than with the crust, and the chief scientist's observation that this type of activity is seen regularly confirms the regularity rather than the activity.
The Department of Unnecessary Research is drawn to the classification. An aftershock is defined entirely by what precedes it, so Wednesday's magnitude-3.2 tremor is formally an attribute of Saturday, filed three days late.
Whether the two overnight tremors of magnitude 2.5 and 1.5 are aftershocks of Saturday, or aftershocks of Wednesday's aftershock of Saturday, remains open. Seismology has no procedure for amending an amendment.
On the official count, Gippsland is still waiting for its second earthquake of October. It is advised to be prepared.