2023 Sunbury earthquake

An earthquake struck approximately 28 kilometres NNW of Melbourne CBD, near the suburb of Sunbury in Victoria, Australia on 28 May 2023, at 23:41 local time (AEST). The earthquake measured 4.0 on the moment magnitude scale. It caused minor structural damage in parts of Melbourne and was felt as far as Tasmania and the Victoria-New South Wales border. Although the earthquake was weaker than the magnitude 5.9 Mansfield earthquake in 2021, this earthquake occurred within metropolitan Melbourne, so it was felt at a similar strength there, albeit for a lesser amount of time.

Tectonic setting
The Australian landmass is situated in the Australian Plate, far from any known active plate boundary, where most of the world's earthquakes occur. Such earthquakes at the plate boundary are known as interplate earthquakes. In Australia, earthquakes occurring within the Australian Plate are known as intraplate earthquakes because they happen within a tectonic plate rather than at the boundary.

The earthquake is situated in the Lachlan Fold Belt, an orogenic belt consisting of folded and faulted strata. This zone formed as a result of plate convergence occurring at the eastern boundary of the supercontinent Gondwana during the Neoproterozoic. From the Neoproterozoic to Early Devonian, the region was dominated by thrusting and some rifting. Crustal deformation were later accommodated by predominantly strike-slip faulting in the Devonian. One of the major strike-slip faults accommodating this deformation is the Baragwanath Transform; a transform fault. Rifting also occurred in this period, leading to volcanism. By the Middle Devonian, the Baragwanath Transform became extinct.

This earthquake occurred near the Clarkefield Fault Scarp – a neotectonic feature with a history of earthquakes between 5.3 and 2.6 million years ago. However, it is unknown whether the earthquake began in a mapped or an unmapped fault. There are many of these historic faults in Victoria, due to its seismically active history.

Earthquake
According to Geoscience Australia, the earthquake occurred at a very shallow depth of 2 km, while the U.S. Geological Survey placed the depth at 9.0 km. The geological fault involved has not been identified. The earthquake was preliminarily determined to have a magnitude of 3.8, but was later upgraded to a magnitude of 4.0 after a manual review two days later.

On the modified Mercalli intensity scale, the earthquake earned a maximum intensity of VI (Strong) by Geoscience Australia, while USGS put it at V (Moderate). Tremors were felt throughout Victoria and as far as Hobart, Tasmania and Albury, New South Wales.

Impact
The quake generated 5 to 10 seconds of shaking which was widely felt in Melbourne. More than 26,000 felt reports were submitted to Geoscience Australia. Slight damages including buckled roads, cracked buildings, cracked plaster and fallen pictures were reported across the northern and western suburbs of Melbourne near the epicentre region. It was the strongest earthquake within 40 km of Melbourne in more than 120 years, with the last stronger earthquake being a magnitude 4.5 quake in 1902.

Response
An earthquake advice was issued by VicEmergency which covered the entirety of Greater Melbourne and parts of Victoria. The earthquake made headlines in Australian news outlets the moment it occurred, due to the rarity of an earthquake within Melbourne and the unusual intensity of a low magnitude earthquake.

Further tremors
On 30 May 2023, a 2.6 magnitude aftershock which happened 2 minutes after the initial quake was identified by Geoscience Australia after manual analysis. Another 2.3-magnitude tremor occurred in the Melbourne Suburb of Croydon, about 33 km east of Melbourne's CBD at 17:03 two days later. It was felt by approximately 400 people.