EXPERT REACTION: Magnitude 5.8 earthquake felt across south-east Australia

Today was the largest earthquake to have been recorded in Victoria since European settlement. 

Australia is fortunate that it lies away from tectonic plate boundaries where the world experiences its largest earthquakes. Australia can still experience small to moderate sized earthquakes as illustrated by the damaging Newcastle Earthquake in 1989.

Previous earthquakes have been felt in Melbourne, most recently of note in 2012, and have caused some minor damage. For example, an earthquake in 1922 saw a collapsed chimney in Glen Iris and several places reported that crockery had vibrated off shelves and broken in Pakenham, Portarlington, Cranbourne and East Malvern. Earthquakes larger than today's event in Victoria have been experienced in other parts of Australia.

Emergency services maintain earthquake plans to direct the response to damaging earthquakes in Victoria. In the event of an earthquake people inside buildings should DROP to the ground; take COVER by getting under a sturdy table or other piece of furniture; and HOLD ON until the shaking stops.

The 5.8 magnitude earthquake this morning in Mansfield, Victoria, with shocks spreading all the way south to Tasmania and north to New South Wales is a reminder that Australia faces seismic risk. The Australian tectonic plate is colliding into the Pacific plate in the Pacific Ocean and generates compressive stress, which when built up is released suddenly, causing earthquakes.

According to Geoscience Australia, on average 100 earthquakes of 3 or more magnitude are recorded each year. The 5.6 Newcastle earthquake in 1989 caused serious devastation and in Victoria regular earthquakes have been recorded. The earthquake today and other earthquakes such as the 5.2 magnitude Moe earthquake in 2012 point to the necessity of preparedness.

It is not possible to precisely predict earthquakes, but mapping of seismic activity and recorded earthquakes indicate an area’s risk, which should be the basis for risk reduction measures including, importantly, construction of buildings to a seismic standard. The oft quoted saying, “Earthquakes don’t kill people, buildings do” points to the importance of safe buildings. This needs to be combined with strong public awareness and institutional training initiatives so that people and organisations are prepared on what to do during and after an earthquake.

Animals may be startled by earthquakes, and as a result they may vocalise (e.g. bark, howl) or may hide or seek safety through escape behaviours. Companion animal owners should check that their animals are safe and secure.

It is important that all companion animal owners ensure their microchip details are up to date so that if their pet is found they can be reunited as soon as possible. While there are a lot of anecdotal reports, there is no conclusive evidence that companion animals predict earthquakes

These earthquakes occur because the continental plate on which Australia sits is moving north at about 7 cm per year; this builds up compressive stress within the Australian plate. This stress is occasionally released (resulting in an earthquake) along preferred areas, typically, pre-existing fault lines, where the earth has sheared (and can shear again) because of these stresses. 
 
Inspection of the geological maps of Victoria shows a large number of faults in the Mansfield, Victoria region, thus it appears like an area where the release of compressive stress via an earthquake might be probable.
 
Earthquakes above magnitude 5.0 usually occur every one-to-two years; magnitude 6.0 earthquakes generally occur every 10 years or so (Geoscience Australia).

At 9:16am local Melbourne time on 22 September 2021, a magnitude 5.8 earthquake at a very shallow depth of 10 km occurred about 38 km south of Mount Buller (population more than 2000) and 130 km north east of Melbourne in Victoria, Australia. The intensity of this earthquake in surrounding regions can be estimated to be VII (known as very strong earthquake) which can potentially cause light to moderate damage to structures, buildings and infrastructure. Traralgon (population more than 20,000), Morwell (population more than 10,000) and Warragul (population more than 15000) which are within 80km radius from the epicentre can be affected severely. It is expected that communities near Melbourne (with population in excess of 4 million) were the most impacted considering the population density. The length of fault rupture for this earthquake is estimated to be about 1.4 km. Based on the initial data, energy released due to this earthquake was estimated to be twice of the 1989 Newcastle earthquake (NSW).

In terms of seismicity and plate tectonic setting, Australia is classified as a stable continental plate, with intraplate stress fields generated due to presence of passive margins imposing forces at the plate boundaries. On average, Australia experiences two earthquakes with magnitudes greater than 5 per year, which is a rather high level of seismicity for a stable continental region. In the past century, about 27,000 earthquakes were recorded in Australia, and over 90 per cent of these earthquakes occurred at a very shallow depth (i.e. shallower than 5 km).

Several regions in Australia had experienced surface-rupturing due to earthquakes in the past. For example, a surface rupture more than 30 km long was observed in Meckering (WA) in 1968 as a result of a 6.6 magnitude earthquake, while one about 20km long was reported in Petermann (NT) in 2016 due to 6.1 magnitude earthquake. Surface rupturing is a significant risk to any structure that is built across a fault zone that may be active, in addition to any risk from ground shaking. Surface rupture can affect large areas of land, and it can damage all structures in the vicinity of the fracture. 

Local roads can be damaged due to earthquake or due to landslides and rock falls. This can hamper the search and rescue efforts and obstruct the access to the affected areas needing urgent attention particularly near villages and towns close to the epicentre. Residents must stay away from the cracked building as aftershocks can further loosen the already compromised sections of structures. Residents should also stay away from temporary excavations particularly near existing buildings.

It should be noted that non-structural components of buildings can be significantly impacted by earthquakes like this one, and thus impact safety of occupants. Building services including mechanical and electrical components which are not secured well can impose risk. Restraining systems such as anchoring, or bracing should be used to secure the loose non-structural components. It is possible ceilings of some buildings may not be compliant with the latest Australian Standard AS 1170.4 – 2007 and it is very important to design or retrofit ceilings, and non-structural elements as required in the standard.

Although we don’t often think of earthquakes as a natural disaster that occurs in Australia, they are not without historical precedent. In December 1989, Newcastle was struck by an earthquake measuring 5.6 on the Richter scale, the biggest recorded earthquake in south-eastern Australia in the twentieth century.

The earthquake fundamentally altered the city, taking 13 lives, injuring 162 people and damaging 10,000 buildings.

Aboriginal people tell of past earthquakes, for example the Awabakal people of Newcastle remember past occurrences recounted in their Dreaming stories. Since British colonisation in 1788, Australia has experienced 20 magnitude 6 earthquakes, with one as large as Newcastle’s occurring every two years (up to 2010), the most substantial in 1837, 1841, 1842, 1868 (5.3 magnitude), 1925 (5.0) and the largest, 1989 (5.6).

Every Australian city has many unreinforced brick masonry (URM) buildings in their central business district which were built long before earthquake design requirements were in place. Those which have parapets along their front pose high life safety risk to the public.

Unlike NZ, Australia has not mandated that existing buildings be strengthened against earthquake loads, however, ongoing research at Adelaide with support from the Australian Research Council & the Bushfire and Natural Hazards CRC is working to develop cost-effective techniques to prevent such damage as was observed in Chapel Street, Melbourne.

At magnitude 5.8, this is the largest onshore earthquake in Victoria in recorded history.

Earthquakes occur when rocks slide against each other along a fault plane to relieve stress that has accumulated within the Earth. The earthquake in Mansfield, Victoria is likely to have occurred along the Governor Fault – one of the deepest faults in Victoria – that separates the Melbourne Zone from the alpine region.