Recent earthquake in Victoria

Geoscience Australia can confirm that a 5.9 magnitude earthquake was recorded at 09.15 AEST, Wednesday, 22 September 2021.

The earthquake epicentre has been confirmed as north of Rawson, Victoria and was recorded at a depth of approximately 10kms.

The earthquake has been followed by several aftershocks. Aftershocks are expected to continue for the coming weeks and months.

The earthquake is the largest event to have occurred within SE Australia in the modern instrumental era.

  • In 1966, a magnitude 5.5 earthquake occurred near Mt Hotham, while a magnitude 5.4 event occurred near Wonnangatta in 1982. These were significant earthquakes and were felt widely across the state.

The earthquake was shallow.

  • The closest seismic station to the earthquake was over 70 km from the epicentre. This means the exact depth of the earthquake is uncertain.

Rapid Deployment Kits (RDKs) are being sent to the earthquake area, and will help scientists better define the source of the earthquake (i.e. the active fault and depth of the main event) and may provide rare strong ground motion recordings that can be used to improve hazard estimates and guide building design.

The earthquake was widely felt across Victoria and New South Wales.

  • Geoscience Australia’s National Earthquake Alerts Centre (NEAC) has received more than 40,000 felt reports in the 24 hours after the quake.
  • Reports range from personal alarm to building damage – including fallen masonry, cracked walls, and buildings shifted permanently over their foundations.

Geoscience Australia activities and next steps

  • The NEAC is continuing to monitor for other earthquakes and aftershocks as part of routine operations.
  • Geoscience Australia’s seismic analysts have undertaken detailed analysis of the earthquake and aftershocks. These details are available on GA’s earthquakes website https://earthquakes.ga.gov.au
  • The NEAC continues to receive felt reports from the community and to assess the shaking intensity of each. These reports are aggregated into the FeltGrid which can be viewed on the website https://earthquakes.ga.gov.au/event/ga2021sqogij
  • GA is responding to a large number of requests from media.

  • Geoscience Australia’s seismologists and technicians have prepared and freighted Rapid Deployment Kits (RDK’s), which we will seek to deploy to the region to acquire more data.
    • Rapid Deployment Kits contain seismic monitoring equipment plus power and communications equipment.
    • The Rapid Deployment Kits will remain in the field for several months, measuring seismic signals from aftershocks. This includes very small events which cannot be seen using Geoscience Australia’s permanent seismic network.

  • Geoscience Australia will undertake InSAR analysis of the area using Sentinel-1 data. The next Sentinel-1 overpass of this region is on Saturday.
    • Earthquakes cause crustal deformation that is some circumstances cause the ground surface to move.
    • The patterns of ground surface movement can be measured and mapped with InSAR.
    • InSAR is an earth observation technique that makes use of repeated satellite radar images of an area.
    • Radar images collected before and after the earthquake by the European Sentinel-1 satellite constellation will help us determine whether there was surface deformation form this earthquake.

Questions and answers

  • Aftershocks from this event will likely continue for weeks or months, with aftershocks up to magnitude 5 possible. Five aftershocks were recorded in the first few hours following the main shock.
  • Its epicentral location beneath the SE highlands, an area of rugged high relief terrain.
  • Landslides may have been triggered by the strong ground shaking.

  • Ten aftershocks were recorded in the 24 hours following the main earthquake (main shock).
    • Magnitude 3.5 at 09:24, 22 September 2021, AEST and
    • Magnitude 4.1 at 09:33, 22 September 2021, AEST and
    • Magnitude 2.5 at 09:47, 22 September 2021, AEST and
    • Magnitude 3.1 at 09:54, 22 September 2021, AEST and
    • Magnitude 2.4 at 10:15, 22 September 2021, AEST and
    • Magnitude 2.9 at 13:18, 22 September 2021, AEST and
    • Magnitude 2.7 at 17:30, 22 September 2021, AEST and
    • Magnitude 2.8 at 18:00, 22 September 2021, AEST and
    • Magnitude 2.4 at 21:40, 22 September 2021, AEST and
    • Magnitude 2.8 at 08:04, 23 September 2021, AEST

  • Geoscience Australia is responsible for the operation of the National Earthquake Alerts Centre (NEAC).
  • The NEAC operates twenty four hours per day to provide rapid alerts of significant earthquakes occurring in Australia and overseas, including earthquakes with potential to cause tsunami.
  • Significant earthquakes are defined as ones with the potential to cause damage, and/or injury, and/or widespread alarm

  • The NEAC provided preliminary assessment to stakeholders approximately 10 minutes after the earthquake occurrence.
  • The NEAC published its final assessment at 11:20 AEST
  • NEAC manual earthquake analysis is undertaken in three stages:
    • initial (preliminary analysis for time-critical alerting);
    • interim (rapid analysis using all available data, usually completed within one hour of the earthquake occurrence);
    • final review (detailed analysis, usually undertaken next business day).

  • On average, Australia experiences an earthquake of this size once every 1-2 years.

  • The 2018 Lake Muir earthquakes were magnitude 5.2 and 5.3
  • The 1989 Newcastle earthquake was magnitude 5.4.

  • The Australian continent is situated in the middle of the Australian tectonic plate. This is known as an ‘intraplate’ setting.
  • Australia is the fastest moving of all the continents (~ 8cm/yr NNE) and is characterised by one of the highest levels of seismicity for a stable continental region worldwide.
  • Although Australia does not lie on a modern tectonic plate boundary, plate tectonic forces still exert stress on the rocks within the Australian crust. Over long periods of time, the rocks deform and break along ancient zones of weakness, known as a fault, a fault line, or a fault zone.
  • Together with the Australian seismological community, Geoscience Australia published the National Seismic Hazard Assessment in 2018. This assessment maps the likely ground shaking acceleration at a given location that has a given chance of occurring (e.g., 1/500 years, for example).

  • This earthquake occurred in Australia’s South-East Highlands region.
  • The SE Highlands region lies within ~500-400 million year old rocks of the Lachlan Fold belt (LFB). The LFB is one of the large geological building blocks that make up south eastern Australia.
  • There are number of suspected active faults in the SE highlands region, although knowledge on their activity rates is limited. The mountainous terrain is testament to several million years of intermittent fault reactivation activity distributed across much of eastern Victoria, continuing today. Combined, this fault activity has seen uplifts of several hundred metres across the region.
  • The Governor Fault Zone is a ~410 million year old structure that is located approximately 15 km to the east of the epicentre. An ongoing research collaboration between Geoscience Australia, Geological Survey of Victoria, Geological Survey NSW and AuScope (project: the Southern Lachlan Crustal Transect) has imaged the Governor Fault and revealed it as the uppermost boundary of a much larger, kilometres-thick, crustal-scale, east-tilted fault zone, the lower boundary of which reaches surface as the previously-mapped Fiddlers Green Fault Zone.
  • The earthquake epicentre lies just to the east of the Fiddlers Green Fault Zone suggesting that reactivation of a portion of this fault, at the base of this major crustal-scale fault structure, was the cause of the earthquake. 30km south of the earthquake epicentre the Yallourn Fault has one of the higher activity rates of Australian faults that have been studied.
  • The SE Highlands region of Australia regularly experiences small earthquakes, with approximately 60 earthquakes of magnitude 2 and greater in the last 20 years.
  • The SE Highlands region ranks as a relatively high zone of earthquake hazard, nationally.