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Mansfield Earthquake: 22 September 2021 Magnitude 5.9

The epicentre of the 22 September 2021 Magnitude 5.9 Mansfield earthquake occurred at 9:21 am local time at latitude 37.491°S, longitude 146.363°E, Depth: 10.0 km. This is the largest earthquake to have occurred in Victoria since European settlement.

VIC earthquake location - Mansfield
Figure 1. Earthquake location. Source: Geoscience Australia

The earthquake did not occur near any identified potentially active faults (Figure 2). The fault that lies to the south of the epicentre is the Yallourn Monocline. The focal mechanism of the earthquake is left-lateral strike-slip on a north-northwest striking fault or right-lateral strike-slip on an east-northeast striking fault. These orientations are compatible with the identified potentially active faults, and with the orientation of the fault that caused the magnitude 5.2 Moe, Gippsland earthquake on 19 June 2012. These earthquakes are primarily caused by stresses generated by the convergence and collision of the Pacific Plate with the Australian plate along the plate boundary faults that run through New Zealand, extending from the Hikurangi subduction zone off the east coast of the North Island to the Alpine fault that runs down the west coast of the South Island.

Potential earthquake fault
Figure 2. Left: Identified Potentially active fault. Source: Geoscience Australia. Right: Focal mechanism in map view showing the orientation of the two possible fault planes. Source: USGS.

The intensity shakemap generated by Geoscience Australia (Figure 3) estimates that MMI Intensity VI (“strong” shaking with light damage potential) occurred in the epicentral region; MMI Intensity V (“moderate” shaking with very light damage potential) occurred on the northeastern outskirts of Melbourne, and MMI Intensity IV (“light” shaking with no damage potential) occurred in Melbourne. The occurrence of some nonstructural damage in Melbourne probably indicates that the affected structures were very weak. This map is consistent with the low level of damage that has been reported to date. Figures 4 and 5 show the peak acceleration and peak velocity maps.

MMI intensity map
Figure 3. Geoscience Australia MMI Intensity map.
Figure 4. Geoscience Australia peak acceleration map.
Figure 5. Geoscience Australia peak velocity map.

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About the author/s
Paul Somerville
Chief Geoscientist at Risk Frontiers | Other Posts

Paul is Chief Geoscientist at Risk Frontiers. He has a PhD in Geophysics, and has 45 years experience as an engineering seismologist, including 15 years with Risk Frontiers. He has had first hand experience of damaging earthquakes in California, Japan, Taiwan and New Zealand. He works on the development of QuakeAUS and QuakeNZ.

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