Victorian Earthquakes

22 March, 2009

Rowsley Fault
Rowsley Fault
Image: Frank Coffa
Source: Museum Victoria

Question: Why has Victoria had a number of earthquakes recently? I didn’t think Australia had earthquakes.

Answer: Although it is true that Victoria and Australia have a relatively low level of earthquake, or seismic, activity compared to other more active areas in the world such as New Zealand, Japan, and the west coast of the United States of America, we do actually experience an above-average number of earthquakes compared with other similarly stable continental regions.

According to Geoscience Australia, Australia experiences, on average, an earthquake of magnitude six or greater every five years, and between two and four magnitude-five earthquakes every twelve months.

The earliest known earthquake in Victoria was recorded in 1837, although instruments to properly measure and record seismic activity in Victoria were not in place until 1888, when a Gray - Milne seismograph was installed at the Melbourne Observatory.

Countries such as New Zealand and Japan experience earthquakes because they are located right on the boundaries of two or more of Earth’s moving tectonic plates. In contrast, Australia is sitting in the middle of one of these plates, but still experiences earthquakes, possibly because the boundaries of our plate to the east and the north are among the most active in the world.

Furthermore, the tectonic plate on which Australia sits is itself in motion, heading in a northward direction at the rate of a couple of centimetres per year. However, the movement of the plate isn’t uniform, causing localised stress to build up in the rocks. When the stress level in the rocks exceeds their resistance (strain), they fracture and slip along what is known as a fault line. An earthquake is the subsequent vibration in the earth released when this fracturing takes place.

The most recent earthquake to occur in Victoria was just this week, on Wednesday 18th March, 2009, and measured 4.5 on the Richter scale. It followed a similar-sized earthquake in the same area on March 6th. The epicentre (the point of origin of the earthquake) for both quakes was near Korumburra in the state’s south-east, within the region identified as having the highest earthquake hazard levels in Victoria. The earthquakes were reported as being felt by many people around the epicentre, and across Melbourne’s city and surrounding suburbs. There were a number of aftershocks (subsequent earthquakes but of a lesser magnitude than the main earthquake) which occurred within the next couple of hours after the main event.

 

Reference

Birch, William D. (ed.), Geology of Victoria, Geological Society of Australia (Victoria Division), 2003, pp. 593-602.

Comments (8)

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Jess 28 August, 2009 12:02
I recken that an earthquakes causes by the two plates moving on eachother by going transform, divergent or convergent.
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Lisa 27 September, 2009 22:33
Apparently Melbourne is quit prone to earthquakes- more than India, Africa, UK and other countries are. A massive earthquake is suppost to happen every 200 years along the Selwyn Fault which runs from Bass Strait up the west coast of the Mornington Peninsula up through the South Eastern Suburbs of Melbourne where it then ends through Mt Dandenong. It has produced earthquakes over magnitude 7.0 which is just about the same size as the 1995 Kobe,Japan earthquake and the 1989 Loma Prieta earthquake. They fear that one is extremly overdue and that it's building up stress and will produce a catastrophic quake in South Eastern Victoria- including Melbourne CBD, suburbs, gippsland, Mornington Penn, the highlands, westernport bay, french island, geelong, torquey, the dandenong ranges etc. I didn't believe it but people have been talking about it and i've read articles on the internt from earthquake experts from Australia and around the world and they think it's gonna happen in our grandparents life time! ive also heard things on tv and radio about it! there are hundreds of faults in South Eastern Vic and smaller fault under Melbourne CBD and there are large faults surrounding melbourne! it'll be much worse than that of previous australian quakes includin the 1989 Newcastle quake which was of magnitude 5.6. So be prepared and if you feel the ground move- get under table or door frame or run outside!
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Robert 7 October, 2010 03:40
I have come across a map of Lynhurst near Dandenong where the Selwyn fault runs very close to a hazardous land fill, if we have a magnitude of 6+ at any time in the future, could somebody be able to advise as to what may happen, and how safe is it,and would it be likely to fracture the cell at all.It just seems silly building so close to a fault.
Karen 2 November, 2013 15:02
Hi Robert if you get this, I'd like to see a copy of where the Selwyn Fault runs, the ones I've seen are a bit vague. Or if anyone else knows where.
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Discovery Centre 7 October, 2010 11:55
Hi Robert, the organisation to contact who may be able to assist in your question is the Seismic Research Centre  http://www.seis.com.au/
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Edwin 28 June, 2011 21:08
How significant is the Selwyn Faultline in Cape Schanck and are there any unconformities that we can see there ? as compared to those in the Werribee Gorge
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Discovery Centre 3 August, 2011 13:47

Hi Edwin, what used to be thought of as the exposure of Selwyn's Fault in the cliffs about 2 km north of the Cape is now known to be another fault. Selwyn’s Fault itself passes beneath Point Nepean about 10 km northwest of Cape Schanck, but there is no apparent surface expression of it (i.e. no outcrop in the rocks along the coast). The map of Fig. 11.14 on page 347 of Geology of Victoria shows its rough position. It's true that Selwyn's Fault forms the boundary between the uplifted Mornington Peninsula and the downfaulted Port Phillip Basin. The Cape is an erosional feature formed in thick flows of Tertiary basalt flows fronting Bass Strait.

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Karen Young 21 February, 2014 15:23
Selwyn's fault is in a few maps, I am not a geologist and have copied a couple of maps for my own use. The Library System does lend out "Geology of Victoria". It's a jolly big book. Still looking.... for something more specific at Lyndhurst/South Dandenong area.
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