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