Tuesday 19 June 2012

Melbourne shaken by Earthquake.

A few minutes before 9 pm local time on Tuesday 19 June 2012 (slightly before 11 am GMT) the Australian state of Victoria was shaken by an Earthquake centered roughly 120 km east of the state capitol, Melbourne, and measured by Geoscience Australia as measuring 5.3 on the Richter Scale and occurring at a depth of 10 km. There are no reports of any serious damage or casualties, but a number of buildings are reported to have suffered minor damage, and there are concerns about damage to gas supplies; local authorities are recommending the evacuation of any building where gas can be smelt.

Map of Australia showing the location of the quake. USGS.

The quake was felt across much of Victoria and neighboring New South Wales, there are even a few reports of people feeling it in Tasmania. This is has been widely reported as the largest earthquake in Victoria for over a hundred years, though this is doubtful. The details of this quake have yet to be finally confirmed by seismologists, but Victoria, while not noted for its tectonic activity, has suffered a number of comparable quakes in the past. In November 1982 an Earthquake in the Wonnangatta Valley in the north of the state was recorded as measuring 5.4 on the Richter Scale. In 1969 an Earthquake near Boolara (about 30 km southeast of today's quake) was measured as 5.3 on the Richter Scale and felt in Melbourne. In 1966 an Earthquake near Mounth Hotham in the east of the state was measured as 5.7 on the Richter Scale.

Victoria is located far from any active tectonic boundary and is not an obvious place to expect earthquakes. The convergent boundary between the Pacific and Australian Plates is at its closest in South Island, New Zealand. The boundary with the Antarctic Plate far to the south is a divergent margin, which is slowly pushing Australia to the north, which accounts for some tectonic stress in the region. 

More relevantly, the geology of the east of Victoria is dominated by ancient fold mountains, formed by pressure on the rocks during the assembly of the ancient supercontinent of Pangea during the Palaeozoic. This caused the flat layers of rock in the Earth's crust too fold up to form mountains, in much the same way as a piece of paper pushed from both ends. These mountains have largely eroded away today, but the layers of rock beneath eastern Victoria are still folded, stacked up against one-another rather than laid flat on top of one-another. This is a less stable arrangement, so tectonic pressures can lead to larger quakes here than might otherwise be expected.

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