Australia, the shaky continent

CANBERRA - Australians may call New Zealand the Shaky Isles, but an earthquake that rumbled beneath the Northern Territory Outback this week has given new pause for thought.

The quake, 4.4 on the Richter scale, was powerful enough to have been felt or heard up to 100km from its epicentre, about 27km southwest of Tennant Creek.

The tremor was in fact nothing new. There have been 14 others around the Tennant Creek area this year, and in 1988 three much bigger quakes that, Geoscience Australia duty seismologist David Jepsen said, each released 1000 times more energy than this week's tremor.

And across Australia, there are on average 10 medium-sized shakes every year, with dozens of smaller tremors and the occasional real thumper.

Australians have been recording them since one shook up the first European settlers in June 1788, five months after the First Fleet landed in Botany Bay.

The biggest was in 1941, at Meeberrie in Western Australia, a magnitude 7.2 shake that had it not been in a remote corner of the continent would have flattened buildings and twisted bridges. It was felt in Perth, 500km distant.

Worse may be in store. Geoscience Australia says that the continent is being squeezed as the Indian-Australian tectonic plate is pushed north, colliding with the Eurasian, Philippines and Pacific plates. Adelaide, South Australia's capital, is being pushed sideways at the rate of about 0.1mm a year and, with more medium-sized earthquakes than any other in the past 50 years, is the most at risk of Australia's major cities.