Ground slips 1 meter during Christchurch quake
The New Zealand city of Christchurch felt the full brunt of a 6.3-magnitude earthquake today, just five months after experiencing a 7.1-magnitude quake.
Based on the magnitude, Gary Gibson, a seismologist at the University of Melbourne, Australia, says ground shaking would have been "severe" in the city centre, which was only 10 kilometres from the earthquake's epicentre.
Details of the damage remain vague, but 65 deaths have been confirmed and there are widespread reports of severe building damage. Two buses were crushed by collapsing buildings, and 75 per cent of the city was left without power.
According to Gibson, the active fault area was approximately 15 square kilometres. "One side of the fault moved about 1 metre relative to the other," he says.
Weaker is worse
Last year's 7.1-magnitude earthquake was more than 10 times as strong as today's but caused no deaths, probably because it occurred at greater depth and further away from Christchurch: its epicentre was 70 kilometres west of the city. And the focus of September's quake was some 10 kilometres below ground – today's was half as deep.
"The ground motion [of the previous quake] had significantly attenuated by the time it reached Christchurch," says Adam Pascale, a seismologist atEnvironmental Systems & Services in Richmond, Australia.
Seismologists suggest today's quake occurred along the same fault line as last, a previously undetected fault line.
More to come?
The question now is: are more quakes likely? That all depends on whether the underlying rocks have settled into a position where they can absorb the stress from tectonic plate movements again. Gibson suspects this has already happened. "An earthquake of this magnitude does a good job of releasing that stress," he says. Although it is possible that other fault lines in the region might burst, "it's unlikely".
New Zealand sits on the tectonic boundary between the Pacific plate and the Australia-India plate. Christchurch is not on that frontier, but it is near to related secondary faults such as the Alpine fault, which runs along the spine of the South Island.
The magnitude 6.3 earthquake that devastated Christchurch was strong enough to shake 30 million tonnes of ice loose from Tasman Glacier at Aoraki Mt Cook National Park.
Passengers of two explorer boats were hit with waves of up to 3.5 metres as the ice crashed into Terminal Lake under the Tasman Glacier at the mountain.
Aoraki Mount Cook Alpine Village tourism manager Denis Callesen said huge icebergs formed in the lake, which were then rocked by massive waves for 30 minutes.
The danger to passengers could have been worse if not for safety proceedures put in place when it was suspected the area was becoming unstable, he said.
"We have procedures to deal with this type of event and for some time have stayed 800 metres away from the Terminal Face as we suspected it was becoming unstable," he said.
It was the third biggest amount of ice to fall in the history of the Terminal Lake, which is now over six kilometres long and two kilomtres wide in places, said Mr Callesen.
He said today's aftershock was the first quake felt at the Aoraki Mt Cook since the major quake last September.
Sources: Newscientist.com and Nzherald.co.nz