New Zealand quakes likely linked
The latest earthquake near the New Zealand city of Christchurch is thought to have occurred along the same fault line that shook the city late last year, say Australian experts.
Despite being smaller in magnitude than last September's quake, it caused more damage because the epicentre was shallower and closer to the city centre.
According to Trevor Allen, an earthquake hazards expert at Geoscience Australia, the epicentre of the magnitude-6.3 quake was 10 kilometres southeast of Christchurch.
"In hazard terms that was almost a direct hit on the city."
While it's too early to confirm, Allen says, the quake may be on the same fault line that caused last September's quake.
"What we think has happened is the stress has been transferred from the [September] quake to a different segment of possibly the same fault," Allen says.
He says it's not uncommon for a large earthquake to be followed by other large earthquakes close by.
"For example, after the 2004 Boxing Day tsunami there was a series of very large earthquakes that occurred essentially like a zipping along a particular fault line. This could be a similar sort of thing we are seeing here, just on a much smaller scale."
Shallow quake caused more damage
Early reports indicate the damage caused by Tuesday's quake is much greater than the magnitude-7.1 event that occurred last September.
Professor James Goff from the Natural Hazards Research Laboratory at the University of New South Wales says the shallow depth of the quake would have contributed to the increased level of damage.
"It was very shallow, only four kilometres deep, and therefore a lot more shaking was involved," says Goff. The hypocentre of the September 2010 quake was located 50 kilometres west of Christchurch at a depth of 10 kilometres.
He says structural damage caused by the September quake may have also played a role in more buildings collapsing.
"To a certain extent this would have had a more serious impact this time because it's not as resistant as it could have been."
He says the removal of buildings damaged by the September quake may have made others weaker in the process.
New Zealand, which sits between the Pacific and Indo-Australian tectonic plates, records on average more than 14,000 earthquakes a year, of which about 20 would normally top magnitude 5.0.
But research has mainly focused on the more active areas, says Goff, such as the Southern Alps and the east coast of the North Island.
"I think through no fault of their own, scientists have been blindsided a bit by this. [We've been] looking in areas where probably it was easier to look, because we knew there were big faults and earthquakes that had happened in the past," he says.
Allen agrees. "If we don't know there is an active fault ... then we generally won't spend time and effort looking at it in detail. Being able to map every single one is going to be a time consuming job and each one has the capability of generating large earthquakes."
According to Goff there has been an increased level of earthquake research in the Christchurch region since the September quake.
"We are just starting to find out a little more about how seismically active or how many faults Christchurch has."
"Now it's most definitely a focus of fairly intensive research."