'Risks remain' after Chile quake

Earthquake damage Damage was widespread, even near Santiago some 350km from the epicentre

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Central Chile still runs the risk of a large earthquake close to the site of last February's Magnitude 8.8 event, scientists say.

It was widely speculated at the time that the tremor would have released pressure on the fault running through the country's Maule region.

But a new study finds the strain has not been released markedly, and has in fact increased in places.

The Nature Geoscience report should aid preparedness, researchers say.

Maule's M8.8 event occurred at the converging boundary between the Nazca and South American tectonic plates. These vast slabs of the Earth's surface are grinding past each other at a rate of centimetres per year.

The quake's epicentre was about 100km from Concepcion city, just off shore and at a depth of about 35km.

It caused widespread damage to buildings and other infrastructure. The tremor also set off tsunami that inundated the coastline. More than 500 people are known to have been killed.

The quake occurred in what scientists referred to at the time as a "seismic gap" - a zone along the tectonic plate interface that had not experienced a major quake since the one witnessed by Charles Darwin in 1835 during his famous Beagle expedition.

The assumption had been therefore that Maule's M8.8 quake would have released the strain that had built up in the rocks over 175 years.

Chile quake map

But Stefano Lorito, of Italy's National Institute of Geophysics and Vulcanology, and colleagues say they have found a very different picture after studying the region closely.

They used a mixture of tsunami, satellite radar, and GPS data to assess where the rocks had slipped in the quake and where they remained tightly coupled.

"The vast majority of the energy released in the earthquake was to the north, overlapping the area of the 1928 earthquake, and further south, overlapping a portion of the 1960 earthquake," explained team-member Professor John McCloskey from the University of Ulster, UK.

Taca damage (AP) Scientists cannot predict quakes, but they can identify regions at risk of future tremors

"The area of the 'Darwin Gap' itself didn't experience very much slip, although the rupture did pass through there; and furthermore when you calculate the stresses that the earthquake generated, we discovered that places within the Darwin gap which were strongly coupled - i.e. they would have accumulated a lot of strain over the past nearly 200 years - have actually experienced an increase in stress, because the slip on them was so small that the interaction with the highly slipping places to the north and south has loaded them more than they were relaxed."

This study suggests that rather than making another earthquake less likely in the Darwin Gap, there is now actually an increased chance of a further big tremor in the area around the epicentre of the February 2010 event and slightly to the south.

Calculations indicate this tremor could be of M7.5 or even M8 - although the team believes an eight would be right at the top end. It is not possible to say when such earthquake might happen.

The researchers believe their study underlines yet again the complexity of plate behaviour and illustrates how unreliable seismic gaps can be in indicating future events.

"We need to study historical earthquakes in much more detail and then we'll start to be able to build up a picture. We'll never be able to predict earthquakes - I don't believe that's a realistic ambition in my lifetime," said Professor McCloskey. "But I think we will be able to identify hotspots in places like this that are likely to go next. And then you can focus your resources on preparation, in terms of building control and the like."

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