Page last updated at 12:33 GMT, Saturday, 27 February 2010

Chile's long experience of quakes

By Jonathan Amos
Science correspondent, BBC News

Map shows ring of fire and tectonic plates

It is not possible to predict the time and magnitude of an earthquake, but certain places on the Earth know they are always at risk from big tremors. Chile is one of those places.

It lies on the "Ring of Fire", the line of frequent quakes and volcanic eruptions that circles virtually the entire Pacific rim.

The magnitude 8.8 event that struck the country at 0634GMT on Saturday occurred at the boundary between the Nazca and South American tectonic plates, just off shore and at a depth of about 35km (20 miles).

The biggest city close by is Concepcion, just over 100km to the south.

Collapsed buildings and widespread disruption will have been unavoidable.

Because the quake occurred below the sea floor, tsunamis were also generated, and alerts were issued not just for the Chilean coast but across the Pacific in general.

The Nazca and South American tectonic plates are vast slabs of the Earth's surface and grind past each other at a rate of about 80mm per year.

The Nazca plate, which makes up the Pacific Ocean floor in this region, is being pulled down and under the South American coast.

It makes the region one of the most seismically active on the globe.

Since 1973, there have been 13 events of magnitude 7.0 or greater.

Gap-filler

Saturday's shock had its epicentre some 230km north of the source of the magnitude 9.5 tremor of May, 1960 - the biggest instrumentally recorded earthquake in the world. Thousands died in that event.

And it was also about 870km south of the 1922 8.5 event which killed several hundred people in central Chile.

Saturday's tremor is therefore something of a gap-filler between two massive historical events.

French and Chilean seismologists had recently completed a study looking at the way the land was moving in response to the strain building up as a result of the tectonic collision. Their analysis suggested the area was ripe for a big quake.

"This earthquake fills in an identified seismic gap," Dr Roger Musson, who is the British Geological Survey's Head of Seismic Hazard, told BBC News.

"The last major earthquake that occurred in this area was in 1835. This was a famous earthquake observed by Charles Darwin during his voyage on the Beagle. This is a place where the stress has been gathering for 170 years, and finally it's gone in another earthquake that's repeated this famous historical quake."

As is nearly always the case, the region was hit by a series of aftershocks. In the two and a half hours following the 90-second 8.8 event, the US Geological Survey reported 11 aftershocks, of which five measured 6.0 or above.

People will no doubt reflect on the scale of this event and compare it with the recent devastation in Haiti which has claimed an estimated 230,000 lives.

Saturday's quake was more than 500 times more powerful than the one to hit Port-au-Prince in Haiti. But size is not in itself an indicator of the likely number of deaths.

One major factor which will limit the number of deaths in Chile will be its greater level of preparedness.

Both the Chilean authorities and the Chilean people are generally well versed in how to cope in such an emergency.

Severe shaking

The Chilean National Emergency Office (Onemi) is responsible for coordinating responses from services such as fire fighters, medical teams and civil defence.

The emergency response system is organised at national, regional and local level.

"Chile is a seismic country. So, we must be prepared!" is the message from Onemi.

The office provides advice on how to prepare for earthquakes and other disasters, and how to behave when one strikes.

Scientists say severe shaking is likely to have been experienced along a 300km stretch of coastline, including in important urban centres such as Concepcion, Arauco, Lota, and Constitucion.

The biggest city close to the epicentre is Concepcion, which forms part of the second largest conurbation in the country with a population of about one million.

It is the capital of Concepcion Province and the Bio Bio region, the name of the river that flows through it.

Concepcion's history has been marked by earthquakes. After a huge tremor in 1751, Concepcion was moved from its original site, currently the town of Penco, to a location further from the sea in the Mocha Valley.

Jonathan.Amos-INTERNET@bbc.co.uk



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