Brian Baptie: This was 500 times more powerful than Haiti tremor

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The Independent Online

The magnitude 8.8 earthquake that struck Chile was more than 500 times bigger than the devastating magnitude 7 earthquake that hit Haiti last month and resulted in at least 200,000 deaths. History has shown that the biggest earthquakes aren't always the deadliest. The world's deadliest earthquakes tend to occur in places such as China, Japan and central and south-east Asia, where population densities are high. On the other hand, many of the world's largest earthquakes, with magnitudes of nine and above, occur in places such as Alaska or Kamchatka.

The earthquake occurred on the boundary between the South American and Nazca tectonic plates which runs down the west coast of Chile and has also resulted in the formation of the Andes. The Nazca plate is moving east at about 8cm a year and is being thrust down beneath the South American plate, causing earthquakes all along the boundary where the two plates slide past each other.

Although Chile has a long history of very large earthquakes, no previous large earthquake had occurred on this section of the plate boundary since 1835, when one was observed by Charles Darwin. This meant that a large amount of strain had been stored up over hundreds of years before being released over a few tens of seconds. The energy released was equivalent to more than 1,000 megatons of TNT.

Earthquakes occur on faults deep inside the earth, where strain accumulates over tens or hundreds of years, gradually deforming the rocks on either side of the fault until the rocks suddenly rupture. Long ruptures with large amounts of slip result in bigger earthquakes. Great earthquakes of magnitude 8 and above can have fault ruptures that are hundreds of kilometres long and the slip on these rupture may be tens of metres.

Seismologists use the amplitude of the seismic waves that spread out from the epicentre and are recorded all around the world to determine the size of an earthquake using a magnitude scale. The original magnitude scale was developed by Charles Richter in 1935.

Earthquakes like this one, which occur under the ocean, lift the seabed and displace huge volumes of water, causing giant waves or tsunami that spread out from the epicentre. In the deep ocean, tsunami travel at several hundreds of miles per hour, about the same speed as a plane. This means the wave caused by an earthquake off the coast of Chile takes more than 10 hours to cross the Pacific. Also, in deep water, the wave amplitudes are quite small, so that a ship at sea might not even notice it. But as it approaches the coast, the wave slows down due to the changing water depth, causing the amplitude of the wave to increase. This means that islands such as Hawaii, which rise suddenly in the middle of the ocean, can be greatly affected by tsunami, whereas areas of more gently shelving coastline can be less dramatically affected.

Dr Brian Baptie is the seismology project leader with the British Geological Survey

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