Chile earthquake | Magnitude 8.8 | 27 Feb 2010
The magnitude 8.8 earthquake that struck Chile on 27 February was more than 500 times bigger than the devastating magnitude 7.0 earthquake that hit Haiti in January 2010.
'The slip distribution in the Haiti earthquake was concentrated near the capital so it was almost like a 'direct hit' on a city of more than 2 million people living in poorly constructed buildings.
In the Chilean earthquake most of the slip was distributed over around 300km releasing much of the energy that caused violent shaking throughout Chile.'
Dr David Kerridge, Head of Earth Hazards, BGS.
The magnitude 8.8 earthquake occurred on the boundary between the South American and Nazca tectonic plates that runs down the west coast of Chile and has also resulted in the formation of the Andes.
The adjacent diagram shows how the earthquakes map out the subducting Nazca plate as it dips down under South America.
Nazca plate
The Nazca plate is moving east at around 8 cm/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 a since 1835, when a large earthquake was observed by Charles Darwin. This meant that 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 over 1000 megatons of TNT.
700 km rupture
The earthquake epicentre was around 100 km north east of the city of Concepcion and 300 km south west of Santiago. However, the earthquake rupture was over 700 km long, as mapped out by the aftershocks with slips of almost 10 metres on the fault plane.
'The Haiti earthquake ruptured the plate boundary over a distance similar to that between Edinburgh and Glasgow. For the Chile earthquake the length of the rupture was more than the distance from Edinburgh to London.'
Dr David Kerridge, Head of Earth Hazards, BGS.
The rupture occurred on the segment of the plate boundary just north of the rupture from the 1960 earthquake. Large aftershocks are always associated with earthquakes of this magnitude. The largest so far has had a magnitude of 7.0.
Tsunami
Earthquakes like this one, which occur under the ocean, uplift the sea-bed and displace huge volumes of water, causing giant waves or tsunami that spread out from the epicentre like ripples on a pond. In the deep ocean, tsunami travel at several hundreds of miles per hour, about the same speed as a jet plane. This means the wave caused by an earthquake of the coast of Chile take over ten 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.
Many oceanic islands across the world have long histories of devastating tsunami that result from earthquakes thousands of kilometres away. The magnitude 9.5 earthquake that struck Chile in 1960, unleashed a devastating tsunami that travelled across the Pacific, reaching Japan over twenty hours later and killing over two hundred people.
South Atlantic magnetometers shaken
As part of the National Geomagnetism service, the BGS operates two magnetic observatories in the South Atlantic, one at Port Stanley in the Falkland Islands and one on Ascension Island.
The magnetometers, which record variations in three components, sit on concrete pillars which shook as the seismic waves arrived. The adjacent diagrams below show how the instrument readings were affected as the sensors were shaken about in the Earth's magnetic field at 06:40 UT on Falkland Island and at 06:45 UT on Ascension Island.
The Earth's magnetic field wasn't changed by the earthquake! These records show that the seismic waves took six minutes to arrive at the Falkland Island and 11 minutes to reach Ascension Island which are 2117 km and 6650 km away from the earthquake epicentre, respectively.
Contact
Contact Brian Baptie for further information.
