The Caribbean is riddled with a complex network of tectonic fault lines resulting from the movement of the Caribbean plate which is slipping eastwards at a rate of about 2cm a year, relative to the vast North American plate further to the north.
The boundary of these two tectonic plates lies off the north coast of Haiti but there are several fault-line systems to the south that cut across the country from east to west. It was the sudden "strike-slip" movement of one of these fault lines, the Enriquillo-Plantain Garden fault zone, that led to the disaster.
Scientists calculate that the epicentre of the earthquake, which measured 7 on the Richter scale, was approximately nine miles south-west of Haiti's capital, Port-au-Prince. The point beneath the Earth's surface where the rupture began – its hypocentre – was just six miles away, making it a relatively shallow shock.
"Closeness to the surface is a major factor contributing to the severity of ground shaking caused by an earthquake of any given magnitude," said David Rothery, a planetary scientist at the Open University. "Furthermore, shaking tends to be greatest directly above the source. In this case, the epicentre was only 15km from the centre of Port-au-Prince, which therefore suffered very heavily."
The seismic waves from the initial shock took about 10 minutes to reach the Edinburgh monitoring station of the British Geological Survey (BGS). Scientists there said yesterday that it was the largest earthquake to affect this area of the Caribbean for more than two centuries.
"The situation in Haiti is similar to the San Andreas Fault in California, in that two plates are sliding past one another. The fault in this case is called the Enriquillo-Plantain Garden fault. This fault has been locked for the last 250 years, gradually accumulating stress which has now been released in a single large earthquake," said Roger Musson, a BGS seismologist. "The rupture occurred over a distance of some 17km and the ground probably moved more than a metre. It was a shallow shock and it probably resulted in the rupture of the whole of the Earth's crust above it.
"Most people in Port-au-Prince probably didn't realise they live in an earthquake zone and earthquake safety was not on their list of priorities because the last one there was not within living memory."
Haiti suffered a number of earthquakes in the 18th century: in 1701, 1751 and 1770. The 1751 earthquake, which was probably about 10 times bigger than the current shock, destroyed the recently founded Port-au-Prince and caused severe damage to buildings in neighbouring areas.
According to the US Geological Survey (USGS), the Enriquillo-Plantain Garden fault accommodates a movement of about 7mm a year, nearly half the rate of movement along the boundary of the North American and Caribbean tectonic plates. The USGS has measured more than 30 aftershocks.
"Earthquakes of this size always have aftershocks that can last for many weeks," said Brian Baptie, a seismologist at the BGS. "These always punch above their weight, affecting buildings that have already been damaged and hampering relief efforts."
Landslides are another hazard as they can block roads and cause devastation to the flimsy homes of mountain villages and shanty towns built on hillsides around the capital, said David Kerridge, head of earth hazards at the BGS.
Professor Roger Searle, an earth scientist at Durham University, said that the earthquake released the energy equivalent of half a million tons of TNT but its impact would have been much greater than in developed countries because of poor building standards.
Deadliest earthquakes since 1900
28 July 1976 A 7.5 Richter scale quake struck the city of Tangshan in north-east China, claiming at least 250,000 lives, believed to be the highest earthquake death toll of the 20th century.
26 December 2004 The Boxing Day tsunami was triggered by an earthquake measuring 9.2 on the Richter scale. Some 230,000 people died, across 14 countries.
16 December 1920 A 7.8 quake killed up to 200,000 people in central China's Haiyuan County.
12 May 2008 Up to 87,000 people were killed in China's south-western Sichuan province when a quake measuring 7.8 struck 50 miles from the provincial capital Chengdu, less than a week after Cyclone Nargis struck Burma, killing 146,000.
8 October 2005 An earthquake measuring 7.6 struck northern Pakistan and the disputed Kashmir region, killing 73,000.
31 May 1970 A quake in the Peruvian Andes triggered a landslide, burying the town of Yungay and killing 66,000 people.
21 June 1990 Around 40,000 people died in a tremor in Gilan province, north Iran.
26 December 2003 More than 26,000 lives were claimed when an earthquake destroyed the historic city of Bam in southern Iran.
7 December 1988 An earthquake, 6.9 on the scale, struck north-west Armenia, killing 25,000.
17 August 1999 A magnitude 7.4 earthquake hit Izmit and Istanbul in Turkey; 17,000 died.
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