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What caused killer quake?

The Caribbean is riddled with a complex network of tectonic fault lines resulting from the movement of the Caribbean plate.

It is slipping eastwards at a rate of about 2cm a year, relative to the vast North American plate 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 seven 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" said planetary scientist David Rothery.

Experts warned that earthquakes of this size always have aftershocks that can last for many weeks.