Deadly factors combined to intensify scale of devastation
The devastation caused by the earthquake in Haiti was the result of a combination of deadly factors.
Its magnitude, shallowness -- just five miles below the surface -- and proximity to the capital combined to create a destructive force equivalent to half a million tonnes of high-explosive TNT.
Experts at the British Geological Survey (BGS) said the impact of the earthquake on Port-au-Prince placed it at the top of the intensity scale.
It is thought to be the most powerful earthquake to have hit the Caribbean republic in about 120 years.
Haiti sits on the boundary of two tectonic plates -- the Caribbean and the North American. This creates a system known as the Enriquillo-Plantain Garden Fault.
It is a minor, southern branch of the fault system covering the region where the latest earthquake occurred. These plates constantly "jostle and grind" together and eventually erupt in an earthquake.
Dr Roger Musson, a seismologist at the geological advisory body, said: "This fault has been locked for the past 250 years gradually accumulating stress, which has now been released in a single, large earthquake."
Notable seismic activity has been recorded on the island since 1751 when an earthquake with a magnitude of 7.3 struck at the same fault.
There was a further powerful quake in 1860. The last major activity was in 1887, with a recorded magnitude of 7.2.
The fault experiences a movement of about three-quarters of an inch per year as the Caribbean plate shifts eastward. This latest shift is thought to have moved the surface along the fault by more than three feet.
Edinburgh-based seismologist Brian Baptie said: "The combination of the earthquake's proximity to Port-au-Prince and its shallowness will result in the terrible damage we've seen. The more shallow an earthquake is, the greater the shaking at the surface."
Each year there are about 100 earthquakes across the world of a size which could cause serious damage, according to the BGS.
Mr Baptie added: "Earthquakes of this size always have aftershocks that can last for many weeks. These always punch above their weight, affecting buildings that have already been damaged and hampering relief efforts."
David Kerridge, head of earth hazards at the BGS, said: "With an earthquake of this size and the mountainous terrain, there is a strong possibility of landslides."