By Daniel Cressey

At 21:53 UTC yesterday, an earthquake of magnitude 7.0 struck the Caribbean nation of Haiti. The US Geological Survey (USGS) says that it was the most violent earthquake to strike the impoverished country in a century.Nature examines the causes and repercussions of the quake.

What caused this earthquake?

The island of Hispaniola, with Haiti on the western half and the Dominican Republic on the eastern half, lies on the northern edge of the Caribbean tectonic plate. The much larger North American plate moves westward relative to the Caribbean plate. There are two major faults between the plates at this point: the Septentrional fault system, which runs through northern Haiti, and the Enriquillo-Plaintain Garden fault system in the south.

This quake seems to have occurred on the Enriquillo-Plaintain Garden fault system, which accounts for nearly half of the overall movement between the Caribbean and North American plates -- around 7 millimetres per year, according to the USGS.

That system has not produced a quake of similar magnitude since 1751, says Richard Luckett, a seismologist at the British Geological Survey, which is headquartered near Nottingham, UK.

What damage did the earthquake cause?

The earthquake's epicentre was just 15 kilometres from Haiti's capital Port-Au-Prince, home to around 1 million people.

A number of deaths have already been reported and although communications networks were severely disrupted by the quake, some reports suggest that there are thousands of casualties. Many buildings have collapsed, including the Presidential Palace in Port-Au-Prince, and it is not clear if planes are able to land at the capital's airport to bring in supplies.

Alain Le Roy, the United Nations Under-Secretary-General for Peacekeeping Operations, told reporters that the headquarters of the UN Stabilization Mission in Haiti had collapsed. "We don't have any [casualty] figures for the time being," he said. "But we know clearly it is a tragedy for Haiti, and a tragedy for the UN."

Haiti has been heavily deforested so the quake is likely to have triggered landslides. The country's poverty might mean that it is ill-equipped to deal with any disease outbreaks caused by the quake.

How deep was the earthquake, and how does that affect the damage?

The damage caused by an earthquake is related to its magnitude and how close it is to densely populated areas. The USGS estimates the epicentre of the magnitude-7.0 quake at a depth of 10 kilometres. The Global Seismic Monitor system, based at the GFZ German Research Centre for Geosciences in Potsdam, puts it at 17 km deep and also slightly stronger at magnitude 7.2.

Either way, the quake is a relatively shallow one, and this makes it more dangerous. "It would do a lot more damage close up. We could easily be looking at 1 centimetre of ground movement," says Luckett.

The geographical proximity of the epicentre to the heavily populated Port-Au-Prince will also have added to the damage caused.

"In cases such as these, the real issue is construction standards," says Chris Rowan, a geologist at the University of Edinburgh, UK. "The reason this has been so devastating is probably it's a very crowded city with many poorly built houses."

A number of aftershocks have also been recorded, ranging from magnitude 5.9 to 4.2. There are concerns that these may cause buildings that are already damaged to collapse.

Did the earthquake trigger a tsunami?

As this earthquake took place on land, it would not be expected to generate a large tsunami. However, an initial tsunami warning was issued for local coasts within 100 kilometres of the epicentre at 22:03 UTC.

The Pacific Tsunami Warning Center in Ewa Beach, Hawaii, recorded a 12-centimetre wave at Santo Domingo in the Dominican Republic and decided that there was no threat to coastal areas away from the epicentre. The tsunami warning was subsequently cancelled at 23:45 UTC.

Warnings such as this are currently provided for the Caribbean by the centre because an independent system for the area is still being established. The UN Educational, Scientific and Cultural Organization (UNESCO) said in 2008 that it hoped such a system would be in place by this year.

Should this quake have been expected?

Accurately predicting when an earthquake will occur is still not possible. However, a team led by Paul Mann at the University of Texas at Austin has been monitoring this fault for some years.

In a presentation to the 18th Caribbean Geological Conference in 2008, the team pointed out that their models showed a slip rate of around 8 millimetres per year on the fault. In their abstract they warned that this, combined with the fact that the last known major earthquake near Haiti was in 1751, could add up to yield "~2 meters of accumulated strain deficit, or a Mw=7.2 earthquake if all is released in a single event today".

One of the team members, geophysicist Eric Calais of Purdue University in West Lafayette, Indiana, said in an e-mail to Nature: "Unfortunately we were pretty much right on."

Is it linked with other recent earthquakes?

Some media reports have suggested that this earthquake might be linked to other recent events in the Pacific, implying a chain of quakes in the area that 'jumped' across central America to hit Haiti.

There is some evidence that distant earthquakes might increase the risk of subsequent quakes around the world (see 'Past quakes cause future shocks'). At a local level, movement on one part of a fault may cause stress to build up in another part of a fault that does not move. However, in this case, there is currently no evidence that the Haiti event was triggered by a distant earthquake, and the idea that there is a linked chain of earthquakes stretching from Haiti to Indonesia is questionable.

"When it comes to what actually triggered the fault, there's no real way of us knowing," says Rowan.

UPDATED: Estimates of the number of people who may have died in the earthquake that devastated Haiti yesterday continue to climb. More "here":