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Haiti quake: The worst of places for a big tremor
By Jonathan Amos
Science correspondent, BBC News
It was immediately obvious that Tuesday's quake in Haiti would be an appalling natural disaster.
This was a large tremor centred on an impoverished country with little recent experience or preparedness for such an event.
The buildings in the quake zones of major industrialised nations sit on damping systems that allow them to ride out tremors that not only shake them back and forth but also twist them in the same movement.
The simplest concrete structures in the capital of Port-au-Prince will have crumpled under the same strain.
Seismometers recorded a preliminary magnitude of 7.0 at 1653 local time (2153 GMT).
The epicentre's proximity to Port-au-Prince - 15km (10 miles) - and the focus (or depth) of just 8km (5 miles) will have ensured the destructive forces were at their most intense.
"Closeness to the surface is a major factor contributing to the severity of ground shaking caused by an earthquake of any given magnitude," said Dr David Rothery, a planetary scientist with the Open University, UK.
"Furthermore, shaking tends to be greatest directly above the source. In this case the epicentre was only 15 km from the centre of the capital, Port au Prince, which therefore suffered very heavily."
A series of strong aftershocks - more than 10 larger than Magnitude 5.0 - will have compounded the devastation.
In 1946, a M8.1 quake hit the Dominican Republic, which shares the island of Hispaniola, and produced a tsunami that killed almost 2,000 people.
The last major earthquake in this part of Haiti was 150 years ago.
How earthquakes happen
The greater geological setting is the northern boundary between the Caribbean and North America tectonic plates where vast slabs of the Earth's surface grind past each other in a horizontal motion.
The Caribbean plate appears to shift eastward with respect to the North America plate by about 20mm per year.
In the region where this quake occurred, this motion is accommodated across a number of strike-slip faults, the two main ones in Haiti being the Septentrional fault in the north and the Enriquillo-Plaintain Garden fault in the south.
Seismic data suggests Tuesday's quake was on the Enriquillo-Plaintain Garden fault.
"It's been locked solid for about the last 250 years," said Dr Roger Musson from the British Geological Survey (BGS).
"It's been gathering stress all that time as the plates move past each other, and it was really just a matter of time before it released all that energy.
The question was going to be whether it would release it all at once or in a series of smaller earthquakes," he told BBC News.
The US Geological Survey says the Enriquillo-Plaintain Garden fault has been the likely source of historical large tremors in 1860, 1770, 1761, 1751, 1684, 1673, and 1618.
Reports suggest that in this latest event, the surface along the fault may have been offset in places by a metre or so.
Haiti is used to weather-related disasters, of course. The Caribbean nation is often battered by hurricanes.
Haiti can at least call on the expertise and response systems put in place to deal with those emergencies, although the people who normally co-ordinate that effort will be reeling with the rest of the population
By Jonathan Amos
Science correspondent, BBC News
It was immediately obvious that Tuesday's quake in Haiti would be an appalling natural disaster.
This was a large tremor centred on an impoverished country with little recent experience or preparedness for such an event.
The buildings in the quake zones of major industrialised nations sit on damping systems that allow them to ride out tremors that not only shake them back and forth but also twist them in the same movement.
The simplest concrete structures in the capital of Port-au-Prince will have crumpled under the same strain.
Seismometers recorded a preliminary magnitude of 7.0 at 1653 local time (2153 GMT).
The epicentre's proximity to Port-au-Prince - 15km (10 miles) - and the focus (or depth) of just 8km (5 miles) will have ensured the destructive forces were at their most intense.
"Closeness to the surface is a major factor contributing to the severity of ground shaking caused by an earthquake of any given magnitude," said Dr David Rothery, a planetary scientist with the Open University, UK.
"Furthermore, shaking tends to be greatest directly above the source. In this case the epicentre was only 15 km from the centre of the capital, Port au Prince, which therefore suffered very heavily."
A series of strong aftershocks - more than 10 larger than Magnitude 5.0 - will have compounded the devastation.
In 1946, a M8.1 quake hit the Dominican Republic, which shares the island of Hispaniola, and produced a tsunami that killed almost 2,000 people.
The last major earthquake in this part of Haiti was 150 years ago.
How earthquakes happen
The greater geological setting is the northern boundary between the Caribbean and North America tectonic plates where vast slabs of the Earth's surface grind past each other in a horizontal motion.
The Caribbean plate appears to shift eastward with respect to the North America plate by about 20mm per year.
In the region where this quake occurred, this motion is accommodated across a number of strike-slip faults, the two main ones in Haiti being the Septentrional fault in the north and the Enriquillo-Plaintain Garden fault in the south.
Seismic data suggests Tuesday's quake was on the Enriquillo-Plaintain Garden fault.
"It's been locked solid for about the last 250 years," said Dr Roger Musson from the British Geological Survey (BGS).
"It's been gathering stress all that time as the plates move past each other, and it was really just a matter of time before it released all that energy.
The question was going to be whether it would release it all at once or in a series of smaller earthquakes," he told BBC News.
The US Geological Survey says the Enriquillo-Plaintain Garden fault has been the likely source of historical large tremors in 1860, 1770, 1761, 1751, 1684, 1673, and 1618.
Reports suggest that in this latest event, the surface along the fault may have been offset in places by a metre or so.
Haiti is used to weather-related disasters, of course. The Caribbean nation is often battered by hurricanes.
Haiti can at least call on the expertise and response systems put in place to deal with those emergencies, although the people who normally co-ordinate that effort will be reeling with the rest of the population
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