Where can you find geological information on the Haitian earthquake that occurred on January 12 2010?

Answer 1

Geological information on the Haitian earthquake that occurred on the 12th of January 2010 at 21:53:10 GMT / 16:53:10 local time is available from both the US Geological Survey (USGS) and the British Geological Survey (BGS). Other information is available from a number of news agencies.

What is the global and regional tectonic setting of Haiti?

On a global tectonic scale, Haiti is located near the northern boundary of the Caribbean plate, close to where it meets the North American plate. In this area, the Caribbean plate is moving eastwards by about 20 mm/year relative to the North American plate.

On a subregional tectonic scale, Haiti is situated on the eastern end of of a smaller plate that separates the Caribbean and the North American plates known as the Gonave micro-plate. This micro-plate and hence Haiti is bounded to the north by the Oriente and Septentrional Fracture Zones and to the south by the Enriquillo-Plantain Garden fault which are both transform faults (very similar to the San Andreas Fault in the USA).

What caused the earthquake?

The Enriquillo-Plantain Garden fault has been locked for approximately 250 years. As such, stresses have accumulated in the earth's crust over this time, causing energy to be stored in the form of elastic strain (like that in a compressed spring). Ultimately this stress has exceeded the shear strength of the crust in the fault zone causing a sudden brittle failure or rupture.

This in turn causes movement and a sudden release of the stored elastic strain energy in the form of seismic waves (as well as heat and sound).

The rupture of the fault zone was approximately 65 km (40 miles) in length with an average slip of 1.8 m (5.9 ft). Analysis of seismometer data showed that the seismic waves produced by the movement on the fault had amplitudes of up to 4 m (13 ft).

Where exactly did the earthquake occur?

The earthquake's epicentre (the point on the surface directly above the earthquake) was 25 km WSW of the Haitian capital city, Port-Au-Prince.

According to the USGS the exact co-ordinates were 18.457°N, 72.533°W.

How deep was the earthquake?

The hypocentre was 13 km below the Earth's surface.

How powerful was the earthquake?

The earthquake had a magnitude of 7.0 (on the Moment Magnitude Scale) and a severity of Extreme (X) on the Modified Mercalli (MM) scale.

Were there any aftershocks?

The US Geological survey report that (as of the 5th of February, 2010) there have been 59 aftershocks of magnitude 4.5 or greater. Sixteen of which had magnitudes of 5.0 or above.

The two largest aftershocks had a magnitude of 6.0 and 5.9. The magnitude 6.0 aftershock occurred 7 minutes after the main earthquake on the 12th of January and the magnitude 5.9 aftershock occurred at 11:03 GMT on the 20th of January.

Where did the Haiti earthquake hit with the greatest intensity?

The USGS indicate that the highest intensity (perceived shaking) with a level of 10 (X) on the Modified Mercalli Intensity Scale occurred in Grand Goave (population 5000). The Haitian capital Port-au-Prince suffered an intensity 9 (IX) event. The scale indicates that at this intensity structures will suffer heavy to very heavy damage.

What area of Haiti was affected by the earthquake?

The whole of Haiti was affected to some extent by the earthquake with the whole country experiencing at a minimum a moderate or level V event on the Modified Mercalli Intensity scale which is capable of causing damage to buildings.

A significant number of the largest cities in Haiti were exposed to level 9 intensity events which are described as being capable of causing severe damage to structures. These cities included the Haitian capital, Port-Au-Prince with a population of 1.235 million people, Carrefour with a population of 442 thousand and Delmas with a population of 383 thousand.

Why was the earthquake so destructive?

The earthquake was so destructive for a number of reasons:

• The earthquake occurred at shallow depth - this means that the seismic waves have to travel a smaller distance through the earth to reach the surface so maintain more of their energy in particular this meant that the surface waves (which cause the vast majority of the damage during an earthquake ans which are the most rapidly attenuated and and have significant reductions in amplitude as the depth of the earthquakes focus increases) produced by the earthquake maintained a large amount of their energy maximising the damage they cause.
• The earthquake occurred in close proximity to a populated area. This again means that there is very little attenuation of the seismic waves (i.e they still have most of their energy and so are more destructive when they arrive).
• The buildings in Port-Au-Prince and other areas of Haiti were in very poor condition in general and were not designed or constructed to be earthquake resistant.

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