The geology behind the Haiti earthquake

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Haiti Earthquake | Photo 05

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On 12th January 2010, Haiti experienced one of the most devastating earthquakes known to mankind. The death toll, according to initial estimates, may be anywhere between 100,000 to 200,000. As of now, the main concern is rescue and relief operations. It will take more time to establish the extent of devastation caused to property and infrastructure.

Haiti has traditionally been an earthquake prone region and has the misfortune of having been directly affected by several devastating earthquakes in the past. The earliest reported earthquake in October 1751 was so powerful that, according to French historian Moreau de Saint-Méry, “only one masonry building had not collapsed” in Port-au-Prince. This was followed in 1770 by a 7.5 magnitude earthquake. The 1842 quake killed 10,000 people. In 1946, a magnitude 8.0 earthquake in nearby Dominican Republic resulted in massive tsunamis that killed over 1700 people.

What accounts for the regularity and magnitude of earthquakes in Haiti? Haiti is situated on the Hispaniola island, which rests at a place where two giant pieces of the earth's crust, the Caribbean and North American tectonic plates, grind against each other as they pass in opposite directions.

In geological terms, the boundary between two tectonic plates that pass horizontally in opposite directions is called a Transform Boundary. Plate movement at transform boundaries gives rise to strike-slip faults, smaller features where the movement of rocks is in opposite directions along the fault.

The San Andreas fault in California is one of the best examples of a transform boundary manifesting in a strike-slip fault. Along this boundary, the Pacific plate and the North American plate move in opposite directions, making the region one of the most seismically active in the world.

Along a strike-slip fault, rocks move in opposite directions and interlock each other. A large amount of stress is built up as the rocks compress each other. Even though rocks are highly elastic in nature, there is a limit beyond which rocks break, much like a rubber band that breaks if stretched too much. When this occurs, the large amount of stress that had been built up gets released in a few seconds or minutes with devastating effects.

This stress is released in the form of seismic waves that pass throughout the earth and also appear on the earth's surface. When the waves reach the surface, vigorous shaking is caused resulting in an earthquake. Larger the amount of stress build up, more powerful the earthquake.

Strike-slip faults give rise to devastating earthquakes because the focus, the point beneath the crust where the “break” occurs, lies closer to the surface with the result that when the seismic waves reach the surface, a larger portion of their energy is retained.

In Haiti's case, things are slightly more complex. The boundary between these two tectonic plates is formed by a complex system of strike-slip faults involving the Enriquillo-Plantain Garden fault zone and the Septentrional-Orient fault zone.

According to geologists, the latest quake was caused when the Enriquillo-Plantain Garden fault zone, that gathered stress for a long period of 250 years, gave away. The earthquake measured 7.0 on the Richter scale, which corresponds to an energy of 32 megatons of TNT. The quake was followed by 33 aftershocks, 14 of them above magnitude 5.0.

At the end of the day, it was only a minor realignment of the earth's rocks. However, the extent of damage caused to life, property and infrastructure overwhelms imagination. Such is the magnitude of the earth's geological processes.