Falmouth (Global Adventures): The magnitude 7.0 earthquake that triggered disastrous destruction and mounting death tolls in Haiti occurred in a highly complex tangle of tectonic faults near the intersection of the Caribbean and North American crustal plates, according to a quake expert at the Woods Hole Oceanographic Institution (WHOI).

Jian Lin, a senior scientist in geology and geophysics, said that even though the quake was “large but not huge,” there were three factors that made it particularly devastating: First, it was centered just 10 miles southwest of the capital city, Port au Prince; second, the quake was shallow—only about 10-15 kilometers below the land’s surface; third, many homes and buildings in the economically poor country were not built to withstand such a force and collapsed or crumbled.

The quake struck on a 50-60-km stretch of the more than 500-km-long Enriquillo-Plantain Garden Fault, which runs generally east-west through Haiti, to the Dominican Republic to the east and Jamaica to the west.

It is a “strike-slip” fault, according to the U.S. Geological Survey, meaning the plates on either side of the fault line were sliding in opposite directions. In this case, the Caribbean Plate south of the fault line was sliding east and the smaller Gonave Plate north of the fault was sliding west.

But most of the time, the earth’s plates do not slide smoothly past one another. They stick in one spot for perhaps years or hundreds of years, until enough pressure builds along the fault and the landmasses suddenly jerk forward to relieve the pressure, releasing massive amounts of energy throughout the surrounding area. A similar, more familiar, scenario exists along California’s San Andreas Fault.

Such seismic areas “accumulate stresses all the time,” says Lin, who has extensively studied the nearby Septentrional Fault, which runs east-west at the northern side of the Hispaniola island that makes up Haiti and Dominican Republic. In 1946, an 8.1 magnitude quake, more than 30 times more powerful than this week’s quake, struck near the northeastern corner of the Hispaniola.

Compounding the problem is the fact that a wide zone between the Caribbean and North American plates is made up of a patchwork of smaller “block” plates that make it difficult to assess the forces in the region and how they interact with one another. “If you live in adjacent areas, such as the Dominican Republic, Jamaica and Puerto Rico, you are surrounded by faults,” says Lin.

Some residents of earthquake zones know that after the quake’s faster, but smaller, primary, or “p” wave hits, there is usually a few-second-to-one-minute wait until a larger, more powerful surface, or “s” wave strikes, Lin says. P waves come first but have smaller amplitudes and are less destructive; S waves, though slower, are larger in amplitude and, hence, more destructive.

The Haiti quake did not trigger an extreme ocean wave such as a Tsunami, partly because it was large but not huge and was centered under land rather than the sea. Aftershocks, some of them significant, can be expected in the coming weeks, months, and years. But now that the stress has been relieved along that 50-60-km portion of the Enriquillo-Plantain Garden Fault, Lin says this particular fault patch should not experience another quake of equal or greater magnitude for perhaps 100 years.

However, the other nine-tenths of that fault and the myriad networks of faults throughout the Caribbean are “active.”

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