Haitian quake displays geology's reach, power


POSTED: Friday, January 15, 2010

The earthquake that devastated Haiti and its capital, Port-au-Prince, on Tuesday is a stern reminder that there are many regions of Earth that are geologically alive.

This region lies on a transform fault similar to the San Andreas but has not recorded a major earthquake in 200 years.

The combination of a magnitude of 7.0, a shallow depth of only 6 miles and an epicenter less than 10 miles from Port-au-Prince virtually destroyed a city of buildings that were not engineered to withstand shaking.

Here the tiny Caribbean Plate slides eastward, grinding against the much larger North American Plate along a boundary that includes a curving arc of subduction that sweeps from Puerto Rico to Venezuela through the volcanic islands of the Lesser Antilles.

The plate movement here is slow by comparison, less than 1 inch per year compared with more than 6 inches per year in some parts of the Pacific.

The Caribbean Plate covers only a few thousand square miles. In comparison, the Pacific Plate covers nearly half the planet. The Caribbean Plate is roughly rectangular, bordered on the east by a subduction trench and volcanoes, on the north and south by transform faults. To the west it loses its identity under the Yucatan of Mexico.

The Caribbean Plate is apparently a piece of the Pacific Plate that got isolated as North and South America moved together over the past 120 million years or so, before the volcanoes along the isthmus of Central America arose 100 million years ago.

The Caribbean is not usually thought of as a geologically active area, but the Lesser Antilles are volcanic islands. Their volcanoes are not often active, but when they are, they are devastating.

In July 1995 Soufriere Hills Volcano on the island of Montserrat began erupting after lying dormant for all of recorded history. It eventually caused the evacuation of virtually one-half of the island.

One of the five most deadly eruptions ever recorded took place on the nearby island of Martinique on May 7, 1902.

The residents of St. Pierre on Martinique heard a shocking roar so loud it shook the land. Dual craters near the pinnacle of the Pelee volcano glowed bright red with a large cloud above the peak releasing flashes of bright lightning and thunder.

The next morning two massive jet-black clouds consisting of hot gas and ash raged out of Mount Pelee, one shooting straight into the air and the other aimed straight at St. Pierre. The glowing avalanche annihilated the entire town of 30,000 people in just a few minutes, the way that Vesuvius had engulfed and buried Pompeii nearly 2,000 years earlier.

The devastation of Port-au-Prince should remind us that there is no place on the planet that is completely safe. Geology is everywhere, and it does not care about us.