Haiti Earthquake

            The 7.0 magnitude earthquake that struck Haiti at 4:53 p.m. on Tuesday, January 12, was the most severe and the most destructive in that nation’s history.  The full extent of damage and loss of life is not yet known, but reports indicate many thousands dead, injured, and missing, plus hundreds of buildings completely destroyed.  Aid is being rushed to the island by the United States, by European, Asian, and Latin American nations, and by non- governmental agencies.

            The earthquake’s epicenter was 10 miles west of Port au Prince at a depth of 5 miles.  The epicenter’s shallowness and proximity to the densely populated city of 2,000,000 are the elements that made the quake so catastrophic.

             The Haiti earthquake occurred on the Enrequillo-Plantain Garden fault line, a transform, or strike-slip, fault that runs for 1,000 miles from Jamaica to the middle of the island of Hispaniola, which is shared by Haiti and the Dominican Republic.  This fault line running east-west marks the boundary between the Caribbean plate and the North American Plate.

A fault is called strike-slip when two tectonic plates meet and move horizontally in opposite directions.  In this case, the North American Plate is moving west and the Caribbean Plate is moving east at the rate of approximately one inch a year.  However, plate movement does not occur at a steady rate.  The friction of one plate pushing against the other can cause the plates to stick, and arrest plate movement.  When this happens, stress on the fault line builds up.  The last major earthquake on this fault line occurred in 1760, so pressure has been building for 250 years.  The fault stress level finally exceeded the strain threshold and a brief episode of violent slippage occurred, resulting in the magnitude 7.0 quake.

          Strike-slip fault earthquakes do not usually produce tsunamis, and none was evident in this case.  However, to the east of this fault line, the Lesser Antilles Volcanic Arc lies on the subduction zone that separates the Caribbean Plate and Atlantic Plate.  This volcanic chain that runs from the Virgin Islands on the north to Grenada on the south includes such famous volcanoes as Mt. Pele and Montserrat.  A fault line rupture along this subduction zone, where the Atlantic Plate is sliding under the Caribbean Plate, would be much more likely to produce a tsunami than the fault slippage that produced the Haiti earthquake.