Jan. 14, 2010— -- Everything went wrong in the great Haitian earthquake -- massive poverty, poor construction of buildings and the proximity of the epicenter to the crowded city of Port-au-Prince.
But there was also this: The center of the earthquake, the spot from which vibrations came, was only nine miles below the surface.
"If you had had an earthquake of similar strength 200 kilometers down [about 125 miles], we probably wouldn't be talking right now," said John Bellini of the National Earthquake Information Center, run by the U.S. Geological Survey in Golden, Colo. "The shallow earthquakes are the ones that do all the damage."
Haiti happens to be located right along the Enriquillo-Plantain Garden Fault, an east-west line between two of the great tectonic plates that support the earth's surface. This particular one is what geologists call a strike-slip fault, which means the two plates grind horizontally against each other.
Late on Tuesday afternoon, they gave way, violently. The earthquake had a magnitude of 7.0 -- "large but not huge," in the words of Jian Lin of the Woods Hole Oceanographic Institution.
But the epicenter -- the spot on a map where the earthquake occurs -- was only 10 miles from Port-au-Prince. And what made it worse was that the hypocenter -- the spot in the ground from which the vibrations spread -- was so shallow.
Earthquakes along the Plantain Garden Fault happen to be rare; the North American and Caribbean tectonic plates are, in effect, locked together, and Haiti probably hasn't had an earthquake this violent since 1770.
But strike-slip earthquakes, by their very nature, tend to be near the earth's surface. "This really wasn't unusually shallow for this type of quake," Bellini said.
Harley Benz of the Geological Survey said, "There was a much stronger earthquake -- with a magnitude of 7.4 -- in 2007, near the island of Martinique. It didn't do much damage because it was 90 miles deep."
Port-au-Prince was vulnerable in three ways, scientists say:
Probably most important, Haiti's rampant poverty meant the people in the disaster zone were unprotected. A strike-slip earthquake causes sideways vibration -- and the cinder block buildings common in Port-au-Prince were, in Benz' words, "brittle."