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:
The earthquake so was close to the city.
It was shallow, probably rupturing the surface above it.
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."
"Buildings -- designed to withstand gravity -- have to be built to withstand lateral motion," said Art Lerner-Lam, head of the seismology division at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y. "This was a direct hit on one of the poorest places."
Could it happen here? It has, often, most notably along California's San Andreas Fault -- a strike-slip fault like Haiti's. The Loma Prieta earthquake near San Francisco in 1989 was stronger than Tuesday's, with a magnitude of 7.1, but 63 people died instead of the thousands believed to have been killed in Haiti after Tuesday's disaster.
"In California, you would get a lot of damage right along the fault," Lerner-Lam said, "but people are so sensitized to earthquakes that they plan for it, and build accordingly."
Earthquake-resistant buildings are often designed to bend instead of breaking. They sway in an earthquake instead of crumbling.
But that kind of building only happens in relatively wealthy countries that can afford it. Haiti, with annual per capita income estimated by the Agency for International Development at less than $400, clearly cannot.
"What you're seeing in Haiti," Lerner-Lam said, "is a whole country destroyed."