MELISSA BLOCK, host:
The Haiti quake may have come as a shock to the millions of people who are now suffering through its aftermath, but it was not a complete surprise to geologists. The large quakes in Haiti are rare. The country sits in the middle of an active seismic zone.
NPR's Richard Harris has that story.
RICHARD HARRIS: If you live in California, you're used to occasional earthquakes reminding you that the big one could come at any time. The Caribbean is considerably quieter than that, but Carol Prentice at the U.S. Geological Survey says don't be fooled.
Dr. CAROL PRENTICE (Western Region Earthquake Hazards Team, U.S. Geological Survey): Well, it definitely is earthquake country. This is a plate boundary between the North American Plate and the Caribbean Plate.
HARRIS: The Earth's tectonic plates are forever in motion. And in this case, the Caribbean plate is gradually moving east in relation to the North American plate. The movement may seem slow, about an inch a year, but the edges of those plates are locked together so they don't move all the time. Instead, stress builds up in fault zones and one of those runs very close to Port-au-Prince.
Dr. PRENTICE: You can see it on Google Earth. It's not that hard to see. It had a very clear signature through the landscape in the area.
HARRIS: So lots of stress, a big fault, to Prentice the implications are obvious.
Dr. PRENTICE: It's not a surprise to have a big earthquake along this fault.
HARRIS: What was a surprise, as is almost always the case with earthquakes, is the timing. Geoff Abers is at Columbia University's Lamont-Doherty Earth Observatory.
Dr. GEOFFREY ABERS (Senior Research Scientist, Lamont-Doherty Earth Observatory, Columbia University): This is a fault segment that has been really quiet for about the last 200 years. There's one earthquake in 1860s, but most of the larger known earthquakes are back in the 1700s.
HARRIS: So when the rock in the fault finally gave way and let off the strain that had been accumulating for more than a century, the resulting earthquake was very powerful. The ground shook for tens of seconds and the fault gave way over a stretch that's currently estimated around 30 to 60 miles.
Abers notes that the epicenter was not only very close to Port-au-Prince, the quake started at a fairly shallow depth, just six miles below the surface.
Dr. ABERS: By being shallow and on land, the strong shaking was, I think, fairly close to population centers here.
HARRIS: The quake had a preliminary magnitude of 7.0. And since then, there have been more than a dozen strong aftershocks above magnitude five.
Dr. ABERS: One of the concerns right now is there's a lot of structures that are badly damaged in the main shock, and they're now being exposed to these late aftershocks, some of which are quite significant.
HARRIS: The quake toppled many buildings that were not constructed to withstand such violent shaking. That includes buildings of brick and concrete that were not adequately reinforced with steel bars. Abers says there's also a lot of construction on Port-au-Prince's steep hillsides.
Dr. ABERS: These unstable slopes tend to fail in strong shaking. And, you know, that's also where a lot of the damage could happen.
HARRIS: Early reports do suggest that buildings tumbled down slopes and that landslides contributed to the death and destruction.
For the moment, all eyes are on the human tragedy in Haiti: the deaths, injuries, trapped people and devastation in one of the poorest countries in the world. And Carol Prentice, from the U.S. Geological Survey, knows this will not be the last major quake to hit the region. She's been studying a fault that runs just to the north of Haiti and cuts through the Dominican Republic.
Dr. PRENTICE: That fault, where we studied it, has not had a big earthquake for about 800 years and it's certainly due for a large earthquake as well.
HARRIS: She says the local governments are aware of that hazard, but she doesn't know how well prepared they would be to deal with it.
Richard Harris, NPR News.