The chance of another big earthquake in Haiti in the near future is great enough that people in Port-au-Prince should sleep in tents — not even in buildings that survived the Jan. 12 quake apparently unscathed, geologists said Monday.

A report by the United States Geological Survey says the probability of an aftershock of magnitude 7 or greater in Haiti in the next 30 days is 3 percent, the probability of one magnitude 6 or greater is 25 percent, and of one magnitude 5 or greater is about 90 percent.

"Three percent may not sound big, but it is pretty big in terms of what we might have expected after a standard earthquake,'' said Dr. Tim Dixon, professor of geophysics at the Rosenstiel School of Marine and Atmospheric Science in Miami.

In a typical earthquake, the probability of another quake of the same magnitude that soon would be "vanishingly small, close to 0," he said.

He went on: "If people are in tents, they're quite safe. I would advise them not to move into buildings -- even ones that have been declared safe. And they shouldn't put up a tent beside a tall building."

In the complex terminology of earthquake measurements, a 6.0 earthquake is only 1/32nd as powerful as a 7.0 quake, and a 5.0 quake is 1/32nd as powerful as a 6.0.

But the USGS report warns: "Any aftershock above magnitude 5.0 will be widely felt and has the potential to cause additional damage, particularly to vulnerable, already damaged structures."

In the three weeks since the Jan. 12 quake of magnitude 7.0, Haiti has had 63 aftershocks, ranging from magnitude 4.0 to 5.9, the USGS says.

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