The New York City area is at "substantially greater" risk of earthquakes than previously thought, scientists said Thursday.

Damage could range from minor to major, with a rare but potentially powerful event killing people and costing billions of dollars in damage.

A pattern of subtle but active faults is known to exist in the region, and now new faults have been found. The scientists say that among other things, the Indian Point nuclear power plants, 24 miles north of the city, sit astride the previously unidentified intersection of two active seismic zones.

The findings are detailed in the Bulletin of the Seismological Society of America.

Shaky history

While earthquakes are typically thought of as a West Coast phenomenon in the nation, strong quakes do occur in the Eastern United States, just much less frequently. Importantly, the geology of the East — lots of hard rock leftover from glacial times — makes any rumbling travel a lot farther and with greater intensity from the epicenter.

A 5.0 temblor in 1737, for example, knocked down chimneys in New York City and was felt from Boston to Philadelphia. A magnitude-5.5 quake in 1884 did similar damage in a wider region around New York. Another quake in this range struck in 1783.

The new study involved an analysis of past quakes, plus 34 years of new data on temblors, most of them perceptible only by modern seismic instruments. The scientists looked at 383 earthquakes from 1677 to 2007 in a 15,000-square-mile area around New York City, using newspaper records in some cases to estimate temblor magnitudes.

"The evidence charts unseen but potentially powerful structures whose layout and dynamics are only now coming clearer," the scientists said. And even though eastern quakes are infrequent, the risk is high, because of the overwhelming concentration of people and infrastructure, said lead researcher Lynn R. Sykes of Columbia University's Lamont-Doherty Earth Observatory.

"The research raises the perception both of how common these events are, and, specifically, where they may occur," he said. "It's an extremely populated area with very large assets."

Based on history, the researchers say quakes at least 5.0 in magnitude should be expected, on average, about every 100 years.

"Today, with so many more buildings and people, a magnitude 5 centered below the city would be extremely attention-getting," said John Armbruster, also from the observatory. "We'd see billions in damage, with some brick buildings falling. People would probably be killed."

Even more serious quakes are possible. The scientists said that the fault lengths and stresses suggest magnitude-6 quakes, or even 7 — which would be 10 and 100 times bigger than magnitude 5 — are "quite possible." They calculate that magnitude-6 quakes take place in the area about every 670 years, and magnitude-7 temblors every 3,400 years.

Evidence mounts

Previous studies have hinted at the potential.

The New York City Area Consortium for Earthquake Loss Mitigation put the cost of theoretically possible large earthquakes in the metro New York area at $39 billion to $197 billion. A separate 2001 analysis for northern New Jersey's Bergen County estimated that a magnitude-7 event could destroy 14,000 buildings and damage 180,000 in that area alone.

The new study revealed a significant previously unknown active seismic zone running at least 25 miles from Stamford, Conn., to the Hudson Valley town of Peekskill, N.Y., where it passes less than a mile north of the Indian Point nuclear power plant. Several small quakes are clustered along its length. It is "probably capable of producing at least a magnitude-6 quake," the researchers said in a statement.

Many eastern quakes are not visible at the surface, so a large quake could hit from a fault no one even knows about.

"The probability is not zero, and the damage could be great," said study co-author Leonardo Seeber. "It could be like something out of a Greek myth."