The risk of earthquakes to New York City is substantially greater than formerly believed, seismologists said today.
Among other things, new research has found that the Indian Point nuclear power plants, 24 miles north of the city, sit astride the previously unidentified intersection of two active seismic zones.
The scientists are based at Columbia University’s Lamont-Doherty Earth Observatory, which runs the network of seismometers that monitors most of the northeastern United States. Their findings were published in the current issue of the Bulletin of the Seismological Society of America.
The researchers analyzed past quakes plus new data on tremors, most of them perceptible only by instruments, to produce “evidence of unseen but potentially powerful geological structures whose layout and dynamics are only now coming clearer.”
Space view of New York City by NASA
Lead author Lynn R. Sykes said the data show that large quakes are infrequent around New York compared to more active areas like California and Japan, but that the risk is high, because of the overwhelming concentration of people and infrastructure.
Combing newspaper accounts and other records, the seismologists compiled a catalog of all 383 known earthquakes from 1677 to 2007 in a 15,000-square-mile area around New York City.
They found that magnitude 5 quakes–strong enough to cause damage–occurred in 1737, 1783, and 1884. Based on this, such quakes should be routinely expected, on average, about every 100 years, they estimate.
“Today, with so many more buildings and people, a magnitude 5 centered below the city would be extremely attention-getting,” said John Armbruster, a co-author of the research paper. “We’d see billions in damage, with some brick buildings falling. People would probably be killed.”
Starting in the early 1970s, data on quakes was collected from dozens of seismometers newly deployed in the New York area. The data have revealed distinct zones where earthquakes concentrate, and where larger ones could come.
The network has located hundreds of small events, including a magnitude 3 every few years, which can be felt by people at the surface, but is unlikely to cause damage, the researchers said.
These small quakes tend to cluster along a series of small, old faults in harder rocks across the region. The results clearly show that they are active, and quite capable of generating damaging quakes, Sykes said.
The Earth Institute’s graphic on the left shows all known quakes in the greater New York-Philadelphia area, 1677-2004, graded by magnitude.
One major previously known feature, the Ramapo Seismic Zone, runs from eastern Pennsylvania to the mid-Hudson Valley, passing within a mile or two northwest of Indian Point. The researchers found that this system is not so much a single fracture as a braid of smaller ones, where quakes emanate from a set of still ill-defined faults.
The researchers also found evidence for a previously unknown active seismic zone running at least 25 miles from Stamford, Connecticut, to the Hudson Valley, where it passes less than a mile north of the Indian Point nuclear power plant.
“Indian Point is situated at the intersection of the two most striking linear features marking the seismicity and also in the midst of a large population that is at risk in case of an accident,” says the paper. “This is clearly one of the least favorable sites in our study area from an earthquake hazard and risk perspective.”
Based on the lengths of the faults, the detected tremors, and calculations of how stresses build in the crust, the researchers say that magnitude 6 quakes, or even 7–respectively 10 and 100 times bigger than magnitude 5–are quite possible on the active faults they describe.
They calculate that magnitude 6 quakes take place in the area about every 670 years, and sevens, every 3,400 years.
The researchers pointed out that previous research, in 2003, put the cost of quakes this size 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 would destroy 14,000 buildings and damage 180,000 in that area alone.
Photo of midtown New York by David Braun
No one knows when the last such events occurred, and no one can predict when they next might come, the scientists said.
Coauthor Leonardo Seeber said that because the faults are mostly invisible at the surface and move infrequently, a big quake could easily hit one not yet identified. “The probability is not zero, and the damage could be great,” he said. “It could be like something out of a Greek myth.”
Any conservative approach will look at geologically similar environments, Seeber said. “If you do that, we are similar to Bhuj, India [where a 2001 magnitude 7 quake killed over 15,000 people]. There was no obvious sign of strain there.”
A 2001 analysis by the Federal Emergency Management Agency ranks New York the 11th most at-risk U.S. city for earthquake damage.
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