Could It Happen Here? Earthquakes In The Tri-State Area
By Ryan Fennell
The risk of earthquakes in the tri-state area is substantially greater than previously believed according to a 2008 study performed by scientists of the Lamont-Doherty Earth Observatory at Columbia University's Earth Institute.
The group of prominent seismologists compiled a catalogue of all 383 known earthquakes from 1677 to 2007 in a 15,000-square-mile area around New York City.
Lynn R. Sykes, lead author of the study, said the data show that large quakes are infrequent around New York compared to active areas like California and Japan, but that the risk is high because of the overwhelming concentration of people and infrastructure.
"The research raises the perception both of how common these events are, and, specifically, where they may occur."
Of the 383 known earthquakes in the region between 1677 and 2007, three have been magnitude five quakes, in 1737, 1783, and 1884. The 1884 quake was centered under the seabed somewhere between Brooklyn and Sandy Hook.
"The 1884 earthquake should've shaken Sandy Hook and Red Bank pretty strongly," said study coauthor John Armbruster. "That is one area that could be examined more carefully. It's difficult to operate seismic instruments in Sandy Hook or Coney Island. There isn't rock to place the seismic instruments on and the background noise in the seismographs is much worse than it is further inland."
Based on the data, researchers said that magnitude 5 earthquakes should be expected in the region every 100 years.
The most recent large earthquake, a magnitude 4.1, struck at the Dobbs Ferry fault in suburban Westchester, N.Y., in 1985.
"Today, with so many more buildings and people, a magnitude 5 centered below the city would be extremely attention-getting," Armbruster said. "We'd see billions in damage. People would probably be killed."
In 1970 the Lamont-Doherty network began collecting data from quakes from dozens of newly deployed seismometers. Since then, the Lamont network, led by coauthor of the 2008 study Won-Young Kim, has located hundreds of small events, including a magnitude 3 quake every few years. The quakes tend to cluster along a series of small, old faults in harder rocks across the region.
Six months after the study was released, a magnitude 3.0 earthquake struck within the Ramapo Seismic Zone in Morris County. Magnitude 3 earthquakes occur 10 times more often than magnitude 4 quakes, 100 times more than magnitude quakes, and so on.
The Ramapo Seismic Zone runs from eastern Pennsylvania to the mid-Hudson Valley and passes within two miles of the Indian Point nuclear power plant. The researchers found that the system is not a single fracture but a "braid of smaller ones, where quakes emanate from a set of still ill-defined faults."
East and south of the Ramapo zone is a set of nearly parallel northwest-southeast faults that includes Manhattan's 125th Street fault, which is theorized to have caused the 1737 quake; the Dyckman Street fault, which caused a magnitude 2 in 1989; the Mosholu Parkway fault; and the Dobbs Ferry fault.
According to Sykes the 1884 quake may have hit on a yet undetected member of this parallel family further south.
The researchers said that frequent small quakes occur in predictable ratios to larger ones, which can predict a rough time-scale for damaging events.
The researchers said that magnitude 6 or magnitude 7 earthquakes are quite possible on these active faults. They calculated that magnitude 6 quakes take place in the area about every 670 years, and magnitude 7 quakes every 3,400 years.
"You could debate whether a magnitude 6 or 7 is possible, but we've already had three magnitude fives, so that is very realistic," Armbruster said.
"We need to step back from the simple old model, where you worry about one large, obvious fault, like they do in California," said coauthor Leonardo Seeber. "The problem here comes from many subtle faults. Each one is small, but when you add them up, they are probably more dangerous than we thought. Once you accept that one fault in a family is active, you better consider that all the faults in that family could be active."
In addition to identifying a substantially greater earthquake risk to the region than previously believed, the study also identified a significant previously unknown structure of faults; an active seismic zone that runs at least 25 miles from Stamford, Conn., to Peekskill, N.Y., where it passes less than a mile north of Indian Point nuclear power plant.
The Stamford-Peekskill structure runs parallel to the other faults beginning at the 125th Street fault. Researchers said that this system is capable of producing "at least a magnitude 6 quake." Further on, the system intersects with the Ramapo seismic zone.
"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," the study said. "This is clearly one of the least favorable sites in our study area from an earthquake hazard and risk perspective."
The earthquakes in the New York region also differ from those in California in that they occur near the surface and not in the more malleable formations where earthquakes are frequent, but in the extremely hard, rigid rocks underlying Manhattan and the lower Hudson Valley.
"It's like putting a rock in a vise," Seeber said. "Nothing happens for awhile. Then it goes with a bang."
"This is a landmark study in many ways," said Associate Director of Lamont-Doherty Art Lerner-Lam. "It gives us the best possible evidence that we have an earthquake hazard here that should be a factor in any planning decision. It crystallizes the argument that this hazard is not random. There is structure to the location and timing of the earthquakes. This enables us to contemplate risk in an entirely different way. And since we are able to do that, we should be required to do that."
"New York is not as prone to earthquakes as California and Japan, but they do happen," Sykes said. "This study takes a more realistic look a the possibility of larger ones, and why earthquakes concentrate in certain places. Too much attention has been paid to the level of hazard, and not enough to the risk. Earthquake risk is much, much higher today, since the number of people, assets, and their vulnerability are so much greater."