EARTHQUAKESBalancing Act: Can Precariously Perched Boulders Signal New York’s Earthquake Risk?
The trouble with big earthquakes is that their subterranean root systems can lurk for centuries or millennia before building enough energy to explode. Among many places, this is true of the New York City area, where scientists believe big quakes are possible—but probably so rare, it is hard to say exactly how often they come, or how big they could be.
The trouble with big earthquakes is that their subterranean root systems can lurk for centuries or millennia before building enough energy to explode. Among many places, this is true of the New York City area, where scientists believe big quakes are possible—but probably so rare, it is hard to say exactly how often they come, or how big they could be.
It was only in the 1970s that researchers began studying the region’s seismicity in close detail. They have mapped many previously unknown faults, and observed dozens of tiny quakes each year, most too small to be felt. The biggest modern quake, a magnitude 4.1 in suburban Westchester County in 1985, did little harm. However, old written records suggest that quakes of about magnitude 5 shook New York and environs in 1737 and 1884. These knocked down chimneys, cracked walls, and shook the ground from upper New England to Virginia. Today, an equivalent event could do great damage to the vastly expanded population and infrastructure of the regional megalopolis. Furthermore, based on the sizes of known faults and the frequency of small quakes along them, some researchers have extrapolated an estimate that a magnitude 6 quake could strike the region every 700-some years, and a magnitude 7, every 3,400 years. A magnitude 6 is 10 times more powerful than the events of 1737 and 1884, and a magnitude 7, 100 times stronger.
But this is just an extrapolation. Have quakes of this size ever actually happened here? No one knows. William Menke a geologist and seismologist at the Columbia Climate School’s Lamont-Doherty Earth Observatory, would like to find out.
Recently, Menke and a student intern have been visiting Harriman State Park, about 30 kilometers north of Manhattan. Not far from Lamont-Doherty’s suburban campus, the mountainous 47,500-acre reserve contains numerous giant boulders plucked from bedrock by glaciers during the last ice age, then dropped when the ice melted. Some are precariously balanced on one irregular surface or another, presumably still in their original positions. Menke’s mission: calculate how much force it would take to tip them over. If they are still standing, it would suggest that an earthquake of that size has not happened since the ice age ended, well over 10,000 years ago.
