US Raises Threat of Quake but Lowers Risk for Towers
New York Times
By SAM ROBERTS
JULY 17, 2014
Here is another reason to buy a mega-million-dollar apartment in a Manhattan high-rise: Earthquake forecast maps for New York City that a federal agency issued on Thursday indicate “a slightly lower hazard for tall buildings than previously thought.”
The agency, the United States Geodetic Survey, tempered its latest quake prediction with a big caveat.
“The eastern U.S. has the potential for larger and more damaging earthquakes than considered in previous maps and assessments,” the agency said, citing the magnitude 5.8 quake that struck Virginia in 2011.
Federal seismologists based their projections of a lower hazard for tall buildings — “but still a hazard nonetheless,” they cautioned — on a lower likelihood of slow shaking from an earthquake occurring near the city, the type of shaking that typically causes more damage to taller structures.
“The tall buildings in Manhattan are not where you should be focusing,” said John Armbruster, a seismologist with the Lamont-Doherty Earth Observatory of Columbia University. “They resonate with long period waves. They are designed and engineered to ride out an earthquake. Where you should really be worried in New York City is the common brownstone and apartment building and buildings that are poorly maintained.”
Mr. Armbruster was not involved in the federal forecast, but was an author of an earlier study that suggested that “a pattern of subtle but active faults makes the risk of earthquakes to the New York City area substantially greater than formerly believed.”
He noted that barely a day goes by without a New York City building’s being declared unsafe, without an earthquake. “If you had 30, 40, 50 at one time, responders would be overloaded,” he said.
The city does have an earthquake building code that went into effect in 1996, and that applies primarily to new construction.
A well-maintained building would probably survive a magnitude 5 earthquake fairly well, he said. The last magnitude 5 earthquake in the city struck in 1884. Another is not necessarily inevitable; faults are more random and move more slowly than they do in, say, California. But he said the latest federal estimate was probably raised because of the magnitude of the Virginia quake.
“Could there be a magnitude 6 in New York?” Mr. Armbruster said. “In Virginia, in a 300 year history, 4.8 was the biggest, and then you have a 5.8. So in New York, I wouldn’t say a 6 is impossible.”
Mr. Armbruster said the Geodetic Survey forecast would not affect his daily lifestyle. “I live in a wood-frame building with a brick chimney and I’m not alarmed sitting up at night worried about it,” he said. “But society’s leaders need to take some responsibility.”