Although New York’s modern skyscrapers are less likely to be damaged in an earthquake than shorter structures, a new study suggests the East Coast is more vulnerable than previously thought. The new findings will help alter building codes.
By Mark Fahey
July 18, 2014 10:03 a.m.
The U.S. Geological Survey had good and bad news for New Yorkers on Thursday. In releasing its latest set of seismic maps the agency said earthquakes are a slightly lower hazard for New York City’s skyscrapers than previously thought, but on the other hand noted that the East Coast may be able to produce larger, more dangerous earthquakes than previous assessments have indicated.
The 2014 maps were created with input from hundreds of experts from across the country and are based on much stronger data than the 2008 maps, said Mark Petersen, chief of the USGS National Seismic Hazard Mapping Project. The bottom line for the nation’s largest city is that the area is at a slightly lower risk for the types of slow-shaking earthquakes that are especially damaging to tall spires of which New York has more than most places, but the city is still at high risk due to its population density and aging structures, said Mr. Petersen.
“Many of the overall patterns are the same in this map as in previous maps,” said Mr. Petersen. “There are large uncertainties in seismic hazards in the eastern United States. [New York City] has a lot of exposure and some vulnerability, but people forget about earthquakes because you don’t see damage from ground shaking happening very often.”
Just because they’re infrequent doesn’t mean that large and potentially disastrous earthquakes can’t occur in the area. The new maps put the largest expected magnitude at 8, significantly higher than the 2008 peak of 7.7 on a logarithmic scale. The scientific understanding of East Coast earthquakes has expanded in recent years thanks to a magnitude 5.8 earthquake in Virginia in 2011 that was felt by tens of millions of people across the eastern U.S. New data compiled by the nuclear power industry has also helped experts understand quakes.
“The update shows New York at an intermediate level,” said Arthur Lerner-Lam, deputy director of Columbia’s Lamont-Doherty Earth Observatory. “You have to combine that with the exposure of buildings and people and the fragility of buildings and people. In terms of safety and economics, New York has a substantial risk.”
Oddly enough, it’s not the modern tall towers that are most at risk. Those buildings become like inverted pendulums in the high frequency shakes that are more common on the East Coast than in the West. But the city’s old eight- and 10-story masonry structures could suffer in a large quake, said Mr. Lerner-Lam. Engineers use maps like those released on Thursday to evaluate the minimum structural requirements at building sites, he said. The risk of an earthquake has to be determined over the building’s life span, not year-to-year.
“If a structure is going to exist for 100 years, frankly, it’s more than likely it’s going to see an earthquake over that time,” said Mr. Lerner-Lam. “You have to design for that event.”
The new USGS maps will feed into the city’s building-code review process, said a spokesman for the New York City Department of Buildings. Design provisions based on the maps are incorporated into a standard by the American Society of Civil Engineers, which is then adopted by the International Building Code and local jurisdictions like New York City. New York’s current provisions are based on the 2010 standards, but a new edition based on the just-released 2014 maps is due around 2016, he said.
“The standards for seismic safety in building codes are directly based upon USGS assessments of potential ground shaking from earthquakes, and have been for years,” said Jim Harris, a member and former chair of the Provisions Update Committee of the Building Seismic Safety Council, in a statement.
The seismic hazard model also feeds into risk assessment and insurance policies, according to Nilesh Shome, senior director of Risk Management Solutions, the largest insurance modeler in the industry. The new maps will help the insurance industry as a whole price earthquake insurance and manage catastrophic risk, said Mr. Shome. The industry collects more than $2.5 billion in premiums for earthquake insurance each year and underwrites more than $10 trillion in building risk, he said.
“People forget about history, that earthquakes have occurred in these regions in the past, and that they will occur in the future,” said Mr. Petersen. “They don’t occur very often, but the consequences and the costs can be high.”