Web Extra    Decmber 3 , 2001

Seismicity from Sept. 11

Seismometers at Columbia University's Lamont-Doherty-Earth Observatory in Palisades, N.Y., detected seismic waves Sept. 11 generated by the impact of two commercial airliners into the World Trade Center and the subsequent collapse of the twin towers. Although the ground motions caused by the buildings' collapse were consistent with the energy produced by a small earthquake, they were probably not strong enough to cause damage to surrounding buildings, says a paper in the Nov. 20 issue of Eos written by twelve Lamont researchers.

The local magnitudes of the aircraft impacts were 0.9 and 0.7, resulting in little ground shaking. The tower collapses, in contrast, registered magnitudes of 2.1 and 2.3, comparable to a small earthquake that occurred beneath the east side of Manhattan on Jan. 17, 2001. "The energy of the collapse came from the stored gravitational potential energy of the buildings. As the towers collapsed, the majority of this energy was absorbed in the destruction of the buildings themselves, producing the debris cloud," says Klaus Jacob, a co-author and senior research scientist for the Division of Geology and Geophysics at Lamont. "Only a miniscule fraction of the buildings' potential energy was actually transformed into seismic wave energy."

While the towers' collapse resulted in relatively small seismographic recordings, it was detected by seismic stations in five states and as far as 428 kilometers in Lisbon, N.H.

The combustion of about 50 to 100 tons of fuel in each aircraft -- equivalent to the energy released by 240 tons of TNT -- caused the observed fireballs and subsequent ignited material in each tower. Less than a millionth of this fuel energy was converted into seismic waves, the authors report.

Vibrations recorded on Sept. 11 were too low to have caused structural damage to buildings. The damage to neighboring buildings was probably caused by projectiles of falling debris or by the pressure exerted from the dust- and particle-laden blast of air, the authors write. However, because the nearest seismographic station was 34 kilometers from the towers, the team could not determine whether or not groundshaking affected the surrounding buildings.

The authors emphasize the importance of placing seismographs in urban areas. Plans are pending for an Advanced National Seismic System (ANSS), a national seismological monitoring initiative supported by the U.S. Geological Survey and partner universities, to increase urban seismic instrumentation. "By measuring ground motions near buildings and, in some cases, the motions of buildings themselves, engineers will have more information to constrain the design parameters of urban structures. The ANSS is designed to provide just that sort of information," says Arthur Lerner-Lam, a co-author and director of Columbia's new Center for Hazards and Risk Research.

Neeta Bijoor
Geotimes contributing writer

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