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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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