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to Buildings Near World Trade Center Towers Caused by Falling Debris
and Air Pressure Wave, Not by Ground Shaking, Columbia Seismologists
Report in November 20 issue of Eos
Researchers Call for Seismographic Stations in Urban Areas
On September 11, seismographs
operated by Columbia University’s Lamont-Doherty Earth Observatory
in Palisades, New York, recorded seismic signals produced by the impacts
of the two aircraft hitting the Twin Towers of the World Trade Center,
and the Towers’ subsequent collapse. While the ground shaking was
consistent with the energy released by small earthquakes, it was not sufficient
to cause the collapse of or damage to the surrounding buildings, as some
have thought. Rather, the buildings around the Twin Towers were impacted
both by the kinetic energy of falling debris and by the pressure exerted
on the buildings by a dust- and particle- laden blast produced by the collapse.
Writing in the November 20, 2001, issue
of Eos, published by the American Geophysical Union, seismologists
from Columbia’s Lamont-Doherty Earth Observatory outline the sequence
of seismographic recordings on that tragic day, and argue that vibrations
recorded on September 11 were of a magnitude thought to be too low to cause
structural damage to buildings, especially in the northeast region of the
However, the authors add that
because there were no seismographic stations in or even near the World
Trade Center, it is impossible to know for sure that the ground-shaking
did not have any impact on the neighboring buildings. Ultimately, they
say, urban officials should consider the importance of placing seismographic
stations in high-density urban areas.
recordings on E-W component at Palisades, NY for events at World Trade
Center (WTC) on September 11, distance 34 km. Three hours of continuous
data shown starting at 08:40 EDT (12:40 UTC). Two largest signals were
generated by collapses of Towers 1 and 2. Expanded views of first
impact and first collapse shown in red. Displacement amplitude spectra
in nm-s from main impacts and collapses shown at right. Note broadband
nature of spectra for collapses 1 and 2, whereas relatively
periodicmotions for impacts 1 & 2.
"Our recordings were made
at considerable distance," said Won-Young Kim, who is in charge of
seismological network operations for Lamont-Doherty. "However, plans
are pending for an Advanced National Seismic System (ANSS) that calls for
placing seismic instruments in such urban areas as New York City. The tragic
events of September 11 show that such instrumentation can serve a purpose
that sometimes transcends strict earthquake applications."
The paper was authored by 12
researchers at Lamont, including Kim; Lynn Sykes; Klaus Jacob, Paul Richards
and Arthur Lerner-Lam. Lerner-Lam is the director of Columbia’s new
Center for Hazards and Risk Research.
Lerner-Lam explained what happened
once the planes hit the World Trade Center, and how they resulted in relatively
small seismographic readings.
"The energy contained in
the amount of fuel combusted was the equivalent to the energy released
by 240 tons of TNT," said Lerner-Lam. "This energy was absorbed
by the buildings and produced the observed fireballs, but did not immediately
cause the collapse. During the collapse, most of the energy of the falling
debris was absorbed by the towers and the neighboring structures, converting
them into rubble and dust or causing other damage– but not causing
significant ground shaking."
Seismographic recordings of the
tower collapses were recorded in five states, as far away as 428 kilometers
[266 miles] in Lisbon, New Hampshire. Lamont’s home station, in Palisades,
New York, is located above the Hudson River, 34 kilometers [21 miles] from
downtown Manhattan, where the towers stood. The aircraft impacts registered
local magnitude (ML) 0.9 and 0.7, indicating minimal earth shaking as a
result. The subsequent collapse of the towers, on the contrary, registered
magnitudes of 2.1 and 2.3, comparable to the small earthquake that occurred
beneath the east side of Manhattan on January 17, 2001.
The Lamont seismographs established
the following timeline: 8:46:26 a.m. EDT [1240 UTC] Aircraft impact - north
tower, Magnitude 0.9; 9:02:54 a.m. EDT [1302 UTC] Aircraft impact - south
tower, Magnitude 0.7; 9:59:04 a.m. EDT [1359 UTC] Collapse - south tower,
Magnitude 2.1; 10:28:31 a.m. EDT [1428 UTC] Collapse - north tower, Magnitude
In addition, the seismic waves
were short-period surface waves, meaning they traveled within the upper
few kilometers of the Earth’s crust. They were caused by the interaction
between the ground and the building foundation, which transmits the energy
from the impacts and the collapses.
The authors also noted that as
seen in television images, the fall of the towers was similar to that of
a pyroclastic flow down a volcano, where hot dust and chunks of material
move in a dust/mud matrix down the volcano’s slope. The collapse of
the WTC generated such a flow, though without the high temperatures common
in volcanic flows.
The Lamont-Doherty Earth
Observatory operates 34 seismographic stations in the northeast in collaboration
with several institutions. Network operations are supported by the United
States Geological Survey. The network is part of the Advanced National
Seismic System, a national seismological monitoring initiative being implemented
through a USGS-university partnership.
The paper by Won-Young Kim, Lynn
R. Sykes, J.H. Armitage, J.K. Xie, Klaus H. Jacob, Paul G. Richards, M.
West, F. Waldhauser, J. Armbruster, L. Seeber, W.X. Du, and Arthur Lerner-Lam, "Seismic
Waves Generated by Aircraft Impacts and Building Collapses at World Trade
Center, New York City," appears in Eos, Volume 82, number 47
(20 November 2001), page 565.
Journalists may request a copy
of the paper from Harvey Leifert, firstname.lastname@example.org,
specifying a pdf or fax version. Please include your name, publication,
postal address, phone, fax, and email address. There is no embargo.
To speak with any of the researchers
listed above, please contact Stacey Gander, Administrative Assistant, at
Columbia’s Center for Hazards and Risk Research: (845) 365-8909 or email@example.com.
Web site for the Lamont-Doherty Cooperative Seismographic Network:
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