Fact Sheet
SEISMOLOGICAL OBSERVATION OF IMPACTS AND COLLAPSES AT WORLD
TRADE CENTER
Prepared by:
Seismology Group
Lamont-Doherty Earth Observatory, Columbia University
Palisades NY 10964
Version of 9/14/01
Seismograph stations in southern New
York, northern New Jersey, western Connecticut, and Pennsylvania, operated by
the Lamont-Doherty Earth Observatory of Columbia University, recorded the
collapse of each of the towers of the World Trade Center on Tuesday morning
September 11 and the subsequent collapse of 7 World Trade Center later that
afternoon. The closest station, at Palisades, New York, is located 21 miles (34
km) north of lower Manhattan in Rockland County. This station also registered
the impacts of the two airliners that crashed into the towers.
The signals generated by the
collapsing North and South towers were much larger than those from the two
airliner impacts. The signals generated
by the collapse of Building 7, however, were smaller than those of the
impacts. In addition, many smaller
signals were registered at Palisades throughout the rest of the day that may
have originated from the further collapse of the Twin Towers and the fall of
walls and other debris in the surrounding area.
The
Palisades recordings of the Twin Tower collapses were comparable in size to the
signals from a small earthquake of seismic magnitude 2.4 that was felt in the
east side of Manhattan and in the western parts of Queens earlier this year, on
January 17.
The seismic signals from the five
events on 11 September differed from a small earthquake in that they were
richer in low-frequency energy and poorer in high-frequency energy. These differences can be attributed to the
short time duration of the fault rupture responsible for the earthquake as
compared to the long and complex collapse of the buildings. The seismic waves
from the five World Trade Center events resemble those produced by the collapse
of a salt mine south of Rochester, in 1994.
The
catastrophic events at the World Trade Center, as might be expected, produced
much larger seismic effects than the bombing of the World Trade Center in 1993.
The seismic effects of the collapses are comparable to the explosions at a
gasoline tank farm near Newark on January 7, 1983, which were detected up to
130 miles away.
The seismographic stations are part of the
Lamont-Doherty Cooperative Seismographic Network, which is operated in
conjunction with several other institutions and is supported by the U.S.
Geological Survey under the National Earthquake Hazard Reduction Program. As part of its agreement with the USGS,
Lamont-Doherty makes this data available upon request without restriction.
Preliminary
measurements made by Lamont-Doherty analysts are summarized in the Table below:
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Information
Based on Seismic Waves recorded at Palisades New York
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Event
origin time (EDT) Magnitude Duration
(hours:minutes:seconds) (equivalent seismic)
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Impact
1 at North Tower 08:46:26±1 0.9
12 seconds
Impact
2 at South Tower 09:02:54±2 0.7 6 seconds
Collapse
1, South Tower 09:59:04±1
2.1
10 seconds
Collapse
2, North Tower 10:28:31±1
2.3 8 seconds
Collapse
3, Building 7 17:20:33±1
0.6
18 seconds
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More
information may be obtained on the World-Wide Web at:
http://www.ldeo.columbia.edu/LCSN/Eq/20010911_wtc.html
For
further information contact
Won
Young Kim wykim@ldeo.columbia.edu
Jeremiah
Armitage jha@ldeo.columbia.edu
John
Armbruster armb@ldeo.columbia.edu
Klaus
Jacob jacob@ldeo.columbia.edu
Arthur
Lerner-Lam lerner@ldeo.columbia.edu
Paul
Richards richards@ldeo.columbia.edu
Lynn
R. Sykes sykes@ldeo.columbia.edu
Jia-Kang
Xie xie@ldeo.columbia.edu