Seismograms of Terror
Under ordinary conditions, seismologists focus their observations
on understanding the causes and consequences of natural disasters. But natural disasters are not the only phenomena capable of
producing dramatic vibrations inside our planet. One year ago, the United States was shaken by terrorist attacks
on New York and Washington.
These events not only created shock waves in the political
and social fabric of the United States; they also created seismic
waves that were recorded by sensitive instruments around the northeastern
U.S. Such observations are of interest not only for historical reasons;
they can be used to provide additional forensic evidence into the
nature of such disasters, and they can help provide critical information
that can help engineers design buildings to withstand future attacks.
Vibrations associated with the collapse of the World Trade Center
were recorded across five states, and as far away as 428 km (265
miles) from New York. The terrorist attacks produced a complex
array of signals from the initial impact of the airplanes to the
collapse of the Twin Towers and subsequent collapse of neighboring
buildings. The North
Tower collapse produced vibrations equivalent to a magnitude 2.3
The plot shows two hours of ground movement
as recorded at Palisades (PAL), New York. Each line is 30 minutes
long and the first line starts at 08:40 (EDT).
Image and text contributed by Lynn Sykes, Paul Richards,
Won-Young Kim, Jeremiah Armitage, Klaus Jacob and Art Lerner-Lam
of Lamont-Doherty Earth Observatory.