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 earthquake.

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.