The collapse of the south tower generated shaking of a magnitude of 2.1. When the north tower fell, the shaking reached a magnitude of 2.3, according to researchers at Columbia University.
The vibrations were similar to those generated by a small earthquake that occurred beneath the east side of Manhattan on Jan. 17. Earthquakes of this magnitude can be felt but they rarely cause damage to structures.
The researchers, reporting in Eos, the weekly journal of the American Geophysical Union, said the collapse of nearby buildings was probably not caused by the shaking that resulted from the fall of the twin towers after they were struck by hijacked airliners.
Instead, damage to other buildings was most likely caused by falling material from the towers striking them and by the great blast of air pressure and debris that burst outward when the towers fell, according to the research team at Columbia’s Lamont-Doherty Earth Observatory in Palisades, N.Y.
The collapse of the towers was measured by several seismographs, one 266 miles away in Lisbon, N.H., according to the 12-member research team, led by Won-Young Kim, Lynn Sykes, Klaus Jacob, Paul Richards and Arthur Lerner-Lam.
They said the type of seismic waves generated by the collapse was similar to that caused by the collapse of a salt mine in western New York in 1994.
The Lamont-Doherty observatory, 21 miles from the towers, was also able to record the two aircraft striking the huge buildings.
The impacts measured 0.7 and 0.9 in magnitude.
“The energy contained in the amount of fuel combusted was equivalent to the energy released by 240 tons of TNT,” Lerner-Lam said.
“This energy was absorbed by the buildings and produced the observed fireballs but did not immediately cause the collapse.”
