While the aircraft crashes caused minimal earth shaking, significant earthquakes with unusual es occurred at the beginning of each collapse. The Palisades seismic data recorded a 2.1 magnitude earthquake during the 10-second collapse of the South Tower at 9:59:04 and a 2.3 quake during the 8-second collapse of the North Tower at 10:28:31.
The Palisades seismic record shows that -- as the collapses began --
a huge seismic " e" marked the moment the greatest energy went into the ground. The strongest jolts were all registered at the beginning of the collapses, well before the falling debris struck the earth. These unexplained " es" in the seismic data lend credence to the theory that massive explosions at the base of the towers caused the collapses.
A "sharp e of short duration" is how seismologist Thorne Lay of Univ. of California at Santa Cruz told AFP an underground nuclear explosion appears on a seismograph.
The two unexplained es are more than twenty times the amplitude of the other seismic waves associated with the collapses and occurred in the East-West seismic recording as the buildings began to fall.
Lerner-Lam told AFP that a 10-fold increase in wave amplitude indicates a 100-fold increase in energy released. These "short-period surface waves," reflect "the interaction between the ground and the building foundation," according to a report from Columbia Earth Ins ute.
"The seismic effects of the collapses are comparable to the explosions at a gasoline tank farm near Newark on January 7, 1983," the Palisades Seismology Group reported on Sept. 14, 2001.
One of the seismologists, Won-Young Kim, told AFP that the Palisades seismographs register daily underground explosions from a quarry 20 miles away. These blasts are caused by 80,000 lbs. of ammonium nitrate and cause local earthquakes between Magnitude 1 and 2. Kim said the 1993 truck-bomb at the WTC did not register on the seismographs because it was "not coupled" to the ground.
Experts cannot explain why the seismic waves peaked before the towers hit the ground. Asked about these es seismologist Arthur Lerner-Lam, director of Columbia University's Center for Hazards and Risk Research told AFP, "This is an element of current research and discussion. It is still being investigated."
"Only a small fraction of the energy from the collapsing towers was converted into ground motion," Lerner-Lam said. "The ground shaking that resulted from the collapse of the towers was extremely small."
Last November, Lerner-Lam said, "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,"
Evidently, the energy source that shook the ground beneath the towers was many times more powerful than the total potential energy released by the falling mass of the huge towers.
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