...further proof of secondary explosions causing the towers to collapse, I have read this at another site, with more technical detail and will look for it. Try this for starters:
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The collapse of the two towers produced larger events, equivalent to a magnitude 2.1 temblor for the south tower and magnitude 2.3 for the north, "which can definitely be felt," Lerner-Lam says. Surprisingly, the energy transferred from the falling buildings into the ground was only a tiny fraction of the total energy involved in the collapse.
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http://www.discover.com/dec_01/feat_shook.html>
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The seismic record from Columbia University’s observatory in Palisades, NY (21 miles away) provides indisputable proof that massive explosions brought down those towers. At the precise moment the South Tower began collapsing, a 2.1 earthquake registered on the seismograph. At the precise moment the North Tower began collapsing, a 2.3 earthquake registered; however, as the buildings started to crumble these waves disappeared. The two ‘spikes’on the seismograph, which both occurred at the exact instants the collapses began, are twenty times the amplitude, or more than 100 times the force of the other waves. If the buildings had simply collapsed, the largest jolts would have occurred when the massive debris struck the earth, not at the beginnings of the collapses. Seismologist Arthur Lerner-Lam of Columbia University stated, “Only a small fraction of the energy from the collapsing towers was converted into ground motion. The ground shaking that resulted from the collapse of the towers was extremely small.” In other words, the collapsing did not cause 2.1 and 2.3 magnitude earthquakes. Furthermore, a ‘sharp spike of short duration’ is how underground nuclear explosions register on seismographs. Underground explosions, where the steel columns meet Manhattans granite would account for both the demolition-style implosions and these ‘spikes’ on the seismograph. Another seismologist at the Palisades observatory, Won Young Kim said the 1993 truck bomb did not even register on their seismographs because the explosion was ‘not coupled’ to the ground. Imagine the magnitude of explosions it would take to register the two earthquakes, when the truck bomb didn’t even show up.
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http://www.prisonplanet.com/analysis_lavello_050503_bombs.html>
Very interesting reading.