WTC Towers Collapse Caused Small Earthquakes

"Though terrorists triggered the collapse of the World Trade Center's twin towers, much of the energy -- calculated by a physicist to have been at least 2 percent that of the atomic bomb dropped on Hiroshima -- was supplied by nature's most ubiquitous force: gravity...When the crumbling towers thudded to the ground, they created ground tremors equal to earthquakes of magnitude 2.1 and 2.3, as measured by a seismic station in Palisades, N.Y., 21 miles north of Manhattan...More surprising, even the impacts of the airliners shook the ground, registering with the equivalent of earthquakes with magnitudes of 0.7 and 0.9."