we are talking about the fall of WTC7Consider the seismic records of the closest seismic recording station, at Palisades, NY (PAL). They show a very similar pattern for the leveling of WTC 1 and 2. In both cases there is about five seconds of high-amplitude movement, followed by about three seconds of movement at less than half that amplitude, and then by about 15 seconds of much weaker movement. In addition there is some still weaker movement starting about 12 seconds before the onsets of the high-amplitude movement. The main difference is that for WTC 1 the initial high-amplitude phase builds in intensity to a much higher spike than any seen for WTC 2.
The fact that the largest movement is followed by smaller movement has been cited as evidence that bombs, detonated at the starts of the collapses, generated the large movement, and that the debris impacting the ground contributed to the smaller subsequent movement. However, bombs, if detonated underground, would have generated strong P waves in addition to S waves. The fact that only strong S waves were reported is consistent with the theory that the largest movement was caused by building remains hitting the ground.
9-11 Research: Speed of Fall
9-11 Research: Rate of Building 7's Fall
that link is better for this discussion