The Palisades seismic record shows that -- as the collapses began -- a huge seismic "spikes" marked the moment the greatest energy went into the ground. The strongest jolts were both registered at the beginning of the collapses, well before the falling debris struck the earth.
The foundation supports the building and as the weight of a 30 story building is quickly removed from the foundation the foundation reacts with an equal an opposite reaction.
So when the several thousand ton weight of the collapsing stories was removed from the typical building weight the foundation would have reacted
upward as the compressive energy which supported 30 stories within the foundation was dissapated.
This motion of the ground would have been detected as the seismic motion that it was, before debris impacted the ground.
>edit>The removal of weight was a more time concentrated change in energy than individual piecees of debris impacting the surface over several seconds and therefore would be a greater spike.