These shockwaves can be clearly seen on the seismograph, they are more than 20 times the amplitude of the rest of the collapse. To put that into perspective, a 10 fold increase in amplitude represents a 100 fold increase in energy. These spikes represent truly massive shockwaves similar to the those caused by the detonation of 80,000 lbs of ammonium nitrate during quarry blasting which can register up to a magnitude 2 earthquake. To further put that into perspective you can also see on this seismic record how tiny by comparison the aircraft impacts were.
Whatever caused these shockwaves clearly had nothing to do with any progressive collapse due to weakened steel, these were massive, almost identical detonations.