We have reviewed several technical articles on the World Trade Center seismic evidence in an attempt to pull together the array of voluminous, technically overwhelming, and often contradictory information and conclusions on this subject, so that we can present the most accurate answer to this important question.
Shortly after the events at the World Trade Center on September 11, 2001, Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, located 21 miles north of lower Manhattan, produced a report titled "Seismic Waves Generated by Aircraft Impacts and Building Collapses at World Trade Center, New York City." It concluded that the seismic signals associated with the time of each airplane impact and the time of the collapse of World Trade Center Buildings 1, 2, and 7 (WTC 1, WTC 2, and WTC 7), respectively, must have been produced by the airplane impacts and by the falling of debris during the three collapses.
Four years later, in its 2005 report on the destruction of WTC 1 and 2, the National Institute of Standards and Technology (NIST) adopted these seismic signal times, for the most part. But NIST also contracted with Columbia University (Section 3.6 Absolute Time Accuracy, page 23) to adjust those seismic signal times by three seconds each in order to fit its own timeline of events, apparently so it could justify its theory that the plane impacts and the debris from WTC 1 and 2 hitting the ground were in fact the actual events causing these seismic signals.
In 2006, researcher Jim Hoffman wrote a detailed analysis supporting NIST's conclusion of debris hitting the ground as the cause of the seismic signals, "Seismic Records of the Twin Towers' Destruction: Clarifying the Relationship Between Seismic Evidence and Controlled Demolition Theories." The same year, Hoffman wrote an analysis criticizing a 2002 article by Christopher Bollyn, “New Seismic Data Refutes Official Explanation.” Later, Hoffman wrote still another critique of Bollyn's work: “ERROR: 'Seismic Spikes Preceded the Towers' Collapses.’” In each of his pieces, Hoffman’s conclusions essentially support the original LDEO and NIST contention that the airplane impacts and the collapses alone produced the seismic spikes.
However, in 2006, researchers Craig Furlong and Gordon Ross published an article, "Seismic Proof – 9/11 Was An Inside Job (Updated Version II)," in the Journal of 9/11 Studies. Furlong and Ross determined that there was, as Bollyn had contended, indeed proof of explosions both before the tower collapses and before the airplane impacts.
In 2009, the destruction of WTC 2 (the South Tower) was exhaustively analyzed by Graeme MacQueen in an article, "Did the Earth Shake Before the South Tower Hit the Ground?," which was published in the Journal of 9/11 Studies. MacQueen concluded that the testimony of numerous witnesses and the shaking of tripod-mounted cameras corroborated the pre-collapse explosions—not only at WTC 2 but at WTC 1 as well. He then correlated those explosions with the timing of the seismic spikes in the data from the Lamont-Doherty Earth Observatory.
Finally, we refer you to a 2012 article by Andre Rousseau, an expert in applied geophysics and the author of more than 50 published papers on progressive mechanical waves and geology. Published in the Journal of 9/11 Studies, his article is titled, "Were Explosives the Source of the Seismic Signals Emitted from New York on September 11, 2001?" We find Rousseau's article to be the most definitive analysis of the seismic evidence to date. In it, he directly challenges the official LDEO interpretations of the seismic waves and concludes that the waves clearly point to pre-airplane impact explosions and pre-collapse explosions as the causes of the seismic signals. He reaches a similar conclusion for WTC 7.
On the heels of the aforementioned literature, preeminent 9/11 researcher and author David Ray Griffin weighed in on the subject in an attempt to make sense of it all. Griffin and Elizabeth Woodworth are the inspiration and workhorses behind the 9/11 Consensus Panel, a research project that documents the “best evidence” challenging the official 9/11 narrative.
The purpose of the 23-member panel is, in its own words, to build "a body of evidence-based research into the events of September 11, 2001. This evidence — derived from a standard scientific reviewing process — is available to any investigation that may be undertaken by the public, the media, academia, or any other investigative body or institution. The Panel regularly features selected excerpts from its Consensus Points, with links to full supporting documentation."
It was with this methodology that the panel produced "Point TT-7: Why Did the Twin Towers Collapse? The Seismic Evidence." This article relies largely on the research laid out in the Rousseau and Furlong/Ross articles mentioned above, and it summarizes, in the most focused way possible, the “best evidence.”
We concur with the 9/11 Consensus Panel’s conclusion and recommend reading its article, reprinted below, as the most efficient way to understand the significance of the seismic activity at the World Trade Center. For ardent students of the subject, we advise reading all the references provided below.
Consensus 9/11: The 9/11 Best Evidence Panel, David Ray Griffin et al, 2015
Andre Rousseau, 2012
Graeme MacQueen, 2009
Craig T. Furlong & Gordon Ross, September 2006
Jim Hoffman, 2006
Lamont-Doherty Earth Observatory, Columbia University, 2001
Point TT-7: Why Did the Twin Towers Collapse? The Seismic Evidence
Note: This article is reprinted with permission from the 9/11 Consensus Panel.
Seismic waves were detected at seismograph stations in New York and four neighboring states on September 11, 2001, during the period when WTC 1 and 2 (the North and South Towers) were struck by airliners and collapsed. Scientists at the Lamont Doherty-Earth Observatory (LDEO) at Columbia University published analyses of the seismographic data from the WTC, based on raw data from the Palisades, NY, station. The Federal Emergency Management Agency (FEMA) and the National Institute of Standards and Technology (NIST) relied upon the LDEO analysis in their publications on the events at the World Trade Center.  The 9/11 Commission Report also cited the LDEO analysis,  although it did not confirm LDEO’s analysis of plane-impact times, basing its own conclusions on ground radar data instead of seismic wave data.
But independent analyses have disputed LDEO’s conclusions and thereby the conclusions reached by FEMA and NIST. These independent analyses dispute even more the conclusions of the 9/11 Commission.
The Official Account
The seismic waves were caused by the airplane impacts into the Twin Towers and the resulting collapses of the buildings.  The magnitudes of the airplane impact shocks at WTC 2 and WTC 1, respectively, were 0.7 and 0.9. The collapse of WTC 2 caused a shock of magnitude 2.1; the collapse of WTC 1 caused a shock of magnitude 2.3.  The signals were used to determine accurately when the plane impacts and collapses occurred. 
The Best Evidence
The results of independent research conflict with the conclusions by LDEO (Lamont Doherty-Earth Observatory) that the waves were caused by airplane impacts and resulting building collapses.
In 2006, engineers Craig Furlong and Gordon Ross showed that the plane impacts could not have caused the seismic signals attributed to them by LDEO, because they originated several seconds before the 9/11 Commission’s radar-based times of impact.
The seismic events, therefore, must have resulted from causes of a different type. The best (and probably only plausible) candidate for these causes would seemingly be explosions in the basements of the Twin Towers, for which there is abundant physical and testimonial evidence. 
Although the present Point deals only with the seismic evidence, much of the physical and testimonial evidence is documented in Point TT-8: “Why Did the Twin Towers Collapse? The Physical and Testimonial Evidence.” 
The conclusion of Furlong and Ross – that seismic evidence does not fit the official story (in any of its versions) – was reinforced in 2012 by a French geophysicist, Dr. André Rousseau, who reanalyzed the seismic wave data.  Rousseau concluded that the LDEO report is flawed in three significant respects:
- The radar-based timing of the airplane impacts does not match the origin-times of the seismic waves (as indicated by the data);
- The lack of explanation of why, although the two towers were destroyed in essentially the same way, the data show large differences between them in terms of released energy;
- The frequencies of the waves are much too low to have been caused by plane impacts and building collapses (although they match those of underground explosions, evidence for which is documented in Point TT-8).
The Timing of the Wave Origins: LDEO in 2001 published a report giving the times at which four wave signals began.  It correlated these times with the two airplane impacts and the two collapses. The LDEO researchers stated that they derived these times by calculation from the times the signals were received at the Palisades station. The 9/11 Commission Report, however, published very different impact times, based on ground radar data, which tracked the airplanes’ approaches to, and collisions with, the buildings. The differences are greatest with regard to WTC 1 (which was first): Rousseau, like Furlong and Ross, pointed out that the radar-based times, being approximately 15 seconds later than the times that could be plausibly inferred from the Palisades data, do not support the correlation of the seismic wave-forms with the plane impacts. 
Event Magnitudes: “[I]t is strange that identical events … at the same location,” said Rousseau, “would have generated seismic sources of different magnitudes.”  This discrepancy occurred both for the plane impacts and the building collapses. For the two waves attributed by LDEO to the impacts, the magnitudes of the signals  are different (0.9 for WTC 1, 0.7 for WTC 2), despite the similarity of the two plane crashes into the virtually identical buildings. The signals assigned to the collapses of the Twin Towers also display significant differences (magnitudes 2.1 for WTC-2 and 2.3 for WTC-1), again despite the similarity of the events resulting in the disintegrations of the two essentially identical buildings. Although the difference between 2.1 and 2.3 might seem minor, the unique (logarithmic) way in which seismic events are measured means that a shock that registers a magnitude of 2.3 releases twice as much energy as a magnitude 2.1 event, so the discrepancy is too large to have been due to an error.  Rousseau concluded that the waves had to have been caused by something else (which, given the evidence provided in Point TT-8, points to explosives). 
Wave Frequencies: The frequencies of waves caused by plane impacts, reported Rousseau, are typically much greater – one to two orders of magnitude higher – than the frequencies of the waves that were, according to LDEO, caused by the plane impacts into WTC 1 and 2. That is, the frequencies of waves typically caused by plane impacts range from (roughly) 10 to 100 Hertz (Hz), whereas the waves that were said by LDEO to be caused by the plane strikes are on the order of only 1 Hz. The idea that the seismic waves in question were caused by plane impacts was, therefore, highly unlikely. Furthermore, the recording equipment at Palisades had a range of only 0.6-5 Hz, so it was incapable of recording waves generated by typical plane impacts. 
The discrepancies described above indicate that the LDEO conclusions about the nature of the events that generated the signals recorded at Palisades cannot be correct. Most strikingly, the ground radar data, which is very precise, showed WTC 1 to have been struck 15 seconds later than the Palisades-recorded seismic activity, which LDEO scientists attributed to an airplane impact. The radar also shows WTC 2 to have been struck later than the seismic activity attributed to it. The seismic activity, therefore, must have been produced by something other than the crashes of the airliners into the two buildings.
Rousseau, like Furlong and Ross, provided reasons to conclude that the signals that the official story attributed to airplane impacts had actually been caused by something else – which, as evidence documented in Point TT-8 suggests, was shocks, explosive in nature, that had occurred at the bases of the buildings. Rousseau further demonstrated that the wave details themselves were characteristic of such explosions, not of plane impacts or building collapses.