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Seismic Records of the Twin Towers' Destruction:

Clarifying the Relationship Between Seismic Evidence and Controlled Demolition Theories

Jim Hoffman
Version 0.9, Oct. 31, 2006


The total destruction of each of the Twin Towers produced seismic signals that were recorded by at least 5 seismic observatories within 60 miles of lower Manhattan. The published charts from the Lamont-Doherty Earth Observatory in Palisades, New York, have been cited as evidence supporting the following two claims:

  • Each Tower fell completely in 10 seconds or less
  • The largest seismic spikes preceded the fall of each Tower

Both of these claims can be found in a large cross-section of literature challenging the official story. The first claim is also accepted by many defenders of official collapse theories, and is even found in the 9/11 Commission Report . In this essay I will examine these widely-repeated claims and show that, despite their popularity, they both lack supporting evidence and are contradicted by verifiable evidence. I will, however, also show that the seismic records do support the case for controlled demolition, but in a way that has been overlooked.

I have previously addressed both claims on these pages:

In this essay I will provide a more focused and complete exposition of these issues.


Do Seismic Records Really Show that the Towers Fell in Ten Seconds?

Because the claim that the Towers fell in ten seconds has historically been uncritically repeated by defenders and detractors of the official story alike, most newer researchers have not taken the time to closely examine its validity.

The idea that seismic records from distant recording stations could be used to establish the duration of a building collapse or demolition is highly questionable on its face given the complexities inherent in interpreting seismic data, such as the indirect relationship between ground movement at a source and and at an observatory miles away. Nonetheless, the assertion that seismic records from observatories such as that in Palisades establish that each Tower collapsed within ten seconds is widespread.

Origin of Claim that Towers Fell in Ten Seconds or Less

The assertion that both Towers fell in ten seconds or less apparently originated with proponents of the collapse theory less than a month after the attack. An article in the Journal of the The Minerals, Metals & Materials Society contains the following:

This started the domino effect that caused the buildings to collapse within ten seconds, hitting bottom with an estimated speed of 200 km per hour. If it had been free fall, with no restraint, the collapse would have only taken eight seconds and would have impacted at 300 km/h. 1
Figure 1: Table 1.1 from Chapter 1 of FEMA's WTC Building Performance Study.

The statement cites reference 1, which is listed as "1. Presentation on WTC Collapse, Civil Engineering Department, MIT, Cambridge, MA (October 3, 2001)."

Most of the more contemporary references to the ten-second collapse times appear to have originated with FEMA's Building Performance Study. Although the Report makes no claim about how long it took the Towers to "collapse", it includes the chart on the right describing the duration of seismic signals. That chart gives 10 and 8 seconds for the durations of the signals corresponding to the South and North Tower "collapses", respectively.

Apparently, the only official report that gives a duration for either of the "collapses" is the 9/11 Commission Report, which states that the South Tower collapsed in ten seconds in Chapter 9, Section 2:

At 9:58:59, the South Tower collapsed in ten seconds, killing all civilians and emergency personnel inside, as well a number of individuals-both first responders and civilians -- in the concourse, in the Marriott, and on neighboring streets. The building collapsed into itself, causing a ferocious windstorm and creating a massive debris cloud. 156

This passage cites reference 156, which is listed as "NIST report, 'Progress Report on the Federal Building and Fire Safety Investigation of the WTC,' June 18, 2004, appendix H, p. 40." However, wtc.nist.gov lists no publication dated June 18, 2004, and NIST-SP 1000-5 June 2004 Progress Report on the Federal Building and Fire Safety Investigation of the World Trade Center , [PDF] does not appear to provide any estimate of either Tower's collapse duration.

Video Records Show 15-Second+ Durations

Unlike seismic records, video recordings of the Towers' destruction allow the conclusive determination of lower bounds for the durations of each event, and those are much greater than ten seconds. Several live television broadcasts showed these events from their precipitous onsets to their explosive dust-shrouded conclusions. In each case, portions of the Tower below the exploding rubble clouds are visible up to at least the 12 second mark.

Figure 2: The North Tower at about 10 seconds into its destruction. About two-thirds of the Tower is still standing.

Lacking access to the uncut original broadcasts, I assembled timelines for each Tower using multiple video clips. These timelines clearly show that, in the case of each Tower, the process of destruction lasted a minimum of 15 seconds, not counting the persistence of fragments of core structures. This is true even accounting for uncertainties in precise times of onsets, greater uncertainties in the times of completions, and margins of error in the compositing of the timelines.

The onsets of the events are different in the two Towers. The South Tower's top leans for about two seconds before the roof starts to plunge downwards, whereas the North Tower's roof begins its plunge about a half second after its radio antenna begins to drop. In both cases I used the first evident motion -- the rotation of the South Tower's top, and the drop of the North Tower's antenna -- to set the timeline origin. Using these timelines, it is clear that large portions of each Tower below the descending debris clouds remained intact at the 10-second mark. Reasonable estimates for the duration of these events are around 17 seconds.

Did Seismic Spikes Really Precede Collapses?

The claim that seismic "spikes" preceded the collapse of each Tower has been uncritically reported by numerous websites, books, and videos challenging the official collapse story, including: WhatReallyHappened.com, Serendipity.li, Synthetic Terror, 9/11 Revealed, Loose Change, and 9/11 Mysteries. The claim is that the greatest ground shaking, whose estimated local magnitude was 2.1 and 2.3 on the Richter scale, occurred before or at the beginning of the destruction of each Tower, not at the end as the falling rubble was impacting the ground.

Origin of Claim that Spikes Preceded Collapses

The claim that the largest seismic signal marked the beginning of the destruction of each Tower apparently originated with reporter Christopher Bollyn, who published in August 28, 2002 the article "Seismic Evidence Points to Underground Explosions Causing WTC Collapse" in the American Free Press, the sister publication of the Hitler-lionizing Barnes Review. That article, commented on here, includes the following:

Seismographs at Columbia University's Lamont-Doherty Earth Observatory in Palisades, New York, 21 miles north of the WTC, recorded strange seismic activity on September 11 that has still not been explained.

While the aircraft crashes caused minimal earth shaking, significant earthquakes with unusual spikes occurred at the beginning of each collapse. The Palisades seismic data recorded a 2.1 magnitude earthquake during the 10-second collapse of the South Tower at 9:59:04 and a 2.3 quake during the 8-second collapse of the North Tower at 10:28:31.

The Palisades seismic record shows that -- as the collapses began -- a huge seismic "spike" marked the moment the greatest energy went into the ground. The strongest jolts were all registered at the beginning of the collapses, well before the falling debris struck the earth. These unexplained "spikes" in the seismic data lend credence to the theory that massive explosions at the base of the towers caused the collapses.
-- Christopher Bollyn, American Free Press

The article mixes facts about the Lamont-Doherty Earth Observatory's charts with unfounded assertions. While it's true that the station recorded 2.1 and 2.3 magnitude quakes, Bollyn provides no evidence that the spikes occurred at the beginning of each "collapse".

This story was reproduced by numerous websites, and continues to be cited even by scholars who apparently have failed to check the validity of its claims. After first providing some background on seismology, I will provide three lines of analysis showing that the largest seismic signals correspond to the times that the rubble was reaching the ground, not to times at or before the onsets of the Tower's destruction:

Interpreting Seismograms

Interpreting the data from seismographs is not as straightforward as implied by the article "Seismic Evidence Points to Underground Explosions Causing WTC Collapse". Seismology is a science typically practiced by scientists with graduate degrees in seismology or geophysics. Correctly interpreting the data from seismometers includes considering such factors as:

  1. Different response characteristics of different seismometers
  2. The various types of seismic waves and their propagation characteristics
  3. Different rates of propagation of a given type of seismic wave through different types of soil and rock

The first factor is typically dealt with by processing the instrument data with various functions such as convolution filters. The graphs presented on this page have already been massaged in some way.

The second factor arises from there being four distinct types of seismic waves, which belong to two categories: body waves, which can propagate deep underground, and surface waves, which propagate near the surface. Body waves come in two types: P or pressure waves, which oscillate in the direction of propagation, and S or shear waves, which oscillate perpendicular to the direction of propagation. Surface waves also come in two types: Rayleigh waves, which cause the ground to move laterally, and Love waves, which cause it to undulate vertically. Each of these waves has different propagation rates, which are similar for the two types of surface waves.

Figure 3: Vertical components of the North Tower event recorded by seismometers at five different observatories

According to the article Seismic Waves Generated by Aircraft Impacts and Building Collapses at World Trade Center, New York City by scientists at the Lamont-Doherty Earth Observatory of Colombia University, seismometers at the various observatories recorded only surface waves arising from events in lower Manhattan on 9/11/01.

The third factor can result in different observatories producing very different seismograms for the same event, because of the differing ground structure between the source and the different observatories.

Consider Figure 3, which shows recordings of the North Tower destruction made at five different observatories. The graphs are arranged in order of the distance of their respective observatories from the World Trade Center, with the Palisades chart being the first. The length of the high-amplitude signal varies from about six seconds for the observatories labeled TBR and ARNY to about 18 seconds for the one labeled MANY. The signal recorded by the observatory at Palisades (labeled PAL) has an intermediate length of about 10 seconds. According to the Colombia University scientists, the differing durations of the signals recorded by the different stations was a function of paths the wave packets took from their source to the various stations. Whereas waves followed relatively simple paths to the TBR, ARNY, and BRNJ stations, their traversal to the MANY and PAL stations up the Hudson was complicated by different strata of rock with different traversal speeds. The scientists concluded that the source event was about 5-6 seconds in duration, meaning that the signal at PAL was spread out by about 4 seconds.

The 'Large Spikes' Were Preceded by Smaller Signals

With the benefit of this background information let's look at a PAL seismogram in Figure 4, below. which represents horizontal ground displacement produced by the North Tower's destruction. At about 5 seconds into the graph we see a transition from background noise to a distinct signal with a period of about 0.8 seconds. That signal continues up until the onset of the large signal at about 17 seconds, becoming slightly stronger at about 9 seconds. The first signal thus starts about 12 seconds before the large signal. Since the spread of about 4 seconds would affect the small and large signals alike, the arrival of the first signal about 12 seconds before the large signal at PAL indicates the onset of an event generating a smaller disturbance about 12 seconds before an event generating the large disturbance at the source. A similar pattern is evident in the seismic records from the other observatories.

Figure 4: East-West component of the North Tower event recorded at PAL

If we now review the video timeline of the North Tower, we see that the rubble begins to reach the ground about 12 seconds after the roof begins to drop. Thus, the PAL chart, and those of the other observatories, show a pattern of seismic activity that fits the following interpretation of the North Tower's destruction. As the Tower breaks up from top to bottom, it generates a seismic disturbance of relatively consistent magnitude lasting as long as it takes the destruction to reach the ground -- about 17 seconds. At about 12 seconds into this event, this signal is eclipsed by the much larger disturbance of thousands of tons of material impacting the ground.

Correlating Seismic and Video Records of the North Tower

Figure 5: Frames from CNN's live broadcast at 2 second intervals

In my earlier analysis of the Towers' speed of fall I attempted to establish a correspondence between the CNN live broadcast of the North Tower's destruction and a seismogram from PAL showing that event. Correlating the timelines of these two pieces of evidence is possible because the CNN broadcast shows a real-time clock, the seismic records include precise time coordinates, and the travel time of seismic waves between the source and seismic observatory can be estimated.

The CNN live broadcast of the North Tower destruction, which is excerpted by this video clip, has a real-time clock on its banner which displays hours and minutes. The minute counter flips from 28 to 29 about 37 seconds after the roof starts to drop. On that basis, I estimated, in 2003, that the North Tower's destruction started at 10:28:23. That is one second from the time NIST assigned to the event in its 2004 Draft Report on the Twin Towers: 10:28:22.

Reports published on colombia.edu estimate that the start of the large signal originated at 10:28:31. The large spikes began to reach the PAL seismometers starting at around 10:28:48. Since Colombia scientists estimated that the surface waves traveled at about 2 km/s, and the station is 24 km from the World Trade Center, they placed the time of the onset of the large signal at the source at 17 seconds before the signal arrived at the observatory.

10:28:31 -- the estimated start of the large signal -- is 9 seconds after 10:28:22 -- the estimated start of the North Tower's destruction. Video recordings show that the rubble started to reach the ground about 12 seconds after the start of the destruction. These times fit the interpretation that the large signal corresponded to rubble hitting the ground, modulo an error of about 3 seconds. That error can be explained by the spread in arrival of wave packets described above.

To summarize, the video and seismic data support the following approximate timeline for the North Tower's destruction:

10:28:22 North Tower roof starts to drop (small signal starts)
10:28:34 Rubble starts to hit the ground (large signal starts)
10:28:36 The heaviest rubble hits the ground (large signal peaks)
10:28:40 The Tower is completely down, except for fragments of the core

Rubble Ground Impact Energy Explains Large Signal

In the article that first claimed that a "huge seismic 'spike'" marked the beginning of each "collapse", Bollyn also asserted that the signals were "strange", "unusual", "unexplained", and support a theory that "massive explosions at the base of the towers caused the collapses." However, none of these assertions are supported by the scientists at Lamont-Doherty Earth Observatory that Bollyn interviewed. Those scientists reported that the seismic energy represented only a fraction of the gravitational potential energy released when the Towers came down:

The gravitational potential energy associated with the collapse of each tower is at least 1011 J. The energy propagated as seismic waves for ML 2.3 is about 106 to 107 J. Hence, only a very small portion of the potential energy was converted into seismic waves.

Visual records of the events themselves and their aftermath clearly show that most of the mass of the Towers fell outside their footprints. Tens of thousands of tons of steel assemblies fell through the air -- some of it from a height of more than a thousand feet -- before hitting the ground. It is difficult to imagine that any events prior to the rubble hitting the ground could have produced anywhere near as much ground shaking. Even the detonation of large bombs in the building would have been dwarfed by comparison. According to the same scientists, the 1993 basement bombing did not produce detectable seismic signals:

A truck bomb at the WTC in 1993, in which approximately 0.5 tons of explosive were detonated, was not detected seismically, even at a station only 16 km away.

The 1993 explosion was so intense it blew a cavity about 5 stories high in the parking garage, constructed of steel-reinforced-concrete. Being in the basement, it was presumably well-coupled to the ground. Cutter charges needed to sever the Towers' core columns would be tiny by comparison.

The claim of "massive explosions at the base of the towers" at the onsets of the Towers' destruction is echoed by the 2005 film 9/11 Eyewitness, and other productions. None of these produce credible evidence to support this idea, which is contradicted by the visual records including photographs and videos, and the most credible and extensive body of eyewitness accounts.

The Seismic Records as Evidence of Controlled Demolition

Although the seismic records from the Doherty Earth Observatory and other recording stations don't support the theory of controlled demolition put forth by Christopher Bollyn and other proponents of basement bomb scenarios, they nonetheless do support the case for controlled demolition, albeit in an indirect sense.

The term controlled demolition has a broader meaning than many people appreciate. It does not require that a building be destroyed from the ground up, as NIST's Answers to Frequently Asked Questions or Protec's Critical Analysis imply. It simply means the destruction of a building in a controlled, or engineered, manner. In the case of the Twin Towers' top-down destruction, it is the only alternative to the premise of the official story that the Towers collapsed due to impact and fire damage. Hence a disproof of this premise would constitute a proof of controlled demolition. A disproof of a particular theory satisfying that premise would not prove controlled demolition since another theory, perhaps yet undiscovered, might explain the collapses. However, it is clear that a substantial number of experts have put a great deal of effort in coming up with the most plausible collapse mechanism that money can buy. To disprove the explanation endorsed by NIST -- the agency that has put more than $20 million into researching the issue -- would be strong evidence for controlled demolition.

A number of different theories of the Towers' collapse have been advanced by proponents of the official story, with the two most extensive government reports endorsing mutually contradictory theories. NIST's theory of "column instability" leading to "global collapse" has replaced FEMA's theory of floor pancaking leading to the buckling of "unsupported columns" as the explanation favored by most informed supporters of the official story. NIST's theory is essentially a pile-driver theory, in which the top of the Tower smashes the intact portion of the building to oblivion, from the crash zones downward. NIST hides the fact that it depends on this theory by failing to acknowledge the progressive nature of the event.

Once the upper building section began to move downwards, the weakened structure in the impact and fire zone was not able to absorb the tremendous energy of the falling building section and global collapse ensued.
-- Final Report of the National Construction Safety Team on the Collapses of the World Trade Center Towers

The pile-driver, AKA sledgehammer, theory explains the destruction of the intact portion of the Tower by the smashing action of the falling block. NIST hasn't described the scenario, but Professor Bazant did in a paper published in the Journal of Engineering Mechanics ASCE:

At that moment, the upper part has acquired an enormous kinetic energy and a significant downward velocity.
The part of building lying beneath is then impacted again by an even larger mass falling with a greater velocity, and the series of impacts and failures then proceeds all the way down.

It seems intuitively obvious that a smashing action sufficient to destroy the building would generate intense shaking. Figure 4 shows that the large signal is about 24 times the magnitude of the small signal that precedes it for about 12 seconds. Assuming that displacement magnitude is proportional to power, it follows that there was about 24 times as much energy being released at the peak of rubble hitting the ground than was being released at any one time in the 12 seconds before the rubble started to reach the ground. Since the Tower was mostly destroyed by the time the large signal started, the small signal has to account for the vibrational energy transferred through the columns to the ground from all of the smashing and crushing that was required to destroy the 95 floors of intact steel and concrete below the impact zone. Yet that energy was dwarfed by the energy released by the rubble hitting the ground.

Most expositions of collapse theories invoke the "tremendous energy" of falling mass impacting the floors below to explain the thorough destruction of the Towers. Yet the seismic records clearly show that the vast majority of this mass did not participate in the destruction of the Towers since it evidently did not encounter substantial resistance to its descent until it reached the ground.

Copyright (c) Jim Hoffman and 911Research.WTC7.net 2006