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Friday, January 20 2012 - Research/Evidence
Were Explosives the Source of the Seismic Signals Emitted from New York on September 11, 2001?
by André Rousseau
When major shocks occur at the Earth's surface or at depth, waves of different types, magnitudes and speeds may move out from the source location. Such waves can be detected by seismometers located at recording stations, and the data from the recordings can be analyzed to learn many details of the source events. Seismic signals were recorded at stations in New York and four neighboring states on September 11, 2001 during the period when the North and South Towers (WTC1 and WTC2, respectively) were struck by airliners and collapsed, as well as during the collapse of Building 7 of the WTC, which had not been hit by a plane.
Data from the Palisades, NY recording station, located 34 km north-north-east of Manhattan, published by the Lamont-Doherty Earth Observatory of Columbia University (LDEO), provide the most detailed seismic waveforms for analysis, particularly for the determination of the locations (aerial, surface, or subsurface) and timing of the events that created the seismic waves.
Seismologists have been puzzled in their analysis of signals recorded for the
events at the World Trade Center, as the contradictions are significant. They
are particularly intrigued by the presence of seismic "peaks" before
the collapses. This text focuses on the study of the seismic signals from Palisades.
The new interpretation presented here renders the assertions of the seismic
analysis of the events at the WTC, presented by the government, null and void.
STUDY OF THE COMPOSITION OF THE DIFFERENT WAVEFORMS
Five waveforms will be analyzed below. They are attributed by the LDEO team to specific causes, as follows:
1. the signals that, according to LDEO, match the moment when the planes hit
WTC1 and WTC2, respectively, shown in figures 1a and 1b;
The analysis presented here will question LDEO's identifications of the causes of the waveforms.
Determination of the Timing of the Signals' Origins
In these five cases the origin of the signals was attributed, by the seismologists who published the data, to the impacts of the planes or the collapses of the buildings (Kim et al. 2001; Irvine, 2001; Hoffman, 2006). Normally in this type of study, the time of origin is known with great precision (to the millisecond), which is necessary in order to calculate the propagation speed of the different waves. Unfortunately, that precision is not possible for the events at the WTC. In this case, timing of the waves must be correlated as well as possible utilizing video evidence.
The video used in this study for the North Tower (WTC1) was from a recording made by CNN with a time stamp on the screen (Hoffman, 2006), and the results were compared with the method utilized by LDEO (Kim et al. 2001). LDEO's method consisted of assigning an estimated speed of 2km/s for a Rayleigh wave (a type of surface seismic wave) that traversed several stations (see Figure 3) situated at various distances from the point of origin. The major inconveniences of this method are that the stations are not situated on a straight line and that the surface terrain, in which the surface waves move, varies and, therefore, the waves don't have the same speed of propagation as they pass through different materials.
The Hudson River is located on a fault line that separates predominantly sedimentary terrain on the west from crystalline and metamorphic rock on the east. These eastern formations permit more rapid surface wave propagation than those found to the west, which explains why the path WTC-MANY (Fig. 3), the only site to the east of the Hudson, was more rapid than all the other paths, situated to the west. In contrast, the stations at Palisades (34 km), at ARNY (67.5 km) and at TBR (51 km), provide similar results because they are situated on similar geological formations. Finally, the enormous indeterminacy of 2 seconds in the calculations attempting to fix the time of origin of each of the signals, admitted by the LDEO authors themselves (Kim, et al.), oblige us to view the official conclusions critically.
Waveforms Attributed to the Planes Crashing Into the Towers
The waveforms that the LDEO team attributes to the impacts of the airliners into the Twin Towers are shown in figures 1a and 1b. Although the waveforms look somewhat similar, they are sufficiently different to raise questions about LDEO's analysis. Although the cause of the two signals is similar -- the crashing of a plane, according to LDEO -- the magnitude (reflected by the amplitudes, or distribution on the vertical axis) of the two signals is different. Further, the waves generated by the two events do not have the same apparent velocity. The calculation of the propagation speeds, as shown in the graphs of Figures 1a and 1b, where the origin was fixed according to the corresponding crash, indicates 2900 m/s for WTC1 and 2150 m/s for WTC2.
A more serious difficulty with LDEO's attribution of the waveforms to plane impacts at the Twin Towers is that even if the impacts had been considerably more energetic, these signals could not have been generated by such impacts. The actual waves generated by the crashes had to have been deadened before hitting the ground. Frequencies of waves generated by explosions are on the order of 1 Hertz (1 Hz, or one cycle per second) -- which is the case with the Rayleigh waves shown in figures 1a and 1b -- while those of crash impacts are above 10 Hz and are often around 100 Hz. Furthermore, the range of the recording instruments (0.6-5 Hz) cited does not allow for the recording of the high-frequency waves that would be created by plane impacts. As to the theory of the oscillation of the Towers to explain these signals, as defended by Irvine (2001), it is inadequate because in such a case we would have had a "square" signal of long duration and a constant amplitude, while in actuality we observe a "bell-like" signal, representing a strong and brief explosion, which is particularly evident in the case of WTC2.
Given that it is geophysically impossible to have two different propagation speeds for two waves of the same type at the same frequency travelling the same path only a few minutes apart, one must bow to the evidence that the supposed origins of the recorded waves are incorrect, and that they are not linked to the plane crashes but to another origin. The waveform data, far from suggesting the conclusion of LDEO that they were caused by plane impacts into the Towers, suggest instead two explosions with different time displacements from the moments of plane impact at each building. Further, the difference in the magnitude of the two signals can only be linked to differences in the volume of explosives and/or their distance from the surface.
Waveforms Attributed to the Collapse of the Towers
While the Twin Towers had approximately the same mass, the same height and size, and the same type of internal structure (as well as essentially identical points of origin of the seismic wave-data in terms of distance to the recording station), the signals attributed to the collapses of WTC1 and WTC2, instead of being similar as one would suppose from the official thesis, are in fact very different. They differ in their form, their composition, and especially in their apparent propagation speed, as calculated from the official origin time.
In fact, the recording for WTC1 (Fig. 2a) demonstrates the three types of wave characteristic of a brief explosive source confined in a compact, solid material: a P wave with a speed of 6000 m/s, the typical value for a very consolidated crystalline or sedimentary terrain (which is the case in the bedrock of Manhattan), an S wave with a speed of 3500 m/s, and a surface wave with a speed of 1800 m/s (a Rayleigh wave). These values match with those registered from an earthquake or seismic prospecting (see for example Kim et al. 2001).
On the other hand, the recording linked to WTC2 (Fig. 2b) does not show the P or S body waves observed for WTC1 but only the surface Rayleigh wave, for which the spreading of the amplitudes over the duration is different from that of WTC1. The propagation speed of 2125 m/s is also markedly different from that of WTC1. Further, this wave seems to be followed by a second Rayleigh wave four seconds later.
We find the same thing for WTC7 (Fig. 2c), where the calculation of the speed of the wave according to the determined origin time indicates a Rayleigh wave with a 2200 m/s speed. Note that the amplitudes are comparable to those of the waves emitted at the time of the crashing of the airplanes into the Towers. This wave seems to be followed by a second Rayleigh wave 6 or 7 seconds later.
In the three cases, the bell-like form points to an impulsive source of energy,
not percussion on the ground due to the fall of debris. The total mass and the
average mass of individual building fragments were relatively small and fell
to the ground over a period of more than ten seconds (which is a very long time
in geophysics). Also note that the duration of a seismic signal does not tell
anything about the source, contrary to the amplitude and particularly the frequency.
The problem of the "displacements" between the times of origin of the seismic waves and the times at which the planes crashed into the Towers, particularly that for WTC1, is certainly a key question and one that is emblematic of all the contradictions of the official version of September 11, 2001. The LDEO published two different timetables of wave-origins (Kim et al. 2001), which are presented in the table below. The first timetable (LDEO ) is that furnished with the published graphs. Then the LDEO modified its timetable (LDEO ). The widely varying but still, somehow, official times given by the 9/11 Commission and by the National Institute of Standards and Technology (NIST) are also presented in the table.
What are the indisputable data here? There are two: the time that the waves reached the Palisades station, which is relatively easy to determine, and the distance from the WTC to Palisades (34 km). If the recorded wave is actually a Rayleigh wave, its (group) velocity is around 2000 m/s. Therefore, this wave was created 17 seconds before its arrival at Palisades. Where the problem deepens for the defenders of the official version is that the time for the source of the Rayleigh wave attributed to the crash into WTC1, which officially arrived at Palisades at 8.46.42+/-1, must in fact be 8.46.25+/-1. Compare that time with the times given in the first column of the table below. Only the revised LDEO timetable (LDEO ) comes close.
The times put forward by the 9/11 Commission come from radar at ground level and are based on the National Transportation Safety Board (NTSB) and Federal Aviation Administration (FAA) data. They are the only reliable times because they are based on ground radar data which do not involve any hypothetical assumptions. They are reliable to one second.
For the time of the impact of the plane into WTC1 furnished by the Commission, 8.46.40 (9/11 Commission Report, p. 7; Ritter, 2002), there is a hiatus of 15 seconds between the plausible time of the origin of the Rayleigh wave based on the Palisades data and the time -- afterwards -- of the crash of the plane into WTC1 based on the ground radar data. What else but an explosion could be the origin for this seismic wave in the absence of an earthquake? A similar discrepancy exists in the data for the seismic wave and impact times for WTC2.
Also, the crash of the plane into WTC2 cannot be the cause for a camera, solidly
on the ground and probably mounted to a tripod, which is filming WTC1 (see 911Blogger.com,
2006) to strongly shake one second before the fireball following this impact
and shake again five seconds later: Only strong explosions can cause such shaking.
This has been discussed at length by MacQueen (Journal of 9/11 Studies
SEISMIC WAVE-GENERATION FROM IMPACTS, COLLAPSES AND EXPLOSIONS
Attributing the transformation of kinetic energy into seismic waves to the crash of a jet into a building would only make sense if such a crash involved two full, solid and non-deformable objects. In this case, the kinetic energy of the moving body would in part be transformed into heat and the rest would be transmitted to the stricken object in the form of vibrations, that is, seismic waves. However, that is not the case here because we have two hollow and deformable objects. During the crash, the whole of the energy is transformed into heat and the envelopes (exterior walls) are deformed. In the case where a little mechanical energy would remain, the waves created in the pierced envelope would be quickly dispersed because of the absence of continuity in this envelope due to the spaces between vertical and horizontal structural members, such as rooms and windows. The necessary condition for the creation of seismic waves by such a crash would be the direct impact into the central columns by a full body. Even if a Boeing engine had hit a core column, it would have been with an energy lessened by passage through the building's envelope. In conclusion, even if a seismic wave could be created in a steel column, it would hit the ground only in the form of seismic noise, and as the passage from metal to rock is a refraction that absorbs energy, there wouldn't be much left to propagate in the ground.
Could the collapse of the Towers be the source of seismic waves as claimed by LDEO and other defenders of the official account? The enormous mass of the Twin Towers could hypothetically be taken into account if the Towers had fallen in a compact block, like a meteorite. But in fact, it was mostly scattered shards, not coherent blocks, that fell, largely transformed into dust, and the fall spanned several seconds. This form of collapse could only produce a force far below that necessary to create seismic waves; in this case, the magnitudes simply don't add up.
Given that neither the crashes into the towers, nor their vibration, nor the fall of debris can be the source of the seismic waves registered 34 kilometres away, as well as the fact that the low frequencies of those waves could not have been generated by such phenomena, we must search for the actual causes of the waveforms observed. Only explosions could produce the waves observed but various possible explosive configurations must be considered. We must distinguish between 1) subterranean explosions, 2) aerial explosions and 3) subaerial explosions (close to the ground without touching it).
Subterranean explosions are similar to earthquakes in that mechanical energy is transmitted to the earth in the form of body waves of two types, P and S (for "primary" and "secondary," or "pressure" and "shear"), and surface waves (either Rayleigh or transverse L) when the signal reaches a solid-fluid interface (for example, the atmosphere at the surface). Another name for Rayleigh waves is ground roll.
Aerial explosions release all of their energy in the air (as P waves, which in the atmosphere are simply sound waves), and what remains upon hitting the ground is thus too weak to create body waves in the solid earth (although there can be surface waves over a small distance).
Subaerial explosions give off energy that splits into sound waves, mainly in
the air, and surface waves in the ground.
EXPLOSIONS THE SOURCE OF 9/11 SEISMIC WAVEFORMS
In brief, a subterranean explosion would not be heard but the ground would shake and initiate a series of waves (body and surface waves). If we hear an explosion, it is either aerial, which doesn't give a seismic signal, or it is subaerial, in which case surface waves could be generated. The seismic wave data provided by Palisades prove the occurrence of surface waves radiating outward from the World Trade Center. In addition, witnesses reported hearing explosions very close to the times at which planes struck the Towers and when they collapsed.
Given these two types of evidence we can affirm that explosions, qualified as subaerial, occurred close to the base of the Towers almost or quite simultaneously with the crashes into the Towers by the planes. The sound coming from these explosions would have been mixed with the sounds generated by the impacts of the planes. The Towers were thus weakened by the breaking of the load-bearing columns. The explosion at the base of WTC1 was heard by witness William Rodriquez (Spingola 2005).
The waveforms produced by the collapses of WTC2 and WTC7 were of a different type than that generated by the collapse of WTC1. Based upon the kind of waves coming from WTC2 and WTC7, they each underwent a very large subaerial explosion, heard by witnesses. For example, in the case of WTC2, a fireman witnessed an explosion before the building collapsed into an enormous cloud of dust (see Testimony , below), apparently not too far from the base of the Tower, accompanied by flashes of light and noise, according to an "Assistant Commissioner" (see Testimony ). Another fireman, present at the base of WTC2, stated there was a large explosion about 20 floors below the impact zone of the plane just before the upper portion of the Tower began to collapse (Testimony ). These explosions were too high above the surface to generate body waves in the ground, and the Rayleigh wave recorded probably comes only from the explosion closer to the surface. Among the other explosions heard at the base of WTC2 (Anonymous, 2009), one of them generated the second Rayleigh wave recorded four seconds after the first. The same thing happened at WTC7. A witness watching this building heard something like a "thunderclap" that caused the windows to explode outwards, while the base of the burning building gave way a second later, before the whole building followed the movement (Testimony ), aided by a second explosion, which generated the second Rayleigh wave 6 to 7 seconds later.
In the case of WTC1, the collapse of which began after that of WTC2 in spite of the fact that it had been hit earlier, it was a subterranean explosion that preceded its collapse. This explosion was thus logically not heard by the witnesses outside at 10:28 EDT, except for those located next to the Tower (Testimony ), but it was "felt" by a camera filming the tower that was solidly on the ground and was shaken by the vibration of the ground at the moment of the explosion (see ). On the other hand, it is also logical that the many explosions shown in videos in the upper floors before and during the collapse didn't provoke any seismic waves, because of the aerial locations and the fragmentation in time of the detonated energy in the series of successive sources, each of which had only a limited force, insufficient to generate seismic waves in the ground.
Even if standard controlled demolitions do not create seismic waves (because the explosions are aerial), it is useful to compare the data from the World Trade Center on 9/11 with seismic data obtained during the controlled demolition of the Kingdome in Seattle (see Anonymous, 2009) and at Oklahoma City (US) (Holzer et al., 1996). The case of the Kingdome is particularly interesting because seismologists expressly asked that the explosions be measured (in order to take advantage of the occasion to gather research data), and those in Oklahoma City were part of a reconstruction, using explosives, of the bombing of the Alfred P. Murrah Federal Building. These two examples involved a powerful subaerial explosion and the emitting of Rayleigh waves. Furthermore, the falling of the debris had no seismic consequences, even at distances well below 34 km (less than 7 km and 26 km respectively). Only the seismic equipment situated close to the source during the reconstruction of the bombing in Oklahoma City was able to record the seismic energy created by the collapse of the building.
The local magnitudes (ML) that the LDEO seismologists calculated from the surface waves gave results that consolidate our analysis. They were higher than 2 on the Richter scale for the waves emitted at the moments of the collapses. It is impossible to get such a magnitude from the falling of the debris alone, especially falling over a duration of ten seconds. Even if an entire Tower had been compacted into a tight ball, it would have necessitated a higher speed than could be caused by the Earth's gravity to even approach such a magnitude. Moreover, we must note that the magnitude attributed to the subterranean explosion at the WTC1 is ML=2.3 -- comparable to the earthquake that hit New York on January 17, 2001 (ML =2.4) -- while the magnitude coming from the WTC2 explosion is ML =2.1, thus weaker. And this disparity, consistent with the explosions described by witnesses in the cited testimonies, is particularly appreciable given the logarithmic scale used to designate event magnitudes. Given that the Twin Towers were of similar height and mass, the falling debris from the collapsing Towers should have generated similar magnitudes, if they were indeed the sources of the waves.
Note that applied geophysicists know how to generate seismic waves in the ground
using non-explosive techniques such as "weight dropping" -- which
consists of letting a three-ton weight fall to earth -- or using vibrators
attached to the ground. But the energy of the waves developed in the ground
by such methods is too low for the waves to go further than several hundred
meters. On the other hand, Won-Young Kim, lead author of the LDEO studies of
the Palisades data, has discussed similar seismic waves generated by subterranean
explosions from mining operations in the region. Kim thus confirmed our result
that subterranean explosions register event magnitudes comparable to those noted
for the WTC events, as well as our finding that subaerial explosions may not
register those event magnitudes if not sufficiently powerful.
THE RELATION BETWEEN THE SEISMIC WAVES AND THE PROCESS OF COLLAPSE
Observation from videos of the collapses of the Towers shows they were each different, and this correlates with the differences between the corresponding seismic waves.
The collapse of WTC7 is the one that comes closest to a classic controlled demolition, with the successive collapsing of the floors starting from the base, which had been weakened by a strong subaerial explosion. As for the Twin Towers, they were first weakened by explosions at their base at the moment the airplanes crashed into them. After that we must distinguish between the parts of the building above the impact zone of the planes and those located below. If the seismic waves couldn't have been generated by the explosions visible in the floors (which allowed for the gradual collapse upwards above the impact zone and downwards below this zone), then only a powerful explosion at the base of WTC2 and a subterranean one under WTC1 could have produced the observed seismic waves. These basal explosions were necessary as well for the total, rapid disintegrations of the buildings.
In the case of WTC1, the Federal Emergency Management Agency (FEMA) implicitly
confirmed this scenario. They noted, "Review of videotape recordings of
the collapse taken from various angles indicates that the transmission tower
on top of the structure began to move downward and laterally slightly before
movement was evident at the exterior wall. This suggests that collapse began
with one or more failures in the central core area of the building" (FEMA,
World Trade Center Building Performance Study, Chapter 2). This transmission
tower was supported by a lattice of large diagonal I-beams, called a "hat
truss," that connected the walls of the perimeter of the building to the
central structure between the 107th floor and the roof, and therefore reinforced
the central structure. Contrary to official findings that it was the hat truss
that transferred the instability of the central columns to those of the perimeter,
which then gave out after they were deformed because of the pulling of the floors,
the logic of the events forces us to consider that the rupture of the central
columns came from an explosive event prior to the collapse of the building.
At the times of the planes' impacts into the Twin Towers and during their collapses, as well as during the collapse of WTC7, seismic waves were generated. To the degree that (1) seismic waves are only created by brief impulses and (2) that low frequencies are associated with an energy (magnitude) that is comparable to a seismic event, the waves recorded at Palisades and analyzed by LDEO undeniably have an explosive origin. Even if the planes' impacts and the fall of the debris from the Towers onto the ground could have generated seismic waves, their magnitude would have been insufficient to be recorded 34 km away and should have been very similar to one another. As we have shown, they were not.
The types and magnitudes of the seismic signals show significant differences. The greatest differences occur in their propagation speeds, even though their paths were essentially identical under identical conditions. This difference is physically unexplained in the interpretation of the events offered by the LDEO researchers, the 9/11 Commission and NIST. Therefore, we must question their calculations of wave propagation speeds based on their assumption that the wave origins are shown on the video images of impacts and collapses. We can only conclude that the wave sources were independently detonated explosives at other times, thus accounting for the variable discrepancies for each wave origin in relation to the videos.
The composition of the waves is revealing both in terms of the location of the source and the magnitude of the energy transmitted to the ground. The subterranean origin of the waves emitted when WTC1 collapsed is attested by the presence of the P and S body waves along with the Rayleigh surface waves, which are present in all five explosions. The placement of the source locations of the four other explosions is subaerial, attested by the unique presence of Rayleigh waves. The aerial explosions visible on the videos of the upper floors of the Twin Towers do not produce seismic waves 34 km from the source.
There is a factor of ten between the power of the explosions at the time of the plane impacts on the Twin Towers (as well as at the time of the collapse of WTC7) and the strength of those more powerful explosions at the times of their collapses, the subterranean explosion under WTC1 being the one that transmitted the most energy to the ground.
Note as well the degree to which the surface waves are dispersive (i.e., their speed depends upon their frequency). The duration of the recorded signal is not representative of the duration of the signal at the source.
Finally, controlled demolition of the three towers, suggested by the visual
and audio witness testimony as well as by observations of video recordings of
their collapses, is thus confirmed and demonstrated by analysis of the seismic
waves emitted at the moments of the plane impacts and at the moments of the
Dr. Andre Rousseau is a former researcher in geophysics and geology at CNRS (the National Center of Scientific Research of France) and specialist in acoustic sound waves. He is a member of Scientists for 9/11 Truth. E-mail: firstname.lastname@example.org. (Tod Fletcher provided editorial assistance by revising an earlier version of this article.)
Testimonies from Witnesses
Note: Testimonies 1, 2, 3 and 5 are taken from "Oral Histories from Sept. 11 Compiled by the New York Fire Department," The New York Times (2005). http://graphics8.nytimes.com/packages/html/nyregion/20050812_WTC_GRAPHIC/met_WTC_histories_full_01.html
(1) Interview of fireman Richard Banaciski, who was in the street facing WTC2
I just remember we were -- initially we were out by the street and they started having jumpers, so they all kind of moved back towards the parking garage, towards the building, so nothing would come down on us.
(2) Interview of Assistant Commissioner Stephen Gregory, who was in the street facing WTC1
At that point in time we heard a rumble, we heard a noise, and then the building came down. ...
(3) Interview of fireman William Reynolds who was in front of WTC2
After a while, and I don't know how long it was, I was distracted by a large explosion from the south tower and it seemed like fire was shooting out a couple of hundred feet in each direction, then all of a sudden the top of the tower started coming down in a pancake. ...
(4) 9/11/2001 radio broadcast, "9/11 Videos: The Controlled Collapse of WTC7"
I was just standing there, ya know... we were watching the building [WTC 7] actually 'cuz it was on fire... the bottom floors of the building were on fire and... we heard this sound that sounded like a clap of thunder... turned around - we were shocked to see that the building was... well it looked like there was a shockwave ripping through the building and the windows all busted out... it was horrifying... about a second later the bottom floor caved out and the building followed after that... we saw the building crash down all the way to the ground... we were in shock.
(5) Interview of EMS Lieutenant Bradley Mann
We were in the staging area the entire time. Shortly before the first tower came down I remember feeling the ground shaking. I heard a terrible noise, and then debris just started flying everywhere. People started running towards the staging area. ...
(6) Video proving the existence of an explosion at the base of WTC1
The 2.6 MB wmv video shows the collapse of WTC1 .... The camera was not hand held, it was directly connected to the ground via a tripod, and this allowed the camera to visually capture a ground shake which occurred ~13 seconds before the building collapsed. The video also shows an object fall from the right hand side of the building moments before the camera begins to shake. The close timing of these two events indicates they are linked."
911blogger.com, u2r2h (2006): "Video proof of inside job (no less!)." http://911blogger.com/news/2006-10-13/video-proof-inside-job-no-less
Bower D.R. (2002): "Radar Data Impact Speed Study, American Airlines Flight 11, United Airlines Flight 175," National Transportation Safety Board, Office of Research and Engineering, February 7, 2002.
Hamburger R., Baker W., Barnett J., Marrion C., Milke J. and Nelson H. "Bud" (2009): "World Trade Center Building Performance Study, Data Collection, Preliminary Observations, and Recommendations," FEMA, Chapter 2. http://www.fema.gov/pdf/library/fema403_ch2.pdf
Hoffman J. (2006): "Seismic Records of the Twin Towers' Destruction: Clarifying the Relationship Between Seismic Evidence and Controlled Demolition Theories," 911research.wtc7.net. http://911research.wtc7.net/essays/demolition/seismic.html
Holzer T. L., Fletcher J. B, Fuis G. S., Ryberg T., Brocher T. M. and Dietel C. M. (1996): "Seismograms offer insight into Oklahoma City bombing," EOS, Transactions American Geophysical Union, 77(41), pages 393, 396-397. http://www.agu.org/pubs/crossref/1996/96EO00269.shtml
Irvine T. (2001): "The World Trade Center Disaster: A Shock and Vibration Perspective," Vibrationdata.com, pages 4-10. http://www.vibrationdata.com/Newsletters/November2001_NL.pdf
Kim W.Y. (2001): "Comparison of E-W component records at PAL (Palisades, NY) from the World Trade Center collapse (North Tower) on Sept. 11, 2001 at 14:28:31 (UTC) and from an earthquake which occurred about 7 km under the upper east side of Manhattan on Jan. 17, 2001 at 12:34:22 (UTC)." In IRIS Newsletter, 2001, number 1, page 8. http://www.iris.edu/hq/files/publications/iris_newsletters/doc/2001-newsletter.pdf
Kim W.Y., Sykes L.R., Armitage J.H., Xie J.K., Jacob K.H., Richards P.G., West M., Waldhauser F., Armbruster J., Seeber L., Du W.X. and Lerner-Lam A (2001): "Seismic Waves Generated by Aircraft Impacts and Building Collapses at World Trade Center, New York City," EOS, Transactions American Geophysical Union, Vol. 82, No. 47, pages 565, 570-571, November 20, 2001. http://www.ldeo.columbia.edu/LCSN/Eq/20010911_WTC/WTC_LDEO_KIM.pdf
Kim W.Y. (2000): "Analysis of Regional Seismic Data for Underground Explosions," Lamont-Doherty Earth Observatory of Columbia University, September 2000. http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA530298
Mackey R. (2007): "On Debunking 9/11 Debunking: Examining Dr. David Ray Griffin's Latest Criticism of the NIST World Trade Center Investigation," Journal of Debunking 911 Conspiracy Theories, Volume 1, Issue 4, pp. 79-82. http://www.jod911.com/drg_nist_review_2_1.pdf
MacQueen, Graeme (2009). "Did the Earth Shake Before The South Tower Hit the Ground?" Journal of 9/11 Studies, July 9, 2009. http://www.journalof911studies.com/volume/2009/MacQueen_EarlyEarthShake.pdf
Pacific Northwest Seismograph Network (2000): "Kingdome implosion seismograms, special Kingdome Implosion event information," University of Washington Department of Earth and Space Sciences. http://www.geophys.washington.edu/SEIS/PNSN/WEBICORDER/KINGDOME/
Ritter J. (2002): "Flight Path Study - American Airlines Flight 11," NTSB, Office of Research and Engineering, February 19, 2002. http://www.gwu.edu/~nsarchiv/NSAEBB/NSAEBB196/doc01.pdf
Spingola D. (2005): "William Rodriguez, a 9-11 Survivor," Spingola.com. http://www.spingola.com/william_rodriguez.htm
WhatReallyHappened.com, Unnamed author (2009): "9/11 Firefighters: Bombs and Explosions in the WTC." http://whatreallyhappened.com/WRHARTICLES/911_firefighters.html
Figures are shown as published by the LDEO without modification of their scales, which are not uniform.
Waves emitted at the times of the aircrafts' impacts
Waves emitted at the times of the towers' collapses
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