2002.09.21 Trying to Understand the Seismic Record
Sept. 21, 2002
This is an experiment to see how long it will take for me, a layman, to understand a technical question which is extremely important.
After reading an article on the Internet by Christopher Bollyn suggesting that the seismogram of the WTC collapse on 9/11 at http://www.ldeo.columbia.edu/LCSN/Eq/20010911_wtc.html indicated nuclear explosions in the basements of the towers to cause their collapse, I found this seismogram of an underground nuclear explosion: http://www.llnl.gov/str/Walter.html (Fig. 2, upper image).
The long, thin, vertical line, apparently characteristic of a nuclear explosion (as opposed, for example, to an earthquake), looks very similar to the long, thin, vertical lines in the seismogram of the WTC collapse.
The long, thin, vertical lines seem to indicate a very short but very intense ground movement that is much greater than the ground movement caused by either the impact of the planes or the falling rubble (as pointed out by Kim et al., "Seismic Waves Generated by Aircraft Impacts and Building Collapses at World Trade Center, New York City, Transactions, American Geophysical Union, Nov. 20, 2001, pages 565-571).
My question is: What caused these very short and very intense ground movements, if it was not the impact of the planes or the falling of the rubble?
Here is what three experts have had to say on the matter.
PM: There is no time scale illustrated on the Livermore recordings and they are recordings made at a distances of about 2500 km compared with 50km from the WT.. The appearance of the Columbia seismogram when the timebase is expanded clearly illustrates a very low frequency signal significantly different from any earthquake or nuclear explosion recorded at 50km. In fact the frequency of the WTO signals is about 1Hz compared with about 20Hz from quakes and often even higher from explosions (Both nuclear and conventional). A collapse signal is exactly that. We have a large number of seismic signals generated by the collapse of underground mines and nuclear post-explosion cavities to know what these signals look like. I have not seen the EOS paper but the largest signals that I saw were from the collapse, the very short duration signals generated by the impact may look explosion like but they are too low in frequency compared with those that would be generated by an explosion in the basement (almost fully coupled to the surrounding rock). Finally, seismological techniques can not differentiate between nuclear and non nuclear explosions at this extremely low level of signal to noise ratio or yield level. The unique identifier is of course the radionuclides.
If I have interpreted your question correctly the long vertical lines on the seismogram are 10 minute time markers. The longish red and black lines show the amplitude of the ground movement recorded by the Palisades seismometer (significantly greater than the impact signal).
MW: There are features in the seismic signals which make it quite clear that they are not the result of an explosion, let alone a nuclear explosion. What you observe as a spike, is in fact, a long drawn out signal of 10 seconds or more (when recorded 34 km away). It is a relatively low frequency signal which contrasts with what is observed in explosive sources. A third piece of evidence is the immergent signal. An explosive source would have had a sharp impulse signal.
In short, the seismic signals show no evidence of any type of explosion and are quite consistent with the building collapse.
The source of your confusion I believe is related to the time scale. The figure you sent of the Indian Nuclear explosion is a classic example. Certainly you see how different the explosion and earthquake look. However, the figure lacks any time scale - a significant omission. These signals are roughly a couple minutes long. The seismograms in figure 1 of the EOS paper however span 30 minutes. Thus the signals are very compressed. To see this, compare the top signal in figure 4 to the fourth line of figure 1. These are the SAME signal shown at different time scales. Essentially they are stretched in different ways to illustrate different features. I believe the difference between an explosive signal (such as the Indian explosion you sent) and the WTC collapse signal in figure 4 is clear even to the lay eye. The "long vertical lines in the graphs" are simply the same signals magnified vertically to illustrate details.
My unnamed colleague was quite articulate about the technical differences. I concur entirely with their statements.
MT: As I checked the Lamont-Doherty registrations and the seismograms, they represent only the collisions of the jets on the buildings and the collapses of the towers, nothing else. If there had been nuclear explosions in the buildings the radioactive fallout should have been detected if not globally, at least in the northern hemisphere. The seismic manitudes correspond well with the energy released to the ground, as the buildings collapsed. The energy release can be estimated to be approximately 3.5*10E+11 Joules which in turn corresponds an explosion power of 100 tonnes of TNT equivalent, and further the seismic magnitude of such an energy is equal to 2,7 which is very close to the values computed in Lamont-Doherty Laboratory from the registrations.
I have to say that despite the kind efforts of the three experts I have not yet succeeded in understanding what the long vertical lines represent, or why the striking similarity between the WTC seismogram and the seismogram of the Indian underground explosion is not significant. As for the lack of radioactive residue, that is a separate question which may not be relevant, since the explosion may not have been nuclear.
The results of my little epistemological experiment are thus negative--so far, at least. I will grateful for further enlightenment.