The Weapon and Science I


In this section, the weapons used to destroy the Twin Towers and the other buildings in the WTC complex, as well as the weapon that caused the massive crater and seismic activity in Shanksville, Pennsylvania, are examined here.

Fatman – bomb dropped on Nagasaki

IMAGE: Nagasaki-type Atomic Bomb “Fatman”. The Nagasaki atomic bomb was nicknamed “Fatman” because of its shape. The TNT implodes on the core of plutonium, causing nuclear fission.

The Bomb Called “Little Boy”

Through self-sustaining nuclear fission (chain reaction), an atomic bomb emits a huge amount of energy instantaneously.
The atomic bomb dropped on Hiroshima was a gun-barrel bomb. The uranium was divided into two pieces, both less than a critical mass and placed at either end of a cylinder. When one piece was fired into the other, critical mass was achieved.
Though the bomb was long and thin in shape, it grew shorter over the course of the project. Hence, the final bomb was called “Little Boy.”

Shockwave video: “Little Boy,” the Atomic Bomb Dropped on Hiroshima

VIDEO: “Little Boy” the atomic bomb dropped on Hiroshima (Dailymotion)

Larger version hereSource URL

1. A detonator was used to trigger the explosion.
2. One mass of uranium was fired into the other.
3. The neutrons created by that firing started a fission chain reaction, generating a tremendous amount of energy instantaneously. URL






Images were taken from the swf file above

RNEP Nuclear Bunker Buster

Dailymotion | Youtube: RNEP Nuclear Bunker Buster (from Union of Concerned Scientists) “RNEPFallout” (SOURCE URL 1 | SOURCE URL 2)

Nuclear fission

The nuclear fission reaction

VIDEO: “Nuclear fission reaction” (Dailymotion)

Caption: “When a neutron strike the nucleus of a uranium (or plutonium) atom, the nucleus splits, releasing two or three neutrons and a large amount of energy (as radiation, heat, etc.). The neutrons thus released strike other nuclei, and the repetition of this nuclear fission is known as a “chain reaction.” When a chain reaction occurs in an extremely short time, a tremendous amount of energy is released. The atomic bomb utilized this synergy as a weapon.” (URL)

Watch larger version of video here


IMAGE: Fat Man – internal components

Fat Man: Implosion-type bomb


IMAGE: “The initial design for the plutonium bomb was also based on using a simple gun design (known as the “Thin Man”) like the uranium bomb. As the plutonium was produced in the nuclear reactors at Hanford, Washington, it was discovered that the plutonium was not as pure as the initial samples from Lawrence’s Radiation Laboratory. The plutonium contained amounts of plutonium 240, an isotope with a rapid spontaneous fission rate. This necessitated that a different type of bomb be designed. A gun-type bomb would not be fast enough to work. Before the bomb could be assembled, a few stray neutrons would have been emitted from the spontaneous fissions, and these would start a premature chain reaction, leading to a great reduction in the energy released. Seth Neddermeyer, a scientist at Los Alamos, developed the idea of using explosive charges to compress a sphere of plutonium very rapidly to a density sufficient to make it go critical and produce a nuclear explosion.” URL:

The Structure and Explosive Mechanism of the Hiroshima Bomb

Caption: “The Hiroshima Bomb (Little Boy). About three meters long and weighing about four tons, the bomb dropped on Hiroshima was called “Little Boy” because it grew shorter over the course of the project … The Hiroshima bomb was designed to use gunpowder to blast one piece of uranium 235 into another, in order to reach a super-critical mass instantly. Roughly 50 kilograms of uranium 235 were packed into the bomb, of which less than one kilogram instantly underwent sustained nuclear fission, releasing energy equivalent to 16,000 tons of chemical high explosives. Highly sensitive radar attached to the bomb enabled it to explode at the altitude calculated to maximize the effects of the explosion.” URL:

The Hiroshima Bomb (Little Boy)

IMAGE: “About three meters long and weighing about four tons, the bomb dropped on Hiroshima was called “Little Boy” because it was relatively thin.” URL:

The Hiroshima Bomb

The Hiroshima Bomb (Little Boy)

Because of its long, thin shape, the Hiroshima bomb was called Little Boy. The fissile material was uranium 235. The uranium was divided into two parts, both of which were below critical mass. It was a “gun barrel-type” bomb that used an explosive device to slam one portion of uranium into the other, instantly creating a critical mass.
When a critical mass is available, a chain reaction takes place instantaneously, releasing energy far beyond the capacity of ordinary explosives. The energy released by the Hiroshima bomb was originally thought to be equivalent to the destructive power of approximately 20,000 tons of TNT. Later estimates based on damage to buildings and studies of the bomb’s structure have reduced that figure to approximately 15,000 tons. It is believed that this enormous energy was released by the fission of slightly less than one of the 10 to 35 kilograms of uranium 235 in the bomb. (original link)

Alternative link:

Little Boy – a gun-type bomb


Caption: “In essence, the Little Boy design consisted of a gun that fired one mass of uranium 235 at another mass of uranium 235, thus creating a supercritical mass. A crucial requirement was that the pieces be brought together in a time shorter than the time between spontaneous fissions. Once the two pieces of uranium are brought together, the initiator introduces a burst of neutrons and the chain reaction begins, continuing until the energy released becomes so great that the bomb simply blows itself apart.” URL:

Fizzle yield

Even a sub-kiloton bomb can cause considerable damage

Reasons for a fizzle are examined. A fizzle yield is a yield that is less than expected. A nuclear bomb with a fizzle-yield could have caused considerable damage in the WTC attacks. It’s possible that the perpetrators deliberately employed a bomb that would “fizzle” so as to limit the scope of the nuclear destruction and disguise the fact that a nuclear device was used. In other words, the perpetrators would have selected a detonation size that would be less than typically seen for a nuclear attack but of a size that would cause the damage they wanted to cause. A “fizzle-yield” nuclear bomb would fit the bill: it would cause a detonation half-way between that caused by conventional explosive devices and a small (1 kiloton) nuclear bomb.

Reasons for a fizzle

There are several reasons why a nuclear test might fizzle. Indeed, “fizzle yield” is a recognized nuclear term, indicating the complexities of igniting a fission bomb, especially one that is made from plutonium.

A fizzle can happen when the bomb literally blows itself apart too fast for the nuclear chain reaction to take place properly and generate the large amount of energy needed for a full-scale explosion. Should that happen a bomb meant to be in the Nagasaki range (20kt) may yield only about a kiloton.

There are two ways this can happen. One is contamination with Plutonium-240. The key ingredient for a plutonium bomb does not exist in nature; it is manufactured in a nuclear reactor when atoms of Uranium-238 capture loose neutrons and are converted to Plutonium-239, the stuff of bombs.

But these plutonium atoms can, in turn, capture neutrons, becoming Pu-240 and even Pu-241. With a bomb contaminated with Pu-240, the probability of a fizzle is very large. The US maintains a standard that none of its bomb-grade plutonium will have more than 6% Pu-240. [..]

The other problem concerns detonation. Plutonium bombs work on the “implosion” principle. A sub-critical core of plutonium about the size of soft ball is surrounded by conventional explosives. The pressure from the explosion squeezes the plutonium into a critical mass, setting off the nuclear explosion.

But the shaped charges must be so precisely engineered that they go off simultaneously. If even one charge explodes prematurely, even by a nanosecond, it may blow the bomb apart, cutting short the chain reaction and reducing the yield.

Of course, even a sub-kiloton bomb can cause a lot of damage. A one kiloton bomb will have the radius of destruction of about a third of the Hiroshima bomb, not exactly a city-flattener, but perhaps something that could work as a terrorist weapon.


1) A fizzle yield, for those just coming in, is a bomb failure where the chain reaction blows the fissionable material apart, curtailing a complete explosion. Interestingly, the typical yield of fizzle explosion is a kiloton or under.

2) a ‘fizzle yield’; that is, the smallest nuclear yield this particular device could provide.

Reasons for a fizzle – more

Plutonium weapons have several ways of misfiring. The first depends on the triggering of the plutonium by an implosion process. The implosion must be extremely symmetrical to be fully successful. Typically a combination of fast and slow conventional explosives surrounds a sphere of plutonium (the “core” or “pit”). Engineers must carefully machine all the pieces that make up this explosive shell into shapes that, when detonated simultaneously, produce a precisely spherical shock wave that compresses the plutonium to two to five times its normal density (the more compression, the greater the explosive yield). At the higher density, what was a subcritical mass of plutonium becomes supercritical–that is, one in which a sustained chain reaction takes place, producing the blast.

If the shock wave fails to be completely symmetrical–for example, if a detonator goes off 100 nanoseconds later than the rest–the compression will be less efficient because the core will tend to squirt out in the directions where the shock wave is weaker or arrives late. Another potential troublemaker is the initiator, a small device at the center of the core that emits a burst of neutrons to start the chain reaction reliably at a precise stage of the implosion. An initiator going off early or late–or not at all–reduces the yield.

Early triggering of the explosion, a kind of fizzle called a predetonation, can also occur when too much of the isotope plutonium 240 is present. The fuel rods of nuclear reactors produce the desired isotope, plutonium 239, but the longer it remains in the reactor, the more of it becomes plutonium 240. Plutonium 240 constantly emits tens of thousands of times more neutrons a second than plutonium 239. Although neutrons are the key particles in producing a nuclear chain reaction, an excess of them early in the implosion is a recipe for predetonation.


Reuters: “Gary Gibson, senior seismologist at Australia’s Seismology Research Centre, said a 4.2 magnitude quake would be the result of a one kiloton explosion.”

Reason for a fizzle – chain reaction starts too soon

One of the key elements in the proper operation of a nuclear weapon is initiation of the fission chain reaction at the proper time. To obtain a significant nuclear yield, sufficient neutrons must be present within the supercritical core at just the right time. If the chain reaction starts too soon, the result will be only a ‘fizzle yield’, well below the design specification; if it occurs too late, there may be no yield whatsoever.



In the underground nuclear testing (the testing of nuclear weapons), decoupling refers to the attempt to prevent some of the bomb’s energy from transmitting as seismic waves. This makes it more difficult for outside observers to estimate the nuclear yield of the weapon being tested.

On North Korea’s nuclear test

“I am still puzzled as to why the possibility of a decoupling cavern has been totally ignored.

Some of the early reports were that the test was conducted in abandoned mines, which would have a network of tunnels and shafts that I would expect could serve to decouple the blast.”

”… even a 1 kiloton explosive would still be a terrifying weapon. Recall that the 1995 Oklahoma City explosion involved only a few tons of ANFO. A 1 kiloton (1,000 ton TNT equivalent) bomb could kill people in an area of about one square mile and would partially destroy a much larger area. Most of these deaths would be from fire or from the prompt nuclear radiation.”

“A fizzle is simply a detonation of lower yield than designed. It does not mean a dud. Wrong words leads to wrong thinking.

The 1/2 kiloton NK fizzle would kill with immediate radiation over a 2 square kilometer area.

The area in the red ring in this image of Manhattan is the >500 REM lethal zone. The yellow arrows show the World Trade Center for scale purposes:”

IMAGE: Aerial view of the WTC site in lower Manhattan

Seismic and decoupling

Detonation of a sub-kiloton bomb would cause destruction of several buildings although it would not flatten a city. The radiation that would have been released was not a desired effect. The attack on the WTC was not a terrorist attack like the one described below, where the aim was to simply cause as many deaths as possible. The terrorists in the 9/11 attack faced restrictions of having to hide the authorship of the attacks. Saudis did not have nuclear capability at the time of the 9/11 attacks so the destruction of the buildings had to look like they had been caused by something else.

South Korean authorities said they monitored a seismic event of 3.6 on the Richter scale that was not a natural occurrence and corresponded in time to the claimed test.

A yield of 550-800 tons (.55-.8 KT) is not too small by any means as an achievable yield.

It’s also worth pointing out that an atomic bomb of .6KT or so is no city flattener, but would work quite spectacuarly as a terrorist weapon. If detonated on the ground or from the top of a building, it would also result in serious fallout, increasing the terror effect and the number of deaths. Further, it would contaminate the terrain at and near ground zero for a long time. Cleanup and decontamination would be lengthy and very expensive. Imagine such a weapon being detonated in an American harbor [..]

Calculating yield from body wave

DefenseTech explains how yields are calculated from seismic readings:

You’re thinking, 3.6, 4.2, in that neighborhood. Seismic scales, like the Richter, are logarithmic, so that neighborhood can be pretty big.

But even at 4.2, the test was probably a dud.

Estimating the yield is tricky business, because it depends on the geology of the test site. The South Koreans called the yield half a kiloton (550 tons), which is more or less — a factor of two — consistent with the relationship for tests in that yield range at the Soviet Shagan test site:

Mb = 4.262 + .973LogW

Where Mb is the magnitude of the body wave, and W is the yield.

3.58-3.7 gives you a couple hundred tons (not kilotons), which is pretty close in this business unless you’re really math positive. The same equation, given the US estimate of 4.2, yields (pun intended) around a kiloton [..]

Using the US Geological Survey figure of 4.2 magnitude body wave of the seismic shock, giving a 1 KT achieved yield, actually buttresses the case that this test was not a fizzle, in my view. For battlefield purposes, say, against the South Korean or US forces on the peninsula, a 1 KT device is more usable than a 20 KT bomb. A 1 KT weapon is smaller, thus easier to conceal, and can be designed to be fired from existing artillery pieces, whether cannons or rockets. A Nagasaki-yield weapon would be of little military utility in fighting against South Korea or American forces. And you much more easily can get from a tested 0.6-1.0 KT proof-of-concept device to a usable terror weapon of the same yield, than from a test of a much larger yielding device. (DEAD URL)

Detecting nuclear tests


Caption: Seismic detectors form the backbone of the nuclear monitoring system.

Seismic detectors

Information is collected from 500 seismic detectors around the world. Scientists are tasked with distinguishing earthquakes and nuclear testing blasts.

Seismic detectors—the same ones used to study earthquakes—form the backbone of the monitoring system. At the National Earthquake Information Center in Denver, information from about 500 seismic stations around the world is collected and analyzed, yielding information on 30,000 earthquakes a year, says Harley Benz, the U.S. Geological Survey scientist in charge of the center. But not every “event” is an earthquake, so the scientists have developed techniques to distinguish quakes from blasts. “We do this all the time in the western U.S.—there’s lots of quarry blasts, and the signal looks quite different,” Benz says.

Explosions tend to blast outwards in all directions evenly, producing a strong compression wave. Earthquakes, in contrast, are produced by rocks sliding against each other along a fault line, yielding strong shear waves. The waves travel at different speeds, with the compression wave arriving at detectors before the shear one. And if that first wave is the stronger of the two, Benz says, it suggests the event was an explosion. Other characteristics of the wave patterns, such as the frequency spectrum and how much of the wave traveled along the surface as apposed to through the bulk of the Earth, can also help distinguish blasts from quakes.

The equivalent magnitude of an earthquake and nuclear explosion

A magnitude 4 earthquake releases energy on the order of a one-kiloton nuclear explosion. – Lawrence Livermore National Laboratory

Decoupling – methods

Disguising nuclear testing can be done by decoupling. Another method is to camouflage the blast with mining explosions (mine masking). Decoupling requires exploding the device near a large underground cavity. The “Bathtub” underneath the WTC complex could have served as the large underground cavity that served to reduce the seismic activity of the nuclear detonations in the WTC attacks.

There are several ways to fool a seismic detection system. One way is to camouflage the blast with conventional mining explosions; another method, known as “decoupling”, involves exploding the device in a large underground cavity designed so that the energy of the explosion will go to compressing the gas in the cavity instead of melting and compressing rock. A U.S. experiment in 1966 showed that decoupling could reduce the seismic signal by a factor of 70. Both decoupling and mine masking are extremely difficult to carry out in practice—but just in case, there are several other detection techniques.

The most definitive signature of a nuclear explosion is the presence of radionuclides, a collective term for the radioactive isotopes produced by a nuclear reaction. While “fallout” was most obvious when early tests were conducted above-ground, underground and underwater tests also tend to leak radiation into the atmosphere. The detection process, which looks for key indicators like the ratio of different xenon isotopes, takes several days and can’t pinpoint the exact origin of the blast. But once a signal is detected, there’s no doubt a nuclear explosion has taken place.

Decoupling – effects

Decoupling can reduce the magnitude of a seismic reading. A fully-decoupled 1 kiloton nuclear test can cause an earthquake of magnitude 2.7. An earthquake of this magnitude is rarely felt by anyone.

Small Earthquake Near Russian Test Site Leads to U.S. Charges of Cheating on Comprehensive Nuclear Test Ban Treaty

On August 28 the Washington Times carried a lead story “Russia suspected of nuclear testing.” It was followed up the next day by the Washington Post and the New York Times, the latter with the headline “U.S. Suspects Russia set off Nuclear Test.” In the body of those and other press reports, however, the nature of event on August 16–whether it was a nuclear explosion or an earthquake at or near the Russian Arctic nuclear test site at Novaya Zemlya–was expressed as being still in doubt. [..]

[An] earthquake of magnitude 2.7 is a very small event, one that is rarely felt by anyone. In his executive summary of 1993 about the 1992 event, Ryall states that its magnitude was appropriate for a fully-decoupled 1 kiloton nuclear test. This begs the question Is fully decoupled or highly decoupled nuclear testing possible at Novaya Zemlya? In a 1996 review paper on decoupled nuclear testing I argued that large decoupling factors could be obtained only for explosions with yields in excess of 1 kiloton when they were detonated in huge cavities constructed in salt domes. The decoupling factor is the amplitude ratio of a well-coupled, non-evasive explosion to that of one of the same yield detonated so as to decouple or muffle the size of its seismic waves. Large cavities constructed in hard rock and used for clandestine nuclear tests are likely to leak bomb-produced radioactive isotopes to the surface by way of joints and faults. Hard rock contains such imperfections on a scale of meters and larger. A cavity with a radius of about 28 meters and at a depth of 1 km in salt is needed to fully decouple a 1 kiloton nuclear explosion. […]

A few kilotons tested with a decoupling factor of 70 times corresponds to an mb of about 3.0. A capability better than that is being achieved now for Novaya Zemlya. Probably decoupling factors no larger than a factor of two, however, are possible at that test site for yields of a few kilotons. An explosion of that size would correspond to mb 4.0.


IMAGE: The Bathtub. This underground basement held six floors of a parking station, a metro line and offices. This space underneath the towers would have acted as a decoupling cavern for any bomb detonated within or above it.





Report on the seismic recordings of Ground Zero

The seismic recordings during the collapse of the towers were 2.1 and 2.3. These recordings are consistent with the detonation of a small nuclear device. In contrast, the recordings obtained during the plane crashes were 0.9 and 0.7. These recordings were not of a sufficient magnitude to cause the collapse of buildings.

Seismic recordings of the World Trade Center collapses

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On September 11, 2001 Columbia University’s Lamont-Doherty Earth Observatory recorded seismic signals produced by the impact of the two aircraft hitting the Twin Towers of the World Trade Center. The ground shaking was consistent with the energy released by small earthquakes, however, it was not sufficient to cause the collapse and damage to the surrounding buildings. The buildings around the Twin Towers were damaged by the kinetic energy of the falling debris and the pressure exerted on them by the debris and particle laden blast produced during the collapse of the two towers. Learn More at the Earth Institute at Columbia University.

Seismographic recordings of the tower collapses were recorded in five states, as far away as 428 kilometers [266 miles] in Lisbon, New Hampshire. Lamont’s home station, in Palisades, New York, is located above the Hudson River, 34 kilometers [21 miles] from downtown Manhattan, where the towers stood. The aircraft impacts registered local magnitude (ML) 0.9 and 0.7, indicating minimal earth shaking as a result. The subsequent collapse of the towers, on the contrary, registered magnitudes of 2.1 and 2.3, comparable to the small earthquake that occurred beneath the east side of Manhattan on January 17, 2001. Source: November 20, 2001, issue of Eos, published by the American Geophysical Union, seismologists from Columbia’s Lamont-Doherty Earth Observatory

Seismic recordings captured


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Figure 1: Seismic recordings on E-W component at Palisades for events at World Trade Center (WTC) on September 11, distance 34 km. Three hours of continuous data shown starting at 08:40EDT (12:40 UTC). Data were sampled at 40 times/s and passband filtered from 0.6 to 5 Hz. The two largest signals were generated by collapses of Towers 1 and 2. Eastern Daylight Time (EDT) is UTC minus 4 hours. Expanded views of first impact and first collapse shown in red.

Seismographic recordings of the tower collapses were recorded in five states, as far away as 428 kilometers [266 miles] in Lisbon, New Hampshire. Lamont’s home station, in Palisades, New York, is located above the Hudson River, 34 kilometers [21 miles] from downtown Manhattan, where the towers stood. The aircraft impacts registered local magnitude (ML) 0.9 and 0.7, indicating minimal earth shaking as a result. The subsequent collapse of the towers, on the contrary, registered magnitudes of 2.1 and 2.3, comparable to the small earthquake that occurred beneath the east side of Manhattan on January 17, 2001.

The Lamont seismographs established the following timeline: 8:46:26 a.m. EDT [1240 UTC] Aircraft impact – north tower, Magnitude 0.9; 9:02:54 a.m. EDT [1302 UTC] Aircraft impact – south tower, Magnitude 0.7; 9:59:04 a.m. EDT [1359 UTC] Collapse – south tower, Magnitude 2.1; 10:28:31 a.m. EDT [1428 UTC] Collapse – north tower, Magnitude 2.3.

In addition, the seismic waves were short-period surface waves, meaning they traveled within the upper few kilometers of the Earth’s crust. [..]

The paper by Won-Young Kim, Lynn R. Sykes, J.H. Armitage, J.K. Xie, Klaus H. Jacob, Paul G. Richards, M. West, F. Waldhauser, J. Armbruster, L. Seeber, W.X. Du, and Arthur Lerner-Lam, “Seismic Waves Generated by Aircraft Impacts and Building Collapses at World Trade Center, New York City,” appears in Eos, Volume 82, number 47 (20 November 2001), page 565.

DPRK Nuclear Test 10-9-06: seismic record


Seismic activity is used to estimate suspected atomic weapon’s yield, or destructive power. The activity measured 4.2 on the Richter scale, as recorded by USGS. Initial reports regarding this event were mixed. Russian authorities said the test was equivalent to 5,000 to 15,000 tons of TNT (5-15 kilotons). The bomb dropped on Hiroshima, Japan in August 1945 had a yield of approximately 15 kilotons. “We have no doubts that it (the test) was nuclear,” Russian Defense Minister Sergei Ivanov said.

Comparisons of signals from different sources

Although the signals from the WTC collapse and earthquakes are of similar magnitude, the character of the seismographs are quite different. Differences in P and S waves are observed.

Comparison of World Trade Center collapse signals with signals from earthquakes, gas explosion and mine collapse

The signals at PAL from Collapse 2 and a small felt earthquake beneath the east side of Manhattan on January 17, 2001 are of comparable amplitude and ML (Fig. 4). The character of the two seismograms, however, is quite different. Clear P and S waves are seen only for the earthquake. The 7-km depth of the earthquake suppressed the excitation of short- period Rg, which is so prominent for the collapse. The difference in the excitation of higher frequencies also can be attributed to the short time duration of slip in small earthquakes compared to the combined source time of several seconds of the complex system of the towers and foundations responding to the impacts and collapses. The waves from the WTC events resemble those recorded by regional stations from the collapse of part of a salt mine in western New York on March 12, 1994 (ML 3.6). That source also lasted longer than that of a small earthquake. 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.

An explosion at a gasoline tank farm near Newark NJ on January 7, 1983 generated observable P and S waves and short-period Rg waves (ML 3) at PAL. Its Rg is comparable to that for WTC collapse 2. Similar arrivals were seen at station AMNH in Manhattan, which is no longer operating, at a distance of 15 km. AMNH also recorded a prominent seismic arrival at the time expected for an atmospheric acoustic wave. [..] [The] seismic waves excited by impacts and collapses at the WTC are short-period surface waves, i.e. seismic waves traveling within the upper few kilometers of the crust.

Authors Won-Young Kim, L. R. Sykes, J. H. Armitage, J. K. Xie, K. H. Jacob, P. G. Richards, M. West, F. Waldhauser, J. Armbruster, L. Seeber, W. X. Du and A. Lerner-Lam, Lamont-Doherty Earth Observatory of Columbia University, Palisades, N.Y. 10964, USA; also Dept. Earth and Environmental Sciences, Columbia University.

The above is an html-version of the following pdf-document:

Differences between seismographs of nuclear explosions and earthquakes

The earthquake source differs appreciably from that of an explosion in that the former involves shearing motion along a fault. Consequently, earthquakes typically generate larger shear (S) waves than explosions when the two sources are normalized by the size of the compressional first-arriving (P) waves.

Detection of nuclear testing has increased in reliability

Detonations of yields of a kiloton can be detected anywhere on Earth.

Advances in Monitoring Nuclear Weapon Testing – 2009 report

Detecting a test of a nuclear weapon has become so effective and reliable that no nation could expect to get away with secretly exploding a device having military significance.

Seismic monitoring can now detect a nuclear explosion with a yield of a kiloton or more anywhere on Earth. In many places, detection is far more sensitive than that. – By Paul G. Richards and Won-Young Kim (March 2, 2009)

Seismic disturbances of nuclear blasts

9/11 WTC collapses produced seismic disturbances equivalent to a 4.2 magnitude earthquake. A magnitude of that size would indicate a nuclear blast of 1-kiloton yield.

How do scientists determine if a nuclear blast has occurred? By Julia Layton

Another method of detecting a nuclear blast is by seismograph, the device that monitors Earth tremors to pinpoint and analyze earthquake activity (among other ground-shaking events). There’s actually a whole network of 500 seismograph stations positioned around the world whose job is to report ground-shaking incidents, and that includes any evidence of bomb blasts. NPR’s “Detecting Underground Nuclear Blasts” reports that the seismic activity recorded on Monday indicated a ground disturbance that would be the equivalent of a 4.2 magnitude earthquake. That magnitude indicates a blast with about a 1-kiloton yield, which is equal to the power of 1,000 tons of TNT.

Figuring out if a seismic event is an earthquake or a bomb blast is relatively easy. Scientists perform analyses of wave patterns that can accurately confirm an earthquake-versus-explosion determination. In highly simplified terms, in an earthquake, the ground starts shaking slowly as plates slide against each other, and then the seismic activity slow picks up as the ground really starts to move. In an explosion scenario, the initial blast is extremely power­ful, and the subsequent shaking of the ground grows progressively less severe. But figuring out it’s a blast and not an earthquake is only part of the process; seismographs can’t really determine if the blast was nuclear or conventional in nature. Also, it’s possible to “hide” a nuclear blast, for instance by detonating it in a tremendous underground cavity, which decreases the effects on the ground because the blast’s energy goes into compressing all that gas in the huge hole.

Seismic recordings of underground nuclear blasts

The explosion recorded in the 9/11 WTC attacks was the equivalent of a 4.2 earthquake and would translate into an explosion of 1,000 tons of TNT.

Detecting Underground Nuclear Blasts – by David Kestenbaum

Earthquake monitors around the world picked up vibrations from North Korea’s nuclear test range, but those signals alone are not enough to prove that it was a nuclear blast. […]

A large underground explosion generates a shock wave that travels great distances through the Earth. Seismic detectors register the vibrations. The U.S. Geological Survey says the explosion they recorded Monday was the equivalent of a 4.2 magnitude earthquake. Assuming the test took place in rock, this would translate into an explosion equivalent to roughly one kiloton, or 1,000 tons of TNT. For comparison, the “Little Boy” bomb dropped on Hiroshima in 1945 exploded with an energy of about 13 kilotons. In 1952 “Mike” — the first hydrogen bomb — created a blast of more than 10 megatons, 1,000 times more powerful. – NPR October 11 2006

P and S waves are the key in distinguishing earthquakes and nuclear explosions

Indian nuclear tests and seismic signature


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As part of DOE’s effort, teams at Livermore and Los Alamost have been working to improve ways to seismically characterize clandestine underground nuclear explosions and differentiate them from other sources of seismicity, such as earthquakes and mining explosions. Much of Livermore’s work has centered on developing regional discriminants, which are characteristic features of a seismic waveform (for example, the peak amplitude at a particular frequency, within a specific time frame) recorded at distances less than 2,000 kilometers away. These discriminants are used to differentiate between explosions and other types of seismic sources. [..]

As seen in Figure 2, the seismogram from a representative earthquake clearly differs from that of the May 11 test. Livermore-refined discriminants based on P and S waves were strongly indicative of an explosion, not an earthquake or other seismic source, at all frequencies tested (0.5 to 8 hertz). Livermore seismologist Bill Walter explains that the differences in seismic P- and S-wave energy provide one method of discriminating explosions from earthquakes. Seismic P waves are compressional waves, similar to sound waves in the air. Shear (S) waves are transverse waves, like those that propagate along a rope when one end is shaken. Because underground explosions are spherically symmetric disturbances, they radiate seismic P waves efficiently. In contrast, earthquakes result from sliding or rupture along a buried fault surface and strongly excite the transverse motions of S waves. Thus, we expect that explosions will show strong P waves and weak S waves and that earthquakes will show weak P waves and strong S waves, as seen in Figure 2.

According to Walter, one way to quantify this difference is by determining the ratio of P-wave to S-wave energy measured from the seismograms. Explosions should have higher P/S ratios than earthquakes.

The seismograph of the World Trade Center tower collapses compared with the seismograph of nuclear test


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Img: URL: Source: Science and Technology Review Dec 1998


Img: URL: Source: Distributed Seismic and Radionuclide Sensor Network

South Tower collapse and North Korean nuclear test


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Caption: Figure 3: Record section of vertical-component seismograms from stations in Fig. 2 following collapse of north Tower of WTC. Zero corresponds to computed origin time of 10:28:31 EDT. Data filtered for passband 0.5 to 10 Hz. Three velocities indicated by dotted lines.

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Caption: Figure 3: Record section of vertical-component seismograms from stations in Fig. 2 following collapse of north Tower of WTC. Zero corresponds to computed origin time of 10:28:31 EDT. Data filtered for passband 0.5 to 10 Hz. Three velocities indicated by dotted lines. Img: URL:

Caption: Seismograph of North Korean nuclear test.  Modified image from URL: – NY Times: Seismic Readings Point to a Small Nuclear Test (May 25, 2009)

Caption: Seismic Readings Point to a Small Nuclear Test (May 25, 2009) Img: URL: – NY Times

Report: Initial seismic readings from the underground blast in North Korea were consistent with a relatively small nuclear test, but it will be days or weeks before the United States and international organizations can determine whether it was a nuclear detonation and how successful it might have been, experts said Monday. [..]

Martin B. Kalinowski, a nuclear expert at the University of Hamburg, said a magnitude of 4.7 corresponded to a nuclear explosive yield of about three to eight kilotons of high explosives, with the most likely yield being four kilotons. A blast that generated a tremor with a magnitude of 4.5 would be smaller.

In contrast, the primitive bombs dropped on Hiroshima and Nagasaki had explosive yields of 15 kilotons and 22 kilotons, respectively.

A senior Obama administration official said Monday afternoon that based on seismic data alone, officials estimated the explosion at “several kilotons.”

URL: – NY Times


Img: URL: Caption: Collapse of South Tower

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Ground-shaking insignificant in Twin Tower collapses

“During the collapse, most of the energy of the falling debris was absorbed by the towers and the neighboring structures, converting them into rubble and dust or causing other damage — but not causing significant ground shaking.” — Dr. Arthur Lerner-Lam, Director of Columbia University’s Center for Hazards and Risk Research, as quoted in Earth Institute News

More discussion at

Plane impacts probably involved explosions


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Caption: Seismic Readings Point to a Small Nuclear Test (May 25, 2009). Modified image from URL: – NY Times.

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Comparisons with other nuclear events

Caption: Onset of P waves from a Soviet underground nuclear test monitored at a relay station in England. Img: URL: (Ground Zero-The Nuclear Demolition of the Twin Towers – Copyright © 2007 by Niall Kilkenny)


Caption: Seismic signature from the North Tower nuclear explosion. Img: URL:

Reversed Rayleigh Waves

Reversed Rayleigh waves are consistent with an implosion rather than an explosion.

Reversed Rayleigh Waves


Caption: Interestingly, the observed Rayleigh waves are reversed, consistent with an implosion rather than an explosion source. Image: URL:


The last underground nuclear tests were conducted by India and Pakistan in May, 1998. [..] The IRIS station NIL (Nilore, Pakistan) is close to a planned IMS primary station and recorded some very interesting seismograms from the May 18, 1998, Indian test. We carefully calibrated the path to NIL using a prior Mw 4.4 earthquake that occurred on April 4, 1995, about 110 km north of the Indian test site. [..] Interestingly, the observed Rayleigh waves are reversed, consistent with an implosion rather than an explosion source. The preferred explanation is that the explosion released tectonic stress near the source region, which can be modeled as a thrust earthquake of approximate Mw 4.0 earthquake plus a pure explosion. This tectonic release is sufficient to completely dominate the Rayleigh waves and produce the observed signal (Walter et al., 2005). We also examined the explosion at high frequencies of 6-8 Hz where many studies have shown that relative P/S amplitudes can discriminate explosions from a background of earthquakes (Rodgers and Walter, 2002). Comparing with the April 4 1995 earthquake we see the classic difference of relatively large P/S values for the explosion compared to the earthquakes despite the complication of the large tectonic release during the explosion. (This is LLNL report UCRL-TR-211315).

Rodgers, A. J. and W. R. Walter, Seismic discrimination of the May 11, 1998 Indian nuclear test with short-period regional data from station NIL (Nilore, Pakistan), Pure Appl. Geophys., 159, 679-700, 2002.

Walter, W. R., D. Bowers, N. Selby, A. Rodgers and D. Porter, Tectonic Release from the May 11, 1998 Indian Nuclear Tests, LLNL report UCRL-JRNL-202983-DRAFT (to be submitted in 2005).

Date Taken: January 29, 2009

Photographer / Contributor: William R. Walter, Arthur J. Rodgers • Lawrence Livermore National Laboratory; David Bowers, Neil Selby • Blacknest, United Kingdom



Caption: Record section of vertical-component seismograms from stations in Fig. 2 following collapse of north Tower of WTC. Zero corresponds to computed origin time of 10:28:31 EDT. Data filtered for passband 0.5 to 10 Hz. Three velocities indicated by dotted lines. Img: URL:



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Comparison between North Korean test and World Trade Center Tower collapses

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Caption: DPRK’s seismic signature of their nuclear test  Img: URL:

Caption: Seismic record of explosion from digital station Vladivostok

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Source: CEME – Geophysical Service of Russian Academy.

TEXT: On October 9, 2006, North Korea’s Korean Central News Agency announced that it had successfully conducted an underground nuclear test. The test was reported to have taken place at 10:36AM, local time, in Hwaderi, near Kilju city, in North Hamkyung province. According to the KCNA statement, no radioactive leakage had taken place as a consequence of the test. [..]

The nuclear test was reported to have had a yield equivalent to between 500 and 2000 tons of TNT. By 15 October 2006 US intelligence estimates of the test put the size of the blast as low as 0.2 kilotons, the equivalent of 200 tons of TNT. According to some reports, the North Koreans had informed the Chinese government that the planned test would produce a 4-kiloton explosion.


Underground nuclear explosion appears in the seismograph

A sharp spike of short duration appears in a nuclear explosion, says a seismologist.

Strange anomaly noted in World Trade Center collapses

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Caption: These unexplained “spikes” in the seismic data lend credence to the theory that massive explosions at the base of the towers caused the collapses [..] A “sharp spike of short duration” is how seismologist Thorne Lay of University of California at Santa Cruz told AFP an underground nuclear explosion appears on a seismograph [..] Experts cannot explain why the seismic waves peaked before the towers actually hit the ground.


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.

However, 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.

A “sharp spike of short duration” is how seismologist Thorne Lay of University of California at Santa Cruz told AFP an underground nuclear explosion appears on a seismograph.

The two unexplained spikes are more than 20 times the amplitude of the other seismic waves associated with the collapses and occurred in the East-West seismic recording as the buildings began to fall.

Experts cannot explain why the seismic waves peaked before the towers actually hit the ground.

Asked about these spikes, seismologist Arthur Lerner-Lam, director of Columbia University’s Center for Hazards and Risk Research told AFP, “This is an element of current research and discussion. It is still being investigated.”

Lerner-Lam told AFP that a 10-fold increase in wave amplitude indicates a 100-fold increase in energy released. These “short-period surface waves,” reflect “the interaction between the ground and the building foundation,” according to a report from Columbia Earth Institute.

“The seismic effects of the collapses are comparable to the explosions at a gasoline tank farm near Newark on Jan. 7, 1983,” the Palisades Seismology Group reported on Sept. 14, 2001.

One of the seismologists, Won-Young Kim, told AFP that the Palisades seismographs register daily underground explosions from a quarry 20 miles away.

These blasts are caused by 80,000 pounds of ammonium nitrate and cause local earthquakes between Magnitude 1 and 2. Kim said the 1993 truck-bomb at the WTC did not register on the seismographs because it was “not coupled” to the ground.

“Only a small fraction of the energy from the collapsing towers was converted into ground motion,” Lerner-Lam said. “The ground shaking that resulted from the collapse of the towers was extremely small.” – American Free Press: New Seismic Data Refutes Official Explanation By Christopher Bollyn (Sept. 3, 2002)

Other graphs of seismic disturbances at the World Trade Center

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Other seismographs of Twin Tower Collapses


Caption: 1st collapse south tower  Img: URL:


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Audio accords with the seismic evidence

Booms heard before collapse

VIDEO: Rick Siegel 9-11 Eyewitness-Booming sounds  Dailymotion

Audio wave and seismic record of the North Tower

VIDEO: 911 Eyewitness video of the audio wave for the North Tower Dailymotion

Audio wave supports explosives

VIDEO: 911 Eyewitness – Sound analysis shows explosives Dailymotion

Witnesses reported rumbling and dust clouds before collapse

VIDEO: 911 Eyewitness – Witnesses reported rumbling and dust clouds before collapse Dailymotion

Complete Rick Siegel documentary 

VIDEO: Rick Siegel 9 11 Eyewitness Video  ArchiveOrg

Distinguishing nuclear explosions from other events

Distinguishing an earthquake from a nuclear event requires a close examination of the seismic waves. Such waves fall into two major categories: surface waves, which move along Earth’s surface, and body waves, which move through Earth and bounce off structures inside. Body waves may be primary (P) or secondary (S). Seismic P waves are compressional waves, similar to sound waves in the air. S waves are shear, or transverse, waves, similar to those that propagate along a rope when one end is shaken. Underground explosions radiate P waves in a relatively symmetric spherical shape. Earthquakes, which result from plates sliding along a buried fault, strongly excite the transverse motions of S waves, producing a distinct radiation pattern. Explosions thus show strong P waves and weak S waves. Earthquakes, in contrast, typically show weak P waves and strong S waves.


Img: URL: Caption: Various kinds of seismic events can be grouped on a source-type, or Hudson, plot based on their ground motion. A perfectly symmetric underground explosion would appear at the apex of the plot. By analyzing the seismic waves produced by the disturbance that rocked the Crandall Canyon Mine in Utah in August 2007, Laboratory seismologists determined that the event was an implosive tunnel collapse, not the sideways slippage of an earthquake.

But this information alone is not foolproof because the structure of Earth imparts an imprint on the signal. One way to quantify the difference between these seismic disturbances is to determine the ratio of P-wave to S-wave energy measured from the seismograms. Explosions should have higher P:S ratios than earthquakes.

Recent Livermore work led by Walter sought to clarify the characteristics of the P:S ratios that distinguish nuclear weapons tests from other tectonic activity. By examining regional amplitude ratios of ground motion in a variety of frequencies, his team empirically demonstrated that such ratios indeed separate explosions from earthquakes. The researchers used closely located pairs of earthquakes and nuclear explosions recorded at monitoring stations at or near the Nevada Test Site; Novaya Zemlya and Semipalatinsk, former Soviet Union test sites; Lop Nor, China; India; Pakistan; and the North Korea test.

“At high frequencies, above 6 hertz, the P:S ratio method appears to work everywhere we looked,” says Walter. “Explosions have larger P:S amplitude values.” For example, a test in India on May 11, 1998, compares well with the October 9, 2006, North Korea test.

Bill Walter is quoted. Lawrence Livermore National Laboratory Operated by Lawrence Livermore National Security, LLC, for the U.S. Department of Energy’s National Nuclear Security Administration

Caption: Seismograms recorded during the explosion (red wave) and a recent earthquake (blue wave) near that experiment show the different seismic patterns produced by these geologic disturbances.

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Differences between nuclear and earthquake seismic signatures

Differences between nuclear explosions and earthquakes

• Explosions produce strong compressional wave (P-wave). Seismic signals are depleted in S-wave.

• Earthquakes produce strong S-waves.

Explosions and earthquakes

• Explosions produce strong compressional wave (P-wave). Seismic signals are depleted in S-wave

• Earthquakes produce strong S-wave.


Explosion Yield and Magnitude

The energy releases (yield Y in kt TNT) of tamped (no decoupling) underground nuclear explosions have been calibrated against their measured seismic magnitudes using some NTS tests:

mb = log Y + 4.0

Ms = log Y + 2.0

So a 1 kt explosion is equivalent to an earthquake of mb 4.0.

Differences between nuclear explosions and earthquakes

• Explosions have much smaller source dimension.

Log R (in km) = (1/3) log Y – 2

It’s ~ 1 km for a 1 Mt device. An earthquake of this size (Ms 8) usually have a dimension of 100 km.

• Explosions have shorter time duration

A Mt device has a duration of ~ 1 sec, as compared to 30 sec for a similar sized earthquake.

• So explosions generate more high-frequency signals than earthquakes.

Difference in Radiation Pattern

• Earthquakes have much more complicated radiation patterns than explosions

• Explosions tend to have nearly isotropic radiation patterns (the same in all directions) while earthquakes have many “lobes” in their patterns

• Therefore the observed radiation pattern of a source can be used as a discriminant


Powerpoint: Detect and Isentify Nuclear Explosions by Lupei Zhu Fall 2004

Comparing the seismic records of World Trade Center collapses and Indian nuclear test

More comparisons with Indian nuclear test URL: Img 2: URL: Alternate URL: URL: Img 2: URL: Alternate URL: URL: Img 2: URL: Alternate URL:

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Caption: Seismic shockwaves from the latest test were recorded in South Korea. IMG: URL:

Cavity decoupling

Because of the drawbacks of all of these scenarios for clandestine testing, partial or full decoupling of subterranean tests is widely viewed as the only serious threat to monitoring a test ban. Decoupling involves conducting a nuclear test so as to reduce the amplitudes of seismic waves to levels below those for a contained, tamped explosion. One less effective way to do this is to conduct the test above the water Table in a large body of dry, porous sediment. Far more effective, in principle, is testing in a large mined underground cavity.

In a tamped underground nuclear explosion, large volumes of rock are exposed to overpressures far beyond the limits of elasticity, causing them to yield and suddenly to be displaced outward a large distance, generating large and easily detected seismic waves. If the explosion takes place within a very large cavity, however, the pressure of the shock wave in the surrounding rock remains below the elastic limit and produces relatively small displacements. Some early work suggested that, with “full decoupling,” seismic amplitudes transmitted into the earth at some frequencies could be reduced by over two orders of magnitude. Later studies indicate that decoupling factors of ≈70 may be achieved at low frequencies but decoupling factors at high frequencies are likely to be much smaller. Hence, broadband seismic instruments can be extremely useful in test ban monitoring. Joints or other fractures in the surrounding rock may lead to somewhat lower decoupling factors. If the cavity is too small for full decoupling, the seismic waves are reduced by a value smaller than that for full decoupling. The effects of a nonspherical cavity can be quite complicated.

Proc Natl Acad Sci U S A. 1999 September 28; 96(20): 11090–11095.

Copyright © 1999, The National Academy of Sciences Geophysics

Geologic constraints on clandestine nuclear testing in South Asia

Dan M. Davis and Lynn R. Sykes

Large factors can be achieved by decoupling


• Another way of decoupling is to fire the explosion in a large cavity.

• A factor of 120 can be achieved in theory if the cavity is large enough so that the shock-wave deformation is elastic (fully decoupled).

• A factor of 70 was achieved in one US test in which a 0.38 kt explosion was detonated in the cavity produced by a 5.3-kt explosion (see movie)


• To reduce seismic signal strength by firing the explosion in low-coupling medium (such as dry alluvium) or in a cavity.

• The decoupling factor is defined as the amplitude ratio of the seismic signals from a fully contained explosion in hard rock to from a decoupled explosion.

• A decoupling factor up to 10 can be achieved for dry alluvium.

• A problem is to avoid leakage of radioactive material and surface deformation. This demands thick alluvial deposit (~600 m for a 1 kt shot).

URL: (Detect and Identify Nuclear Explosions – Lupei Zhu Fall 2004)

Seismic signature

IMG: Seismic waveform nuclear vs. earthquake URL:

Richter magnitude and TNT-equivalent seismic energy yield

Richter Magnitude TNT for Seismic Energy Yield Example (approximate)
-1.5 6 ounces Breaking a rock on a lab table
1.0 30 pounds Large Blast at a Construction Site
1.5 320 pounds Large Quarry or Mine Blast
2.0 1 ton
2.5 4.6 tons
3.0 29 tons
3.5 73 tons
4.0 1,000 tons Small Nuclear Weapon
4.5 5,100 tons Average Tornado (total energy)
5.0 32,000 tons
5.5 80,000 tons Little Skull Mtn., NV Quake, 1992
6.0 1 million tons Double Spring Flat, NV Quake, 1994
6.5 5 million tons Northridge, CA Quake, 1994
7.0 32 million tons Hyogo-Ken Nanbu, Japan Quake, 1995; Largest Thermonuclear Weapon
7.5 160 million tons Landers, CA Quake, 1992
8.0 1 billion tons San Francisco, CA Quake, 1906
9.0 32 billion tons Chilean Quake, 1960
10.0 1 trillion tons (San-Andreas type fault circling Earth)
12.0 160 trillion tons (Fault Earth in half through center, OR Earth’s daily receipt of solar energy)


Seismic waves


The mechanical properties of the rocks that seismic waves travel through quickly organize the waves into two types. Compressional waves, also known as primary or P waves, travel fastest, at speeds between 1.5 and 8 kilometers per second in the Earth’s crust. Shear waves, also known as secondary or S waves, travel more slowly, usually at 60% to 70% of the speed of P waves. P waves shake the ground in the direction they are propagating, while S waves shake perpendicularly or transverse to the direction of propagation.

Seismic detection of nuclear tests can pose some challenges

Seismic detection of nuclear tests


Meeting Monitoring Challenges Zucca points out that under the current Threshold Test Ban Treaty (banning explosions exceeding 150 kilotons), determining accurate explosive yield is the critical issue. Most nuclear tests near the threshold treaty’s limit generate seismic magnitudes of about 6 or greater on the Richter scale. Seismic signals from these tests travel thousands of miles through Earth’s relatively homogeneous core and mantle and are readily picked up by far-away seismic stations for relatively straightforward characterization (Figure 2a). Under the CTBT, however, the critical issues will be to determine that a nuclear explosion no matter its size took place and to pinpoint its location accurately. A nation attempting to conceal a test could attempt to minimize the seismic signals. Such signals from a small nuclear test could be well below magnitude 4, with resulting measurable signals traveling 1,000 miles or less. What’s more, the signals would likely be confined to Earth’s upper mantle and crust, an extremely heterogeneous environment that distorts, and even blocks, parts of the signals (Figure 2b). Accurately locating and characterizing signals at these so-called regional distances pose a significant challenge, says seismologist Bill Walter. It’s a much harder job because we can’t use global models of Earth. We have to calibrate region by region, seismic station by seismic station. Successfully meeting the regional distance challenge, says seismologist Marv Denny, has been the most difficult aspect of the Livermore effort over the past several years. Denny says that complicating the task is the huge number of events that, at first cut, can resemble a small nuclear detonation. Stations will be recording a constant stream of background noise that includes earthquakes, lightning, meteors, sonic booms, navy armament testing, mining explosions, construction activities and other industrial operations, nuclear reactor operations and accidents, natural radioactivity, and even strong wind and ocean waves.

Analysis of nuclear test seismic wave pattern

Indian nuclear test seismic wave pattern


Figure 5. An international monitoring station in Pakistan detected the Indian nuclear test of May 11, 1998, about 740 kilometers away. (a) Analysis of the seismogram showed a P-wave-to-S-wave ratio strongly indicative of an explosion and not (b) nearby earthquakes [..] Key algorithms provide discriminants, characteristic features of a waveform (peak-to-peak distance, height, width, or some ratio). A particularly useful discriminant, for example, is the ratio of P-wave amplitude to S-wave amplitude. The P (or primary) wave is a compressional wave that is the first to arrive at a station. The S wave or shear wave has a slower propagation speed and arrives behind the P wave. The seismogram from the Indian nuclear test of May 11, 1998, as recorded by an international monitoring system station in Pakistan about 740 kilometers away, showed a P-to-S ratio strongly characteristic of an explosion and not an earthquake (Figure 5).

Comparison of the aircraft impacts and collapse of towers

The aircraft impacts registered local magnitude (ML) 0.9 and 0.7, indicating minimal earth shaking as a result. The subsequent collapsing of the towers, on the contrary, registered magnitudes of 2.1 and 2.3, comparable to the small earthquake that had occurred beneath the east side of Manhattan on January 17, 2001. The Lamont seismographs established the following timeline:

8:46:26 a.m. EDT [1240 UTC] Aircraft impact – north tower Magnitude 0.9

9:02:54 a.m. EDT [1302 UTC] Aircraft impact – south tower Magnitude 0.7

9:59:04 a.m. EDT [1359 UTC] Collapse – south tower Magnitude 2.1

10:28:31 a.m. EDT [1428 UTC] Collapse – north tower Magnitude 2.3

In addition, the seismic waves were short-period surface waves, traveling within the upper few kilometers [miles] of the Earth’s crust. They were caused by the interaction between the ground and the building foundations, which transmitted the energy from the impacts and collapses.

Ground shaking

Oral reports of ground shaking before collapse

Brian Becker — Lieutenant (F.D.N.Y.) [Engine 28] We felt — our whole building that we were in, when World Trade Center 2 collapsed, that was the first one to collapse. We were in World Trade Center 1. It was a tremendous explosion and tremendous shaking of our building. We thought it was our building maybe collapsed, there was a collapse above us occurring. It was tremendous shaking and like everybody dove into this stairwell and waited for, I guess, 20, 30 seconds until it settled, and that was our experience of the other building collapsing.

Interview, 10/09/01, New York Times

Michael Beehler — Firefighter (F.D.N.Y.) [Ladder 110] I was by I guess the outer part of the building and I just remember feeling the building starting to shake and this tremendous tremendous like roar and I just — I kind of didn’t even notice it, but like out of the corner of my eye, I saw out of the building, I saw a shadow coming down. At that point I thought it was the upper part of the north tower that had just basically like toppled over, fell off. I didn’t actually see the building part go by me, because I think I was on the opposite side. But I just remember feeling this tremendous tremendous shake and hearing this, like, noise. Again I can’t describe. What I did was I ended up running out.

Interview, 12/17/01, New York Times

Jody Bell — E.M.T. (E.M.S.) I lost track of time. You start to hear this rumble. You hear this rumble. Everything is shaking. Now I’m like, what the hell could that be. I’m thinking we’re going to get bombed. This is an air raid. You hear this thunder, this rumbling. Then you see the building start to come down. Everybody’s like, “Run for your lives! The building is coming down!” At that moment when that building was coming down, I was strapping a patient onto a stair chair.

Interview, 12/15/01, New York Times

David Blacksberg — E.M.T. (E.M.S.) I lost track of time of when the second building was coming down. It sounded like one big rumble, and then it just sounded like it just continued, and I was — I wasn’t really paying attention. I was looking at the sound.

Interview, 11/23/01, New York Times

Nicholas Borrillo — Firefighter (F.D.N.Y.)on 23rd floor of North Tower:
Then we heard a rumble. We heard it and we felt the whole building shake. It was like being on a train, being in an earthquake. A train is more like it, because with the train you hear the rumbling, and it kind of like moved you around in the hall. Then it just stopped after eight or ten seconds, about the time it took for the building to come down.

Interview, 01/09/02, New York Times

Peter Cachia — (E.M.S.) [Battalion 4] I was like a little too close to the tower when it started coming down, because when I started running, I knew I was too close and I really didn’t think I was going to get out of there. So about halfway up Liberty Street I saw a truck, I guess an SUV. It wasn’t a police or a fire vehicle. It was just a car that was parked there. I went under the truck while the tower came down and the ground was shaking and the truck was shaking and I thought that was it for me. I thought I was done. I stayed under there until I guess everything was over.

Interview, 10/15/01, New York Times

Paul Curran — Fire Patrolman (F.D.N.Y.)North Tower:
I went back and stood right in front of Eight World Trade Center right by the customs house, and the north tower was set right next to it. Not that much time went by, and all of a sudden the ground just started shaking. It felt like a train was running under my feet.

The next thing we know, we look up and the tower is collapsing.

Interview, 12/18/01, New York Times

Timothy Julian — Firefighter (F.D.N.Y.) [Ladder 118] You know, and I just heard like an explosion and then cracking type of noise, and then it sounded like a freight train, rumbling and picking up speed, and I remember I looked up, and I saw it coming down.
I made it right to the corner, and there’s a column right there, and I was with my guys. We all made it to like the column, and I remember it was plate glass behind me, and I’m thinking I’m going to get hit by this glass and like a porcupine. I’m going to get it, you know, but nonetheless, it rumbled.
It was the loudest rumbling I ever heard. The ground shook, and I got thrown down, and I remember it just got black, and I got knocked down. I remember geing buried.

Interview, 12/26/01, New York Times

Bradley Mann — Lieutenant (E.M.S.) 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.

Interview, 11/07/01, New York Times

Keith Murphy — (F.D.N.Y.) [Engine 47] At the time, I would have said they sounded like bombs, but it was boom boom boom and then the lights all go out. I hear someone say oh, s___, that was just for the lights out. I would say about 3, 4 seconds, all of a sudden this tremendous roar. It sounded like being in a tunnel with the train coming at you. It sounded like nothing I had ever heard in my life, but it didn’t sound good. All of a sudden I could feel the floor started to shake and sway. We were being thrown like literally off our feet, side to side, getting banged around and then a tremendous wind starting to happen. It probably lasted maybe 15 seconds, 10 to 15 seconds. It seemed like a hurricane force wind. It would blow you off your feet and smoke and debris and more things started falling.

Interview, 12/05/01, New York Times

The “Bathtub” and slurry wall of the World Trade Center

Genesis of the Bathtub

The WTC complex consisted of seven buildings on a 16-acre site in lower Manhattan. The deep basement (bathtub) portion of the site covers a four-city block (980 foot) by two-city block (520 foot) area some 200 feet from the east shore of the Hudson River (Figure 1). The deep basement occupies only about 70 percent of the 16-acre WTC site and is just west of the place where the Dutch landed in 1614. The size and depth of the deep basement and the alignment of the perimeter wall were dictated by several requirements: the construction of a new interstate commuter railroad (PATH) station parallel to the Greenwich Street east wall; support for an operating New York City subway tunnel located just outside the east wall; protection of the entry points of two 100-year old, 17-foot diameter PATH tunnels on the east and west; and the foundation of the twin towers (WTC 1 and WTC 2) on bedrock within the excavation (Figure 2).

The geology of the WTC site varies from east to west. On the east (Greenwich Street), 15 to 30 feet of fill cover as much as 20 feet of glacial outwash sand and silt, below which are 5 to 20 feet of glacial till/decomposed rock. The Manhattan schist bedrock is found at depths of 65 to 80 feet. A knoll of quartzite rock intrudes into the site at the southeast corner. On the west (West Street), the fill is 20 to 35 feet thick and is underlain by 10 to 30 feet of soft organic marine clay (river mud). Below the river mud is a 20-foot thick layer of glacial outwash sand and silt and 5 to 20 feet of glacial till/decomposed rock. Bedrock is found at depths of 55 to 75 feet. Groundwater levels were within several feet of ground surface. [..]

The basement was bounded by a 3,500-foot long, 3-foot thick slurry wall (perimeter wall) constructed from grade and socketed into rock located at depths of as much as 80 feet.

[The slurry walls contained bentonite at first which was replaced with concrete.]


DIAPHRAGM WALL CASE STUDY #4: World Trade Center Recovery, NY, NY

The tragic events of 9/11/01 have left few people untouched. One of the greatest “heroes” of the day has been the “bathtub” of the World Trade Center, being able to resist the tremendous forces generated by the destruction while holding back the Hudson river from entering the New York City tunnel system. The “bathtub”, is actually a 3 ft thick (0.91 m) structural slurry wall that keeps water and soil out of the excavation.

Description: Img: URL: World Trade Center Bathtub and Original Conditions (adapted from NY Times 9/18/01)

Caption: The bathtub contained the footprints of the Twin Towers and Buildings 3 and 6.

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Caption: This illustration shows a cross-section of the bathtub tall. The diagonal lines represent the post-tensioned tie-backs: cables that resist pressure pushing inward on the bathtub.

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Caption: This photograph shows the bathtub wall after most of the rubble from the 9/11/01 attack was removed. Portions of basement floors are visible in the right-hand side of the photograph.

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Bathtub covered 9 blocks

About half of the superblock that the World Trade Center occupied contained a deep basement, the so-called bathtub. It was a skewed rectangle with sides about 980 and 520 feet, and a depth of about seven stories.

The bathtub is the 9-block area of the World Trade Center site that is excavated down to bedrock and hard soils and ringed by the slurry wall. The bathtub was created to enable the building of the Twin Towers’ foundations, and was ultimately filled with seven stories of basements housing the parking garage, mall, and building services. Since the ground water level at the World Trade Center site was just a few feet below the surface, while bedrock was about 70 feet below the surface, creating the bathtub required first building a 7-story dam below the water level of the adjacent Hudson River — the slurry wall. [..]

Four of the World Trade Center buildings — the Towers and Buildings 3 and 6 — rested on foundations entirely within the bathtub.

Building the slurry wall

The question was how to keep the Hudson out.

Jack Kyle, chief engineer at the Port Authority, came up with an answer. It was known as the slurry trench method. Excavating machines with clamshell buckets dug a three-foot-wide trench right down to bedrock 70 feet below. They did it in 22-foot-wide sections all the way around the site. As they removed fill from each section, they pumped in a slurry of water and bentonite, an expansive clay. The clay naturally plugged any holes in the sides of the dirt walls.

When they had fully excavated a section of the trench, workers slid a 25-ton, seven-story-high cage of reinforced steel into the section, then filled that portion of the trench with concrete from the bottom up. The yard-thick wall became known as the “bathtub,” though this bathtub was meant to keep water out, not in.


Caption: (1) Builders of the “bathtub” wall first excavated a three-foot-thick trench segment that was 65 feet deep by 22 feet wide and filled it with a stabilizing slurry. (2) They then lowered a giant steel cage into the trench, with attachment points for reinforcing tiebacks that were later anchored to bedrock outside the wall. (3) Finally, they poured in concrete, which, as it rose from the bottom up, forced out the temporary slurry.

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The World Trade Center was built on bedrock

Caption: The soil on the island of Manhattan is perfect for building skyscrapers. In midtown Manhattan, BEDROCK is only about 10 feet or 3 meters deep. However, at the location of the Twin Towers in the lower part of Manhattan BEDROCK is about 100 ft or 30 meters deep. There is also the problem of water from New York harbor and the Hudson River. Midtown Manhattan was thus the IDEAL place to build the Twin Towers.

When it comes to demolition by nuclear weapons the deeper the foundation the better because depth of soil hides the effects of RADIATION: “By testing underground, man has placed a shield around most radioactive by-products from nuclear explosions. In contained underground detonations of fusion devices, radioactive products are trapped in cracked rock and rubble beneath the ground’s surface. The gaseous products such as water vapor, carbon dioxide, iodine, and tritium gradually cool. Most of the water and iodine condenses to a liquid. Neutrons promptly interact with lithium in the rock minerals to produce the radioactive isotope tritium (see Box 2.1), and this heavy isotope, in part, replaced ordinary hydrogen in water to form tritiated water. The radioactive water mixes with natural ground water and moves with it through the surface rock.”(Bolt, Nuclear Explosives and Earthquakes, pp. 83-84).


The Hiroshima Bomb

Because of its long, thin shape, the Hiroshima bomb was called Little Boy. The fissile material was uranium 235. The uranium was divided into two parts, both of which were below critical mass. It was a “gun barrel-type” bomb that used an explosive device to slam one portion of uranium into the other, instantly creating a critical mass.
When a critical mass is available, a chain reaction takes place instantaneously, releasing energy far beyond the capacity of ordinary explosives. The energy released by the Hiroshima bomb was originally thought to be equivalent to the destructive power of approximately 20,000 tons of TNT. Later estimates based on damage to buildings and studies of the bomb’s structure have reduced that figure to approximately 15,000 tons. It is believed that this enormous energy was released by the fission of slightly less than one of the 10 to 35 kilograms of uranium 235 in the bomb.

The Hiroshima Bomb (Little Boy)

Hiroshima Bomb Diagram

Length: Approx. 3 meters (120 inches)
Weight: Approx. 4 tons (9,000 lbs)
Diameter: Approx. 0.7 meters (28 inches)
Element: Uranium 235 (dead link)

Alternative link:

The Nagasaki Bomb

Compared to one used on Hiroshima, the Nagasaki bomb was rounder and fatter, so it was called “Fat Man.” The fissile material was plutonium 239. The plutonium was divided into subcritical portions and packed into a spherical case. To cause the chain reaction, gunpowder around the periphery of the case was used to force the units to the center. Thus, it was called an “implosion-type” bomb.
The fission of slightly more than one kilogram of plutonium 239 is thought to have released destructive energy equivalent to about 21,000 tons of TNT.

The Nagasaki Bomb (Fat Man)

Nagasaki Bomb Diagram

Length: Approx. 3.2 meters (128 inches)
Weight: Approx. 4.5 tons (10,000 pounds)
Diameter: Approx. 1.5 meters (60 inches)
Element: Plutonium 239 (dead link)

Project Plowshare

Caption: Project Plowshare sought to use peaceful nuclear explosions for civil engineering purposes. One proposal envisioned the nuclear excavation of a new Atlantic–Pacific canal in Central America. Some of the routes considered are shown in this map.

Plowshare and the quest for peaceful nuclear explosions became one of Livermore’s major programs in the 1960s. Initially, the program focused on large-scale earth-moving, or nuclear excavation, with the long-term goal of using nuclear explosions to excavate a new Atlantic-Pacific canal through Central America (Figure 6).

Peaceful nuclear explosions – Sedan test

Caption: Sedan test (underground test) only a small amount (of radioactivity) released into the atmosphere. (at 5:50 min). (Screen capture)

“(4:35) The results of Plowshare work in controlling and limiting the rate of radioactivity released by a nuclear explosion have been and continue to be highly encouraging.

Scientists have already determined that when a nuclear explosives are buried at a depth that will produce the biggest crater, a major part of the radioactivity is swallowed up in the rubble of the crater. But the most promising results have been gained by improving the design of the explosive itself and by improving emplacement techniques. Both of these specifically calculate to reduce the release of radioactivity.

In the 1962 Sedan event, for example, only a small fraction of the radioactivity produced by the explosion was released to the atmosphere. If Sedan were conducted with the improved design and techniques available today (1965), there would be even less radioactivity and more reduction is still expected. A hundred-fold reduction from the radioactivity levels from 1962 through more improvement in explosive design and emplacement techniques.

When this prediction is realized, it will mean that the main limitation of nuclear excavation projects will not be radiation. It will be other potential hazards of explosions, nuclear or conventional: the ground shock, the air blast, and the dust cloud (6:22 min).”

Layer cake design

Image from movie (dead link)

“In Andrei Sakharov’s Layer Cake design, several layers of light and heavy elements were alternated. High explosives surrounding the Layer Cake would be used to implode and ignite the atomic bomb at the center of the device. The atomic explosion would then set off a fusion reaction in the deuterium.”

Caption: “Implosion-type weapon. Fat Man, the Nagasaki bomb, used 13.6 lb (6.2 kg, about 12 fluid ounces or 350 ml in volume) of Pu-239, which is only 39% of bare-sphere critical mass. (See Fat Man article for a detailed drawing.) Surrounded by a U-238 reflector/tamper, the pit was brought close to critical mass by the neutron-reflecting properties of the U-238. During detonation, criticality was achieved by implosion. The plutonium pit was squeezed to increase its density by simultaneous detonation of the conventional explosives placed uniformly around the pit. The explosives were detonated by multiple exploding-bridgewire detonators. It is estimated that only about 20% of the plutonium underwent fission; the rest, about 11 lb (5.0 kg), was scattered.” URL:

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Camp Desert Rock


Image from video below:

VIDEO: Camp Desert Rock  Dailymotion

At Camp Desert Rock in Nevada, the U.S. military began using smaller atomic blasts to learn how to fight a nuclear war. On April 22, 1952 approximately 2,000 Army personnel conducted maneuvers beneath the mushroom cloud of the Charlie nuclear detonation. The 31-kiloton explosion was one of the largest ever conducted in Nevada to that date. This government film claimed that with a few basic precautions, U.S. troops could fight and survive an atomic attack.

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Video: “Survival Town”

VIDEO: Dailymotion | Youtube

Caption: Survival Town atomic bomb blasts. Army destroys a fabricated American town. Real-size dolls are the only inhabitants in “Survival Town”.

“Survival City was built by the Army for Operation Cue. The goal of the test was to observe how houses, shelters, power lines, a radio tower, metal buildings, food, clothing, and actual people would survive at various distances away from a nuclear blast. Mannequins sourced from J. C. Penney, populated the city.

The May 5th 1955 explosion was dubbed Apple-2. The all-clear was finally given at 8:10am.

The 31 kilotons explosion blast area extended 3 miles out from Ground Zero. Approximately 6,000 spectators watched from about 6 miles. Army troops in tanks and trenches were positioned at a mere 2 to 3 miles from the epicenter.”

Stills from the video:

Video: “Atomic Doomtown”

VIDEO: Atomic Doomtown. Big dolls were the sole inhabitants of this town. Filming was done inside the house to show the effects of nuclear explosions on the interior of nearby houses. Nevada desert in 1955. Dailymotion | Youtube

Some stills from that video:



Operation Cue

VIDEO Dailymotion | Youtube: Beginning in 1953 the Federal Civil Defense Adminstration, working with the Atomic Energy Commission, set up an atomic test program to investigate the effects of nuclear weapons on typical American homes and their furnishings. (“OperationCue1955”) Original movie link | Original URL


U.S. nuclear reactor locations (dead link)

May be archived under

Caption: The yield of a hydrogen bomb is controlled by the amounts of lithium deuteride and of additional fissionable materials. Uranium 238 is usually the material used in various parts of the bomb’s design to supply additional neutrons for the fusion process. This additional fissionable material also produces a very high level of radioactive fallout.

Caption: Nuclear energy can also be released by fusion of two light elements (elements with low atomic numbers). The power that fuels the sun and the stars is nuclear fusion. In a hydrogen bomb, two isotopes of hydrogen, deuterium and tritium are fused to form a nucleus of helium and a neutron. This fusion releases 17.6 MeV of energy. Unlike nuclear fission, there is no limit on the amount of the fusion that can occur.