“We have successfully conducted another nuclear test on 25 May as part of the republic’s measures to strengthen its nuclear deterrent. (…) The results of the test helped satisfactorily settle the scientific and technological problems arising in further increasing the power of nuclear weapons and steadily developing nuclear technology.”
Official statement — Democratic People’s Republic of Korea
In the spring 2009, I wrote a short analysis of North Korea second nuclear test. A few top experts read and commented the document. As the note has never been published, I reproduced it here. Keep in mind that the analysis is entirely based on open source information available before mid June 2009. Follow us on Twitter: @INTEL_TODAY
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On May 25th 2009, a violent explosion occurred in the North-Eastern part of North Korea. The seismic waves produced by the event clearly reveal its man-made origin. The magnitude of the recorded body waves is significantly greater than the one associated with the 2006 nuclear test. As neither CTBTO noble gas stations nor CTBTO Member States own national stations have reported nuclear radiation, the possibility of a chemical explosion can not be ruled out but appears unlikely. If the North Korean scientists tested a Nagasaki type weapon, the device did not detonate properly as the yield amounts to no more than about 3 kilotons of TNT equivalent. The successful detection of both North Korean low yield tests raises hopes that the US Senate will soon ratify the Comprehensive Nuclear-Test-Ban Treaty (CTBT)[i].
FACTS AND ASSESSMENTS OF THE TEST
At 00:54:43 GMT Monday May 25th 2009, a seismic event occurred in the North-Eastern region of North Korea in the vicinity of P’unggye near the town of Kilju, in the North Hamgyong province.[ii]
North Korea had warned officials in Washington and Beijing of the test about an hour before the detonation.
According to the US Department of State, the warning did not cite a specific timing and was immediately passed to Japan, South Korea, China, and Russia.
The North Korean warning made no prediction regarding the yield of the device. The October 2006 warning predicted a 4 kilotons explosion.
“We have successfully conducted another nuclear test on 25 May as part of the republic’s measures to strengthen its nuclear deterrent.”
“The test was safely conducted on a new higher level in terms of its explosive power and technology of its control,” the KNCA news agency said.
“The results of the test helped satisfactorily settle the scientific and technological problems arising in further increasing the power of nuclear weapons and steadily developing nuclear technology.” [iii]
Recent reports from South Korean intelligence sources had reported unusual activity at the Kilju test site.[iv]
“This is absolutely predictable, even though I thought they would do it later, allowing some time for tension to mount,” said Andrei Lankov, a Seoul-based expert on North Korea who teaches at Kookmin University. [v]
Location and Time
The International Data Center (IDC) using the International Monitoring System’s seismic stations (IMS) located a seismic event at 41.2896 degrees North and 129.0480 degrees East at 00:54:43 GMT (08:54 local time).[vi]
CTBTO scientists are confident that the event occurred in a 4.5 km radius circle centered on that position.
The US Geological Survey confirmed the location of the event: 41.306° North and 129.029° East with a horizontal accuracy of +/- 3.8 km.[vii]
5 km represents about 3 seconds of time arrival difference between the P and the pP waves. With proper filtering, such depth may be detectable.[x]
For instance, in 1968, the Russians exploded a 47 kilotons device at a depth of 2.45 kilometers near Bukhara. The British-Canadian array YKA, at a distance of 79 degree, detected a time difference arrival of the P and pP waves of 1.7 second. Such time difference corresponds to a depth of 2.6 kilometers.
The statement that the North Korean shot was 5km deep is highly suspicious. There is no reason for the North Koreans to drill to a depth of 5km to detonate a nuclear device as the cost would be ridiculously high and the benefits minimal.[xi]
Satellite images have revealed a tunnel entrance at 1500 meters under a 2200 meters high mountain.
The tunnel is believed to run horizontally. It is therefore likely that the test site is located at a depth of 700 meters which is more than enough to prevent venting if the tunnel was adequately plugged.
Explosion, not Earthquake
In all likelihood, the explosion is man-made as the area is not active seismically and North Korea issued a warning about one hour prior to the event. Moreover scientific tests can distinguish an explosion from an earthquake.
According to a diplomat close to the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), the tremor was not an earthquake.[xii]
P waves vs S waves
Two types of elastic waves can propagate through the body of an elastic material such as the earth. The P waves consist of pushes and pulls along the direction of travel of the wave. On the contrary, S waves move at right angle to the direction of travel.
In theory, the spherical radial expansion created by a perfect nuclear explosion produces mostly compressional P waves while the sliding of a plate against another which gives birth to an earthquake produces similar amounts of S and P waves.
According to general Aoki of the Japan Meteorological Agency in Tokyo, the S-wave component was just one-fifth that of the P-wave. It is not impossible for a natural earthquake to produce such waves, but it would be very rare.
Body Waves vs Surface Waves
The ratio of surface – Raleigh – waves to body P waves can also help to distinguish between an explosion and an earthquake.
Both signals could of course coexist if, for instance, the collapse of the ceiling of the cavity created by an explosion produced significant seismic waves.
CTBTO experts have concluded that the recorded signals contain distinct characteristics of an explosion. In addition, they also identified simultaneous signals with characteristics atypical for explosions.
The comments about complex seismic signals are interesting. However, the CTBTO website does not elaborate on the possible implications. Dr Jeffrey Park of Yale University suggested three possibilities.[xiii]
First, explosions from North Korea are new phenomena, without a large number of past earthquakes to give the signals a frame of reference. The 2006 event was much smaller, with signals that lay below the noise level of a typical seismic observatory. By contrast, the May 2009 nuclear test was large enough to be clearly observable around the globe, so that details of the signal were more resolvable. Through many geologic factors, such as crustal structures, near surface micro-cracks and elastic anisotropy in the rock, compressional waves from an explosion are partly converted to shear waves.
For stations close to the event, the conversion to shear is less obvious than for stations farther away, because the conversion accumulates as the wave travels.
The second possibility is that there was some tectonic stress in the immediate region of the explosion, and the event induced slip motion to relieve the shear stress on one or a collection of pre-existing faults.
The third possibility is probably not an issue in this case, but may rank highly in concerns as a general matter. One of the evasion scenarios that seismologist chewed over in the 1980s is exploding a nuclear device in a chamber of highly distorted shape. Long tunnel designs with side chambers, for instance, were predicted to couple to the explosion in such a way to release more shear energy than would a spherical or squarish chamber. There was some concern that a budding or established nuclear power would build such a chamber not to muffle a blast, but to misrepresent it. For many reasons, there is little reason why the North Koreans would try to evade detection or masquerade their blast as an earthquake.
The detonation occurred near the site of the October 2006 test which the US Geological Survey Quick Determination of Epicenters estimated as 41.294°N, 129.094°E.
This area consists of Cretaceous granite intrusions and Pre-Cambrian granite gneiss and schist.
Satellite imagery has revealed tunneling activity in this area since the late 1990s.
Body Wave Magnitude
The system of seismic stations operated by the CTBTO detected signal from an event with the magnitude 4.52 on the IDC magnitude scale.[xiv]
According to a message posted on the CTBTO website, “the event’s magnitude is slightly higher than in 2006, measuring 4.52 on the Richter scale, while in 2006 it was 4.1.”
Considerably more seismic stations picked up the signal this time: 61 IMS seismic stations compared to 22 in 2006.
South Korea officials reported “an artificial earthquake” measuring at 4.4 on the Richter magnitude scale. The agency initially said it measured 4.5.[xv]
The US Geological Survey put the magnitude as equivalent to a 4.7 earthquake.[xvi]
The variance of the Richter-magnitude estimates is very common. The first estimates are usually based on a limited number of stations in a limited region, and biases in the efficiency of seismic wave propagation (geometric focusing, frictional attenuation) lead to different estimates.
The most reliable estimates are the ones with the most stations, namely the USGS and CTBTO.
Beside the time and the location of the “event”, seismic waves can also reveal the magnitude of the explosion.
Based on Nevada underground explosion tests conducted in granite, the magnitude of body waves is proportional to the logarithm of the yield in kilotons: Mb = 4.26 +0.97 log (Y).
A kiloton is equivalent to 1,000 tons of TNT.
In October 2006, various authors used this equation to estimate the yield of the first North Korea nuclear test at less than one kiloton.[xvii]
The office of John Negroponte, the US National Intelligence Director confirmed that the size of the explosion was indeed less than 1 kiloton, and thus well below the yield predicted in the 2006 North Korean warning.[xviii]
According to the equation mentioned above, the magnitude reported by South Korea, the CTBTO and the US Geological Survey translate in yields of 1.39, 1.76 and 2.84 kilotons of TNT equivalent respectively.
Actually, if there is any systematic bias in the yield estimates, the Nevada explosion-in-tuffs and alluvium formula might be biased high. Nevada is a tectonically active region which attenuates seismic waves more than most continental regions. Therefore, the North Korea test yield could possibly be lower than the above estimates based on the US-Nevada formula.[xix]
The US bomb dropped over Hiroshima was between 13 and 18 kilotons.  For reference, the first French test device had a yield of 65 kilotons.[xx]
Although, the yield of the second presumed nuclear explosion appears slightly greater than the first one, it is still rather low for a typical plutonium fission device. For instance, the yield of Fat Man, dropped on Nagasaki, was estimated between 20 and 22 kilotons of TNT equivalent.
Alexander Drobyshevsky, a Russian Defense Ministry spokesman, confirmed that North Korea had conducted a subterranean nuclear test and stated that the strength of the explosion was between 10 and 20 kilotons.
It is rather difficult to understand the statement of the Russian Defense Ministry as a yield of 20 kilotons corresponds to a magnitude of about 5.5.[xxi]
Neither Dr Zerbo nor CTBTO scientists would comment on the issue of the yield.
However, the ratio of the magnitude of the body waves reported by the CTBTO in 2006 and 2009 and the widely accepted fact that the 2006 test yielded less than a kiloton unambiguously implied that the 2009 test released, indeed, less than about 3 kilotons of TNT equivalent.
Official US Confirmation of the Low Yield
The low yield estimated in this analysis appears to be in excellent agreement with the current US official analysis.
A senior U.S. military official has stated that “the seismic data indicated a relatively small bomb of around 1.5 kilotons.”[xxii]
On June 15 2009, the Office of the Director of National Intelligence released the following statement.
“The U.S. Intelligence Community assesses that North Korea probably conducted an underground nuclear explosion in the vicinity of P’unggye on May 25, 2009. The explosion yield was approximately a few kilotons. Analysis of the event continues.”[xxiii]
No Radiation detected
Neither the US nor South Korea authorities were able to confirm the nuclear nature of the test.
“The first test results came in inconclusive. They did not find anything that could confirm a nuclear device was detonated,” said the U.S. official, who declined to be named.[xxiv]
The CTBTO website promised to make an announcement regarding detection of radiation if it was detected which would unambiguously confirm the nuclear nature of the test.
In an interview conducted on Friday June 12, CTBTO scientists told the author that it is possible that there is a release of radiation from the explosion but it is below the detection capability of the IMS network and that the measurement could not reveal with certainty that the explosion was nuclear.
A Chemical Explosion?
If no radiation is detected, one is left with the following alternative. Either the nuclear radiations were well contained and no detectable venting of radioactive gas occurred, or the explosion was of chemical nature.
Setting up a single chemical explosion of this magnitude is technologically possible but is a formidable task.
For instance, in the Minor Scale experiment, US scientists detonated 4.8 kilotons of ANFO. The cost of the explosive was about US$ 1.1 million.
On June 12 2009, Dr Zerbo[xxv] told the author that the CTBTO is not making a decision concerning the nuclear nature of the event.
Possible Explanations of a Low Nuclear Yield
Firstly, the North Koreans could have detonated a larger device in a large cavity to muffle its yield.
Indeed, if the bomb is fired inside a cavity large enough to not expose the walls to stresses that exceed elastic range, the energy of the explosion can be drastically decoupled.
The diameter of the cavity required for decoupling depends on the depth of the explosion and the yield of the bomb.
An experiment conducted in 1966 demonstrated that a 0.4 kiloton device exploded at 900 meters depth inside a quasi-spherical 17 meter radius cavity produced seismic signals about 70 times smaller than those produced by a similar but non-decoupled device.
Secondly, the device could have been a higher-tech device designed for smaller yield with less fissile matter. Indeed, their prediction for the yield of their first test could indicate that they are not working on a Nagasaki-type device either to save plutonium or to reduce the weight of the device. The weight of Fat Man was about 4 tons and greatly exceeds the capacity of the Scud and Nodong missiles which is in the 500 − to 1000 − kilogram range. Thirdly, the device may have failed to detonate properly. If a neutron starts a chain reaction before the plutonium reaches super-criticality, only a small fraction of the potential energy of a bomb is released. The phenomenon is known as fizzling. US scientists experienced their first fizzling on their 18th test while the Russians encountered their on the 15th experiment.
The first option makes absolutely no sense in the present context. Although the second one can not be ruled out, it is rather unlikely as no country has ever attempted to weaponize a nuclear device before having successfully detonated the natural design of a plutonium implosion bomb, i.e. the Nagasaki-type design.
In all likelihood, the third explanation is the correct one, namely that the North Korea second presumed nuclear explosion fizzled.
If the explanation suggested above for the fizzling is correct, there may be no solution immediately available to North Korea scientists as the problem lies in the property of the plutonium produced and extracted thus far, namely the percentage of Pu 240.
About one hour after warning the US and China, North Korean scientists conducted an explosion which released between one and 3 kilotons of TNT equivalent.
Although a chemical “bluff” has not been scientifically discounted, such scenario appears implausible. In all likelihood, North Korean scientists have detonated an implosion Plutonium device.
As of June 15 2009, neither CTBTO noble gas stations nor CTBTO Member States national stations have reported the detection of nuclear radiation. The absence of radiation detection does not however rule out that the explosion was a nuclear test.
Underground nuclear explosions always produce an isolating lawyer of molten rock. However, this lawyer can be broken by radial cracks and slips down to the bottom in many cases.
Finally, if North Korean scientists tested a Nagasaki-type device, the bomb failed to detonate properly as the yield amounts to about one tenth of the energy expected to be released by such device.
On June 15 2009, Dr ElBaradei appeared to have confirmed the nuclear nature of the test conducted by North Korea.[xxvi] In his opening statement to the IAEA Boards of Governors, Dr ElBaradei stated: “I am greatly concerned at subsequent news of a second nuclear test by the DPRK last month.”
Consequences Regarding the CTBT
On October 13 1999, the United States Senate rejected the ratification of the CTBT. The US Senate expressed fears that the reliability of the US nuclear arsenal could not be assessed without testing and that countries could either hide or disguise small nuclear tests.
A 2002 report by a committee of the National Academy of Sciences (NAS) concluded that the ongoing stockpile stewardship program can maintain and verify the reliability of U.S. nuclear weapons without explosive testing.[xxvii]
In January 2007, George P. Shultz, William J. Perry, Henry A. Kissinger and Sam Nunn urged leaders of the countries in possession of nuclear weapons “to turn the goal of a world without nuclear weapons into a joint enterprise.”[xxviii]
Among the urgent steps that would lay the groundwork for a world free of the nuclear threat, the authors included “initiating a bipartisan process with the Senate, including understandings to increase confidence and provide for periodic review, to achieve ratification of the Comprehensive Nuclear-Test-Ban Treaty, taking advantage of recent technical advances, and working to secure ratification by other key states.”
In 2008, Congress established the Congressional Commission on the Strategic Posture of the United States to “examine and make recommendations on strategic policy and force structure, as well as to consider other ways to counter the nuclear threat.”
During his 2008 election campaign, President Barack Obama stated that as president, he will “reach out to the Senate to secure the ratification of the CTBT at the earliest practical date.”[xxxi]
The Chair and lead author of the 2002 NAS report was Dr. John Holdren who currently serves as Science Advisor to President Obama.
The North Korean tests should convince US lawmakers that the verification-monitoring system does work as it sweeps aside all the arguments made in 1998 that it might be possible for a rogue nation to conceal a nuclear weapons program.
Testifying before Senate Foreign Relations Committee, Ellen Tauscher said that she would work towards the ratification of CTBT.
“I share the administration’s commitment to obtaining the Senate’s advice and consent to ratify the Comprehensive Nuclear-Test-Ban Treaty and to launch a diplomatic effort to bring states that have not signed the Treaty on board so that it can be brought into force,” Tauscher said in her testimony.[xxxii]
According to Dr. Zerbo, Ellen Tausher visited the CTBTO headquarter in Vienna last year and showed great interest in the work of the organization and deep knowledge of the nuclear proliferation issue.
Ellen Tausher has also alluded to the fact that the US administration will try to convince other Annex-II states, whose ratification is required for the Treaty to enter into force, to ratify the CTBT. Tausher explicitly mentioned India, Pakistan and Israel.
In conclusion, the North Korea presumed second nuclear test has provided the US administration with the perfect political opportunity to resubmit the CTBT for ratification and to engage the International Community in the increasingly complex and urgent fight against Nuclear Proliferation.
The author wishes to thank Dr. Lassina Zerbo, Dr. Jeffrey Given, Dr. John Coyne, Dr. Mika Nikkinen and Dr. Ivan Kitov for a thorough review of the CTBTO scientific assessment of the presumed North Korea second nuclear test as well as Mrs. Silvy Thomas who helped setting up the video conference.[xxxiii]
I am indebted to Dr. Jeffrey Park of Yale University for many valuable comments and suggestions regarding this analysis.[xxxiv]
[ii] On October 9 2006, the test was conducted at 01:35:27 GMT. It would thus appear that North Korea does not conduct its nuclear test on an exact five minute mark.
[iii] Reuters, “Text of the North Korean Announcement of Nuclear Test,” May 25, 2009.
[iv] Blaine Harden, “N. Korea Conducts ‘Successful’ Underground Nuclear Test,” The Washington Post, May 25, 2009.
[vii] USGS, Earthquake Hazards Program, Magnitude 4.7 – NORTH KOREA, 2009 May 25 00:54:43 UTC.
[viii] Peter Alford, “North Korea’s nuclear defiance with bomb test,” The Australian, May 26, 2009.
[ix] Robert Johnston, “Nuclear tests: height/depth of burst vs. yield,” last updated 19 June 2005.
[x] For earthquake monitoring networks, the depth is often determined more from a comparison of P wave times for a collection of stations. The internet available stations are all at least 300 km away, and determining depth relies on the P-pP time delay. This delay cannot be determined visually, because the source pulse is too long. The explosion is very short, but the pulse that emerges from the source region contains more than just the blast, mostly caused by near-source scattering. One important cause of near source scattering is the free surface, which causes the pP wave. The pP time delay is theoretically resolvable via an autocorrelation method that tries to detect multiple time-delayed copies of the source pulse in the P-coda, but the reliability of this method is not demonstrated. The authority on such matters is Paul Richards of Lamont-Doherty Earth Observatory. In a 2006 review paper, Richards and co-authors focused on determining locations with an array rather than with single stations, and cast some doubt on determining depth accurately with single-station records.
Review paper: The Applicability of Modern Methods of Earthquake Location
Paul G. Richards, Felix Waldhauser, David Schaff, and Won-Young Kim, Pure appl. geophys. 163 (2006) 351–372.
[xi] One figure used in determining how deeply the device should be buried is the scaled depth of burial, or -burst. This figure is calculated as the burial depth divided by the cube root of the yield in kilotons. It is estimated that, in order to ensure containment, this figure should be greater than 100. So a 27-kt blast should be detonated at a depth of 300 meters.
McEwan, A. C. (1988). “Environmental effects of underground nuclear explosions,” in Goldblat Jozef and Cox David. “Nuclear Weapon Tests: Prohibition or Limitation?” Oxford University Press, pp. 75–79. ISBN 0198291205.
[xii] Geoff Brumfiel and David Cyranoski, “North Korea’s bigger blast,” Nature News, May 26, 2009.
[xiii] Dr. Jeffrey Park, Director of the Yale Institute for Biospheric Studies, e-mail correspondence with the author, June 2009.
[xv] Sam Kim, “N. Korea conducts second nuclear test following rocket launch,” Korean Center, May 25, 2009.
[xvi] USGS, Earthquake Hazards Program, Magnitude 4.7 – NORTH KOREA, 2009 May 25 00:54:43 UTC.
[xvii] Dr. Ludwig De Braeckeleer, “N.K. Nuclear Test: Evidence and Unknowns,” OhMyNews International, October 12, 2006.
[xix] Dr. Jeffrey Park, Director of the Yale Institute for Biospheric Studies, e-mail correspondence with the author, June 2009.
[xx]Dr. Ludwig de Braeckeleer, “History of French Nuclear Tests in the Pacific: [Part I of IV] 1966-1974: Atmospheric tests,” OhMyNews International, October 29, 2006.
[xxi] RIA NOVOSTI, “Russia confirms N.Korea nuclear test,” June 19 2009.
[xxii] Evan Ramstad, Jay Solomon and Peter Spiegel, “Korean Blast Draws Outrage,” The Wall Street Journal, May 26, 2009, p. A1.
[xxiii] ODNI News Release No. 23-09, “STATEMENT BY THE OFFICE OF THE DIRECTOR OF NATIONAL INTELLIGENCE ON NORTH KOREA’S DECLARED NUCLEAR TEST ON MAY 25, 2009,” June 15, 2009.
[xxiv] REUTERS, “U.S. says North Korea test results inconclusive,” May 29, 2009.
[xxv] Dr Zerbo serves as the Director of the International Data Centre Division, Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO PrepCom) in Vienna.
[xxvi] Elizabeth Dobie-Sarsam, IAEA Public Information Assistant, email correspondence with the author, June 15, 2009.
[xxvii] Indeed, the National Nuclear Security Administration announced last year that an eight-year program to refurbish B61 nuclear bombs and ensure their reliability was completed without explosive testing. In fact, the program was completed a full year ahead of schedule.
[xxviii] George P. Shultz, William J. Perry, Henry A. Kissinger and Sam Nunn, A World Free of Nuclear Weapons,” Wall Street Journal, January 4, 2007, p. A15.
Their statement was endorsed by Martin Anderson, Steve Andreasen, Michael Armacost, William Crowe, James Goodby, Thomas Graham Jr., Thomas Henriksen, David Holloway, Max Kampelman, Jack Matlock, John McLaughlin, Don Oberdorfer, Rozanne Ridgway, Henry Rowen, Roald Sagdeev and Abraham Sofaer.
[xxix] Walter Pincus, “Panel Cites ‘Tipping Point’ On Nuclear Proliferation,” The Washington Post, December 16, 2008, p. A17.
[xxx] The panel however recommended not ratifying the CTBT until the nation’s nuclear laboratories and U.S. Strategic Command submit clear statements on the risks involved by such a measure.
[xxxi] The Editors, “Nuclear Testing Is an Acceptable Risk for Arms Control,” Scientific American, March 2009.
[xxxii] Lalit K. Jha, “US to push countries to sign CTBT,” Business Standard, June 9, 2009.
[xxxiii] Mr. Jeffrey Given is Chief of Software Applications Section at the International Data Centre Division. Mr. John Coyne is Programme and Project Coordinator at the International Data Centre Division. Mr. Mika Nikkinen is the Head of the Scientific Methods Unit -Software Applications Section – at the International Data Centre Division. Mr. Ivan Kitov is Fusion and Review Officer – Office of the Director – at the International Data Centre Division. Mrs. Silvy Thomas is the personal assistant of Dr Zerbo.
[xxxiv] Dr Jeffrey Park is professor of geology and geophysics. He is the current Director of the Yale Institute for Biospheric Studies. His biography can be consulted at http://earth.geology.yale.edu/~jjpark/