7 January 2016

Seismic waves from North Korea suggest a repeat of the 2013 nuclear test

Jeffrey Park

Jeffrey Park

A seismologist, Park is professor of geology and geophysics at Yale University. Previously, he served as the chair of Yale's...

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In the mid-morning, local time, of January 6, an underground blast in a remote corner of North Korea sent seismic waves worldwide, leaving clear recordings on thousands of seismometers. This is the fourth detonation that has been detected at the North Korean nuclear test facility. Prior events occurred in 2006, 2009 and 2013. The yields of North Korea’s previous explosions grew in sequence, by factors of 3 to 5 in size, until the 2013 detonation achieved the size expected for a first-generation plutonium fission device. The 2006 and 2009 detonations were widely regarded as failures to achieve North Korea’s technical goals for explosive yield. The 2013 detonation was large enough to conclude that North Korea had built a nuclear device equal to that built by the United States in 1945, by the Soviet Union in 1949, and by several regional nuclear powers since.   

Seismic signals from the January 2016 detonation at the North Korean nuclear facility bear an uncanny resemblance to the signals recorded for the February 12, 2013 detonation. The vertical ground motions of the first-arriving compression wave (the P wave) for the closest station of the Global Seismographic Network, station MDJ in Mudanjiang, China, follow the ground motions of the 2013 event almost wiggle by wiggle. Slight changes in the relative timing of signals, recorded at Internet-accessible seismic observatories in northeast Asia, suggest that the 2013 and 2016 events were separated by three kilometers or fewer. Chinese seismologists, with ready access to additional seismic observatories in neighboring Manchuria, have estimated that the separation is roughly a kilometer. The simplest explanation for the seismic data is that the January 2016 seismic event was generated by a nuclear device with explosive yield identical to the February 2013 device, suggesting a follow-up test of a “gadget” with identical design.

In stark contrast to the seismic evidence, North Korea has claimed to detonate a thermonuclear device, or H-bomb. The classic A-bomb (“A” stands for “atomic”) relies on the fission of unstable heavy atomic nuclei, using the radioactive elements uranium or plutonium. The “H” in H-bomb stands for hydrogen, the lightest element, which releases enormous energy when two of the heavy isotopes of hydrogen, deuterium and tritium, are fused together to form the element helium. The general outlines of H-bomb designs involve an initial fission detonation that generates the heat and pressure necessary for fusion to occur. The standard Teller-Ulam configuration, thought to underlie all Cold War H-bomb designs, starts with a “boosted” fission detonation, in which an infusion of tritium supplies extra neutrons to split heavy atoms more efficiently. In the most credible reading of North Korea’s claims, between 2013 and 2016 their nuclear program developed a workable Teller-Ulam configuration and a tritium-boosted fission starter-device without any intervening detonation tests, but produced an explosion that matches the size, frequency content and (even) the detailed vibrational oscillations of a simple fission device from 2013. 

A successful thermonuclear detonation would be at least 30 times larger than a first-generation fission detonation, leading to a seismic event of Richter magnitude M=6 or more. The January 2016 seismic event is not larger than the February 2013 event; Chinese researchers estimate the yields of both events to be equivalent to 11-12 kilotons of TNT, with a correlated uncertainty of 4 kilotons, far smaller than thermonuclear yields. Could the starter-device of a fusion bomb have detonated alone, with the fusion component failing, and produce the observed signal? This interpretation is possible, but faces two obstacles. First, a fission device that is designed as a fusion starter would exploit different physics, e.g., tritium-boosted fission, and very likely would explode with a different yield than a first-try fission-only device. Second, suppose the North Koreans had attempted to use their original fission “gadget” as a fusion-device starter, as a way to accelerate their weapon development. This might explain the near-identical seismic signals in a failed thermonuclear detonation, but would also suggest that North Korea is incapable of designing a successful thermonuclear device. If an engineering company bolted a V-6 engine from a Chevrolet Impala onto the wing of a Boeing 737, would you expect the plane to fly?

The United States, the Soviet Union, and China developed H-bombs from simple fission devices with multiyear research programs that involved numerous explosive tests of successive detonation designs. The logistics of these efforts were enormous, but militarily futile. Since 1945, nuclear weapons have not been expended in any military conflict. To achieve the political objective of deterrence—that is, to discourage unprovoked attacks from neighboring countries—advanced nuclear weapons do not appear to be necessary. The regional nuclear powers India and Pakistan have settled for a few explosive tests of first-generation fission devices, along with ballistic missile tests. These rival states detonated multiple devices during 1998, partly, if not entirely, to demonstrate resolve to each other. Since 1998, neither state has continued testing as part of its nuclear program. After decades of flirting with a nuclear program, Iran chose to change course, thereby gaining a diplomatic recognition that, in the Cold War, the bomb was intended to secure.

Why would North Korea overstate not only its technical results, but also its technical goals? Officials in Pyongyang can watch Fox News, too. Any assertion by an American presidential candidate, no matter how ridiculous, gets reported unfiltered by the global news media. By the time experts can contribute their skepticism that, e.g., undocumented Mexican immigrants are mostly rapists, the news cycle will have passed onto another shiny object. The timing of the North Korean nuclear test was no accident. Arms-control specialists have been bracing for a test since the spring of 2014, motivated by media reports of tunneling and construction at the North Korean nuclear facility. If the main benefit of a nuclear capability is political, a shrewd power would not expend its scarce plutonium while American politicians are focused on Obamacare and tax cuts for “job creators.” To garner the greatest global attention and domestic prestige, a shrewd power would wait until national security concerns dominated the US news cycle. With the Iowa caucuses only a month away, no US presidential candidate, of either party, can risk a perception of weakness by offering a nuanced nuclear-security stance. Pyongyang officials have very few cards to play in a game of nuclear poker, but they are playing them well.

The danger posed by North Korea is significant, but should not be overstated.  Any use of a nuclear weapon by the Pyongyang regime would be an act of self-destruction, but insecure dictatorships are capable of desperate actions when cornered. The shrewd timing of North Korea’s detonation relative to the US presidential campaign, paradoxically, suggests that the regime is desperate only for attention. 

The rapid detection and characterization of the North Korean blast—literally overnight in the United States, and by dinnertime in China—tells us that the international monitoring of nuclear explosions is an effective deterrent to a serious nuclear-weapons program. As I discussed in detail after the 2013 North Korean nuclear test, any seismologist with Internet access can use data from hundreds or thousands of seismic sensors, installed and maintained worldwide to detect earthquakes and to explore Earth’s subsurface rocks, to characterize a rogue explosion. This means that any state with the technical capability of a university earth-science department can verify independently whether North Korea’s claims are credible.  Some examples of comparative data assessments can be viewed at web pages maintained by the US Geological Survey Earthquake Hazards Program and the Incorporated Research Institutions for Seismology.

In the 1990s, the proponents for US ratification of the Comprehensive Test-Ban Treaty argued that open sources of seismological data would foster an international “neighborhood watch” to deter nations from embarking on the arduous technical project of developing sophisticated nuclear weapons.  The CTBT remains unratified by the United States, but the neighborhood watch is nevertheless in effect.