02/26/2013 - 10:31

A broader reading of seismic waves from North Korea

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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A few minutes short of noon, local time, on February 12, an underground blast in a remote corner of North Korea sent seismic waves worldwide, leaving clear recordings on thousands of seismometers. Some of these seismological recorders belong to clandestine intelligence-gathering networks in the service of individual nations. You and I will probably never see the data gathered by these networks, but we don't need to. Primarily to monitor earthquakes within active fault zones, thousands of seismometers around the globe record ground motion and distribute that information to the public. And in the scramble to determine North Korea's nuclear capabilities and intentions, one important implication of these seismological networks has been largely overlooked: This event demonstrated once again how effectively the international community can monitor a comprehensive nuclear test ban treaty, as soon as it is ratified by the United States and placed into force.

Station MDJ of the IC subnetwork of the Global Seismographic Network (GSN) lies in Mudanjiang, China, just across the border with North Korea and less than 400 kilometers from that country's nuclear test site. Seismic waves from the North Korean nuclear blast exceed the background noise at station MDJ by a factor of 200 in the raw unprocessed data, revealing the wave motion in all its detail. Seismic waves from a man-made explosion contain much high-frequency energy, because most of such an explosion's force is transmitted to the surrounding rock in hundredths of a second or less. An earthquake as large as the most recent North Korean test would rupture a fault a few kilometers from end to end, a process requiring a half-second or more. Explosions exert more compressional force on the surrounding rock, while earthquakes exert more shear force. As a result, the seismic P wave – that is, a wave that compresses in and out, in line with the direction the wave is moving -- typically predominates in an explosion signal; the seismic S wave, which shakes back and forth perpendicularly to the wave's travel, is typically larger in an earthquake signal. The P wave dominates the seismic signals from the North Korean nuclear test; no experienced observer could mistake it for an earthquake.

How large was the test? Estimates for the Richter magnitude of the event, determined mainly from its P wave amplitudes, cluster around 5.1. This exceeds the Richter magnitude of North Korea's May 2009 test by roughly 0.4, and its October 2006 test by roughly 0.9. The relationship between Richter magnitude and explosive yield is described well by a logarithmic equation. With it we can conclude that the yield of the 2013 nuclear test was more than three times larger than the yield of the 2009 test, and more than 15 times larger than the yield of the 2006 test. We can estimate the relative sizes of the North Korean tests with better confidence than we can estimate their absolute sizes, because the relation between Richter magnitude and explosive yield depends on the local geologic setting. Past research has shown that, at a test site similar to the US site in Nevada, a seismic event of magnitude 5.1 would imply a yield of 25 kilotons. At the Semipalatinsk test site of the former Soviet Union, now located in eastern Kazakhstan, the same magnitude would imply a yield of 7.4 kilotons. This wide range arises from extremes of local geology. Earth's crust and mantle beneath Nevada are actively deforming and warmer than average, a situation in which seismic waves lose amplitude more rapidly as they travel through it. Earth's crust and mantle are stable and cool beneath Semipalatinsk, so seismic waves lose less amplitude. The North Korean test site has few earthquakes locally, like Semipalatinsk, but is surrounded by actively-deforming crust in China and Japan. It seems reasonable to say that the yield of the 2013 nuclear test lies between the calibration extremes of 7.4 and 25 kilotons.

There is no question that the February North Korean "seismic event" was a nuclear explosion. Attempting to determine the precise type of nuclear explosion is a more ambiguous undertaking. A first-generation fission device, similar to the plutonium bomb detonated in 1945 over Nagasaki, would have a yield in the estimated range. A first-generation fission device based on enriched uranium, similar to the 1945 Hiroshima bomb, would yield in this range as well. A second-generation device designed to boost fission efficiency via the use of tritium, the radioactive isotope of hydrogen, could generate explosions over a larger range of yields, depending on the technical goals of the test. A second-generation nuclear device is necessary for missile-launched nuclear weapons. The nuclear-weapon designs utilized in World War II are too heavy and bulky to deliver without a large airplane, truck, or boat.

In their public announcement of the February nuclear test, North Korea claimed to have detonated a second-generation device, but the international community should be skeptical of this claim. The North Koreans' 2006 test had a likely yield far less than 1 kiloton, consistent with the failure of a first-generation plutonium device. The yield of their 2009 test was estimated to lie between 2 and 7 kilotons, but this was also smaller than the first successful tests of emerging nuclear powers, which historically have been in the 10- to 25-kiloton range. Was this latest test meant to demonstrate to the international security community that North Korea has perfected the technology to detonate fully a first-generation plutonium bomb? That is the easiest interpretation of the seismic data.

If the North Koreans had switched to an enriched-uranium weapon design, a first-generation device would be a simple gun design, almost guaranteed to work. An enriched-uranium nuclear arsenal would be more sustainable for the North Korean regime, because the closing of its Yongbyon nuclear reactor in 2007, as part of a six-party agreement negotiated by the Bush administration, limits its supply of both plutonium and tritium. The regime made no mention of a design shift to enriched uranium, but the seismic data certainly do not rule it out.

Testing a second-generation device, on the other hand, would have risked failure by leaping to a new bomb design, and North Korea's nuclear scientists had already failed twice before. In fact, no member of the second wave of nuclear powers has yet detonated an explosion that demonstrates their possession of a boosted-fission or thermonuclear weapon. Despite their press releases, the seismic data argues that the nuclear tests by India, Pakistan, and North Korea all involved only first-generation devices. In May 1998, both India and Pakistan claimed to have detonated a variety of weapon designs in multiple explosive tests, but seismic data detected only three explosions, one in India and two in Pakistan. All three explosions had Richter magnitudes consistent with the explosion of first-generation nuclear devices.

With more than 2,000 nuclear tests spread over half a century, the acknowledged 20th-century superpowers (the United States, the Soviet Union, China, the United Kingdom, and France) demonstrated the breadth of their arsenals by repeatedly testing explosive devices with diverse yields. They learned that a warfare-ready nuclear-weapons arsenal is both expensive and technically difficult to deploy. But history since 1945 has not shown these nuclear arsenals to have much utility in battle, and nuclear politics have changed.

Deterrence and propaganda value are the main benefits, perhaps the only benefits, of joining the nuclear club. Saddam Hussein showed the world how easy it can be for a secretive government to persuade world leaders and the public that it possesses weapons of mass destruction. Adroit manipulation of the world press is far cheaper than developing a sustainable supply of tritium gas, which has a half-life of only 12 years. By firing a missile, exploding a nuclear device, and creating web videos of Barack Obama in flames, Kim Jong-un, the young, untested ruler of North Korea, hopes to project himself as a serious nuclear player. North Korea is dangerous, but an overestimate of its capabilities would be foolish.

In the second term of the Clinton administration, an August 1997 Arctic earthquake mistaken by the CIA for a Russian nuclear test, the May 1998 nuclear tests by India and Pakistan, and the Monica Lewinski scandal formed the backdrop to an October 1999 US Senate vote on the ratification of the Comprehensive Test Ban Treaty (CTBT). Ratification failed, partly because Republican senators argued that seismic and other technical means for monitoring treaty compliance were inadequate.

Fourteen years later, we can see how short-sighted this skepticism was. Seismographic networks worldwide include thousands of advanced motion sensors that detect and locate hundreds of small seismic events, mostly earthquakes, each day. North Korea's sub-kiloton explosive dud in 2006 was detected, located, and characterized almost instantly by the Preparatory Commission for the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO) in Vienna, which has installed and operated monitoring systems in anticipation of the treaty's eventual ratification. Reports from seismological networks showed the weak explosive signals from the 2006 event clearly, in data available to anyone with an Internet connection. Two university seismologists found P waves from the failed 2006 test racing across the western United States, recorded by dozens of seismometers that were deployed to study rocks beneath the sensors, not to detect clandestine explosions half a world away. Other technical means, such as radionuclide detection and infrasound sensors, have also proved their mettle as treaty-compliance monitors.

As a technical matter, we are ready to implement the Comprehensive Test Ban Treaty (CTBT) immediately, at a time when world politics includes only one nation still claiming to develop new nuclear weapons. We should seize this chance now.