Wednesday, February 27, 2013

A broader reading of seismic waves from North Korea

For all of you "techies" who look at the data.

A broader reading of seismic waves from North Korea
Article Highlights
  • Based on seismic data, the explosion North Korea detonated on February 12 was consistent with a successful test of a crude, Nagasaki-type bomb.
  • No regional nuclear power has conducted an explosive test that demonstrates its ability to deploy nuclear-tipped missiles.
  • With only one nation still conducting nuclear tests, the time is ripe to place the Comprehensive Test Ban Treaty into force.
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.
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