North Korea's official mouthpieces are crowing about a successful nuclear weapons test. But how do we know what really happened? By checking the seismic data, scouring the satellite images – and sending in the "Constant Phoenix."

Measurement and signature intelligence – a.k.a. MASINT – is one of the key disciplines for detecting a nuclear event. It starts with seismic data. The U.S. Geological Survey yesterday reported an earthquake in North Korea that measured 4.7 in magnitude. That's more powerful than the tremors that resulted from North Korea's first nuclear test in 2006 – which the U.S. Geological Survey estimated at 4.2 (the Comprehensive Test Ban Treaty Organization gave it 4.0; The South Koreans put it between 3.58 and 3.7).

Seismic measurements, however, don't provide a direct calculation of yield. As our own Jeffrey Lewis commented after the first North Korean nuclear test, "estimating the yield is tricky business, because it depends on the geology of the test site." Helpfully, though, the event took place near Poongkye-ri in North Hamkyong Province, which, as China's Xinhua news agency notes, is the same area where the North Koreans conducted the first nuclear test on Oct. 9, 2006. So in terms of geography, we may have something of an apples-to-apples comparison. Figuring out if a seismic event is an earthquake or a bomb blast is relatively easy.

Scientists do more than look at the strength of seismic impact. They also examine the wave patterns from seismic sensors, to spot the difference between nuke-inspired tremors and earthquakes. __"__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," *How Stuff Works *explains.

Of course, seismic data is not the only information we might have. If, for instance, there is venting from the test site, that would mean there is a radioactive plume that could also be measured. That would help rule out the use of conventional explosives to simulate a small nuclear bomb.

In addition, one wonders if there are any clandestinely based sensors around the North Korean test site that would help give some more precise information. Satellite or aerial images overhead can also provide some clues by probing the layout of the test site (and if it happens, spot any serious venting.) And let's hope that somewhere, somehow, someone is collecting human intelligence inside the North Korean nuclear program.

You can bet that the U.S. Air Force Technical Applications Center (AFTAC), which conducts nuclear treaty monitoring and nuclear event detection, is pretty busy with this one. They've got a tanker aircraft, dubbed the "Constant Phoenix," equipped with air-samplers and filters which can pick out the signature particles of a nuclear plume. (Think ionized xenon particles, to start.) During the Cold War, America maintained a whole fleet of these airplanes. Now, there's only a single one of the aircraft left. In 2006, it flew near North Korea, to confirm Pyongyang's first nuclear blast. No doubt, it's been busy again.

[Photo: USAF]

*– Nathan Hodge and Noah Shachtman

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