Suspicious quake gives away North Korea's third nuke

Some earthquakes just aren't natural. North Korea's third nuclear test on 12 February registered as a magnitude 4.9 quake on instruments operated by the Comprehensive Nuclear-Test-Ban Treaty Organization.

The CTBTO is calling the quake ? which occurred at around 3am GMT ? a "suspicious event" as North Korea has low seismic activity. It seems to have occurred close to the location of two previous nuclear tests carried out in 2006 and 2009, which caused earthquakes of magnitude 4.1 and 4.52 respectively.

Meanwhile, the nation's official news agency confirmed the nuclear blast, saying North Korea had tested a smaller and lighter nuclear device than in previous tests, with "greater explosive force".

NORSAR, which operates the Norwegian part of the CTBTO monitoring network, puts the test bomb at approximately 10 kilotons. That's an increase from the nation's tests in 2006 and 2009, which are estimated at 1 and 5 kilotons respectively, says NORSAR seismologist Tormod Kv?rna. By comparison, the bomb dropped on Hiroshima in 1945 had a yield of about 16 kilotons.

Smoking gun

The CTBTO is now monitoring for signs of radioactive gas released by the explosion. The organisation dubs this type of signal a "smoking gun" and it can reveal whether the explosion was powered by uranium or plutonium.

Not detecting the gas doesn't rule out a nuclear culprit, however, as the test may have taken place too deep underground for the gas to escape. This is thought to explain why no gas was seen following the 2009 test.

Barack Obama called the act "highly provocative" and a threat to US security and international peace. He is expected to propose significant cuts to the US's nuclear arsenal in his State of the Union address tonight.

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