North Korea: When is an earthquake a nuclear test?

The first sign that North Korea had carried out its fifth nuclear test last month was when a massive earthquake was detected by international scientists. As North Korea marks 10 years since its first test, geophysicist and disaster researcher Mika McKinnon explains how scientists have learned to identify these world-shaking events.

Weapons tests cannot hide from science.

Every stage of North Korea's nuclear weapons development programme is under global observation, with scientists using data to extract the true state of their progress.

A worldwide network of sensors is constantly collecting data, seeking the faintest rumble of a weapons test.

When one takes place, the explosion slams into its surroundings.

The vibrations propagate through water, air, and earth, too low frequency to be heard by human ears but picked up by sensors and relayed to control centres for analysis.

A disturbing pattern

On 9 October 2006, seismometers around the world lit up, detecting a significant explosion in North Korea.

The infrasound network stayed silent, the lack of an atmospheric blast indicating it happened underground away from prying satellites. Yet more silence on underwater hydro acoustic stations confirmed it had been muffled by rock, not water.

The scientist triangulated the direction and time of seismic waves to pinpoint the explosion's origin under mountains in North Korea.

All the signs suggested a nuclear test.

In the following days, monitoring stations sniffed the air looking for the faintest trace of radioactive isotopes.

If any isotopes escaped from the test tunnels to drift in the wind, they'd be unmistakable relics of radiation from a nuclear blast and rule out any chance of a conventional weapon.

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