Earthquake or nuclear bomb? Telling the difference just got easier

A group of Canberra-based geoscientists has developed a method to detect secret underground nuclear tests.

Nations have tested nuclear weapons beneath the surface since the 1960s, but while it avoids some of the pitfalls of above-ground testing, they still releases substantial energy.

This can make it difficult to distinguish nuclear explosions from earthquakes, which may display similar seismic characteristics.

Typically, seismologists use two types of seismic energy produced by an earthquake or large explosion to differentiate between the two. Body waves spread out from the centre of an event, whereas surface waves – those that cause the shaking and destruction of ground-level buildings – travel along the planet’s surface.

Usually, the ratios between these waves are used to differentiate between an earthquake and an explosion.

“What the existing method that worked really well does is it looks at the amount of energy that’s being produced in the body waves versus the amount that’s coming in, in the surface waves,” says Mark Hoggard, an earth scientist at the Australian National University.

But as Hoggard points out, that method couldn’t detect a major nuclear test in 2017 when North Korea burrowed inside a mountain to detonate its nuclear weapon.

While technically beneath the Earth’s surface, blowing up a bomb inside a mountain effectively occurred at a higher level than many of the region’s surrounding valleys and depressions. As a result, the waves produced were at ratios more like an earthquake event –the usual surveillance systems couldn’t distinguish between the two.

Aerial photo of a north korean nuclear test site
Punggye-ni Nuclear Test Facility in North Korea. Credit: DigitalGlobe via Getty Images

To resolve this, Hoggard and his colleagues have turned to mathematics.

Collaborating with Janice Scealy, a statistician also at ANU, and Brent Delbridge, a geophysicist at the Los Alamos National Laboratory in the US, the group’s new method is designed to evaluate the distinct conditions of a weapons test against a natural earthquake.

They’ve achieved this by drawing on years of high-quality US nuclear testing and seismological data. Rather than wave ratios, they’re using moment tensors – calculations of earth deformation at the source of an event – to discriminate between atomic bomb and tectonic plate movement.

“They’re essentially a mathematical description of what’s happening at the very moment that thing goes off; what the displacements are around it,” Hoggard says.

Hoggard explains that an earthquake typically involves tectonic fault movements, where a nuclear detonation radiates energy from a single source point.

“We’ve exploited those differences to make this technique,” Hoggard says.

Once designed, the trio took data from the 6 most recent nuclear tests in North Korea and ran it through their new method.

“And sure enough, they come out as explosions.”

A man standing in front of trees
Dr Mark Hoggard. Credit: ANU, Supplied

Published today in Geophysical Journal International, Hoggard says he’s already been approached by several parties looking to adopt the new technique.

It’s also been made available as open-source data to parties looking to incorporate it within their surveillance methodology.

“Ideally, we would live in a world with no more nuclear weapons’ testing but unfortunately that doesn’t seem to be happening. There are treaties in place trying to achieve that, but so far not everyone’s agreed to them,” Hoggard says.

“So, I think having a healthy and well-funded monitoring programme, using as many different tools as you can, all these different techniques that are available, they’re absolutely key to kind of ensuring there’s accountability if you are a government conducting these things in secret.”

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