The Hunt for Secret Nuclear Tests Digs Up Scientific Gold
When North Korea detonated the bomb that was probably not a hydrogen bomb on Wednesday morning local time, the blast tripped dozens of instruments belonging to the Preparatory Commision for the Comprehensive-Test-Ban Treaty Organization. The Vienna-based organization has a vast global monitoring system to confirm secret (or in North Korea’s case, not-so-secret) nuclear tests.
But the CTBTO is in an odd position these days. It exists to detect and thus deter tests of new nuclear weapons, but its eponymous treaty hasn’t been ratified by key nuclear powers like, well, the United States (that’s why it’s a Preparatory Commission). India hasn’t signed it. Pakistan hasn’t signed it. And, obviously, neither has North Korea. Absent power to enforce the treaty, the commission has to prove its worth in other ways. So it’s courting scientists.
It works because the organization’s 280 monitoring stations around the world collect a lot of data—seismic, infrasound, hydroacoustic, radionuclide, and so on. That dataset, 16 GB a day, is a goldmine for researchers in fields as disparate as seismology and whale biology. “Basically we want to use the scientific framework to convince diplomats this is a solid organization with the technological capacity to go beyond the day-to-day monitoring of potential nuclear explosions,” says executive secretary Lassina Zerbo. This year, the commission held its fifth science and technology conference in Vienna, drawing over 1,000 scientists from 104 countries.
“Nothing at all is comparable,” says Margaret Campbell-Brown, a physicist at the University of Western Ontario. She uses the network’s infrasound arrays to study meteors that whiz through the Earth’s atmosphere. The magnitude of the meteors’ blast waves let scientists calculate the falling rock’s energy. When the Chelyabinsk meteor streaked across the Russian sky in 2013, Campbell-Brown’s colleague was able to start downloading the sensors’ infrasound data immediately—except, in some cases, he had to wait for the blast wave to reach a listening station halfway around the world. “The speed of sound is more of a limit than getting data off the network,” she says.
That openness is unusual in the world of nuclear policy. At first, Zerbo says, it wasn’t easy to convince the commission’s members to share their data. But the massive 2004 South Asian tsunami broke it all open. After the tsunami caught Indian Ocean countries by surprise, the commission began sharing its seismic data for a tsunami warning system. With the data-sharing infrastructure in place, scientists have since used it to map whale migrations, track nuclear fallout from Fukushima, and detect submarine and plane accidents.
Today, one of the network’s biggest applications is seismology. While other global seismic networks exist (the United States Geological Survey is a notable example), the CTBTO is unique in having more than one detector at every station. To detect how waves bounce inside the Earth and in some cases travel all the way through it, you need a whole array of detectors to pick up the very weak signals, says Miaki Ishii, a seismologist at Harvard. With this data, she’s able to take an “X-ray” of the planet: The waves bounce when they hit different layers, and they travel at different speeds in areas where it’s hotter or cooler.
For a wave to travel so far and still be detectable, the original source of the wave needs to be powerful: usually an earthquake or a big explosion. In fact, explosions are better. “Earthquakes tend to last for some time,” says Ishii, “whereas an explosion is more or less one time, so the waveform is a fairly simple spike.” Data from old Russian nuclear tests, she says, were the basis of early studies of the Earth’s interior.
The specific mystery Ishii wants to solve with nuclear network’s seismic data is what happens at the boundary between the Earth’s solid inner core and liquid outer core. The solid inner core is growing, but no one knows exactly how fast. “Eventually, after a very long time, the core will be completely solid and then we’ll lose Earth’s magnetic field,” she says. Which, you know, is an apocalypse humans will only ever need to worry about if we don’t destroy the planet with nuclear weapons first.
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