North Korea Nuclear Weapons Test Moved a Mountain, Letting Scientists Track Depth of Site

On September 3, 2017, the world shuddered, figuratively and literally. That day saw North Korea's sixth and largest nuclear test to date. Even as analysts rushed to calculate what the test meant for global security, scientists began gathering what information they could to determine what precisely had happened.

One set of results from that effort has just been published in the journal Science. The team behind the study used two types of geological data to calculate the detonation's location—crucial information for figuring out just how powerful the explosion was.

"I have never seen such large displacement caused by human activity via SAR [Synthetic Aperture Radar] imagery before," first author Teng Wang, a remote sensing specialist at the Nanyang Technological University in Singapore, wrote in an email to Newsweek.

That radar data is gathered by satellites circling Earth, bouncing beams of microwave light down to the surface and measuring the amount of time it takes them to return. Scientists can use the data produced by different passes of those satellites to form a picture of how the surface of the Earth changed in between those two times.

"It's like a snapshot with a few days difference," Meng Wei, a geoscientist at the University of Rhode Island, told Newsweek. Wei wasn't involved with this study, but he attempted to do similar analysis of a previous North Korean test, held in January 2016. "Whatever deformation we saw in the image happens in these few days. But whether it's an hour, a few seconds, one day, we don't know."

In the case of the September 3 nuclear test, that meant looking at changes to the mountain under which the detonation was held, called Mt. Mantap. All of North Korea's nuclear tests to date have been conducted deep inside this mountain to reduce the probability of radioactive materials leaking out to pollute the air or water around a detonation.

But six nuclear tests had rattled the mountain. In the case of the last, most powerful test, the radar data showed an incredible amount of movement from before the test to after it: land sank as much as about a foot and a half and was pushed as much as 11 feet sideways.

North Korea held a celebration in September to mark its sixth nuclear test. Six nuclear tests had rattled Mt. Mantap. North Korea's Korean Central News Agency via Reuters

That's quite a lot of movement. "The size of the event is exciting," Wei said. "This event is the largest one so far conducted by North Korea, and it's created meters of deformation, which we've never seen before at least for the past 20 years or so."

By pairing the radar data with information gathered by seismometers about movements in the Earth triggered by the event, Wang and his colleagues could piece together an estimate of where the test took place. They calculated both the latitude and longitude of the detonation and its depth, about 1,500 feet below the mountain's tip.

A location isn't just nice to have; it affects calculations of the single most important characteristic of a detonation, its yield, or the energy produced. But without a fairly accurate estimate of the detonation's location, it's impossible to calculate the yield. Wang and his colleagues estimate the detonation produced energy equivalent to the explosion of between 120,000 and 300,000 tons of dynamite.

And even more important than producing a yield for this test, the work done in this paper should help scientists revisit other past nuclear tests and learn more about what happened.