North Korea made a show of shutting down one of the world’s most closely watched places Thursday as it blew up the tunnels leading to its nuclear test site—but not before the mountains gave up some important clues to the progress of the North’s weapons program.
Scientists poring over satellite and seismograph data from the site found the country’s last bomb test, in September 2017, was powerful enough that it caused the test site at Mount Mantap to bulge outward and flatten slightly. They found the blast left the mountain about 3.5 meters (11 feet) wider and half a meter shorter than it was before, said Teng Wang, a geodesist at the Earth Observatory of Singapore.
That suggests the blast was likely a bit over 200 kilotons, or about 15 times the force of the bomb the United States dropped on Hiroshima in 1945, Wang and his colleagues reported in a paper published this month in the research journal Science.
The 2,200-meter Mount Mantap is one of three peaks at the Punggye-ri test site, about 230 miles (375 km) northeast of Pyongyang. The North set off six blasts in the area between 2006 and 2017, becoming the world’s eighth declared nuclear power.
Wang, whose specialty is geographic measurements, and a team from Germany, China, and the United States used data from Germany’s TerraSAR-X satellite to gauge the Sept. 3 test. The satellite is equipped with a radar system sensitive enough to capture shifts as small as a few centimeters, and it recorded the change in the mountain’s contours after the explosion, which showed up on seismometers as a magnitude-6.3 earthquake.
Combining the data from those instruments allowed the team to estimate the size and depth of the detonation—and the technique can be used again if the North or another country tests another bomb, Wang said.
“As long as it can produce surface displacement, we can know it from this kind of technology married to the imagery,” he said. “There are more and more and more satellites available, and this German satellite has very high resolution that allows us to verify, with high accuracy, the displacement.”
The size of the blast supports North Korea’s claim that the device was its first hydrogen bomb, said Jeffrey Lewis, the head of the East Asia Nonproliferation Program at California’s Middlebury Institute of International Studies and publisher of the Arms Control Wonk blog. Seismic events recorded soon after the Sept. 3 test led to reports of a cave-in in one of the tunnels at the test site, but Lewis said studies afterward point to the cavity gouged out by the blast collapsing on itself.
“That doesn’t mean that the tunnels leading to it collapsed, and it doesn’t mean the tunnels in the other mountains collapsed,” he said.
North Korea announced in April that it didn’t need to conduct any more tests and that it was willing to discuss denuclearization at a planned summit between its leader, Kim Jong Un, and U.S. President Donald Trump. Trump called off that summit Thursday amid increasing acrimony between Washington and Pyongyang, which have faced off across the tense border between North and South Korea since the truce that ended the 1950-53 Korean War.
Before Trump’s announcement, however, the North Koreans set off a series of conventional explosions it said would close off the tunnels at the test site. It invited journalists, but not nuclear experts, to observe the process: North Korea kicked out inspectors from the International Atomic Energy Agency at the end of 2002.
“This is not something where they want to give up sensitive national security information,” Lewis said. “They just want to detonate some tunnels and put on a show. This is the Cirque du Soleil of nuclear disarmament.”
Lewis said further radar images from satellite instruments like the one Wang’s team used would provide a clearer picture should provide a clearer picture, “and we should see all the explosions in the seismic data, too.”