North Korea carries out its nuclear tests in a complex of tunnels at its Punggye-ri site and images of the mountains, in this case Mount Mantap, above it can give experts a sense of where the device was tested exactly and how powerful it was.
The new Synthetic Aperture Radar satellite images, captured before and after Sept. 3, showed "significant changes at Mount Mantap's peak elevation. Prior to the test, Mount Mantap was 2,205 meters high; the mountain has since diminished in height," wrote Jeffrey Lewis, head of the East Asia program at the James Martin Center for Nonproliferation Studies in California.
"You can see that the explosion visibly displaces the mountain, which demonstrates both how large the explosion was but also that it occurred in the same tunnel complex as the preceding four nuclear tests," Lewis wrote on the Arms Control Wonk website. "This is useful because the relationship between the size of the explosion and the magnitude of the seismic signals is sensitive to the overburden - how much rock is above the explosion."
The images were taken by Airbus, a space technology company that makes earth observation satellites, using its TerraSAR-X satellite, and provided to experts at the center.
The device, which North Korea described as a hydrogen bomb capable of being placed on a ballistic missile, was the most powerful tested to date. Original estimates had put its yield in the 100 kiloton range, but updated seismic data analyzed by experts this week put it closer to a whopping 250 kilotons, or nearly 17 times more powerful than the bomb that flattened Hiroshima.
The new images are "additional proof that the September 2017 explosion was much larger than ever before at this site," said Melissa Hanham, senior research associate at the Center for Nonproliferation. In comparison, radar images of last year's nuclear test did not show a noticeable change in the surface area of the same mountain, she said.
The sunken area corresponds with some of the highest peaks of Mount Mantap, Hanham said.
"It makes sense that they would use their existing tunnel network attached to the North Portal entrance, because this leads to where the overburden is the greatest," Hanham said. "If they used a tunnel with less overburden, they might have blown the top off the mountain."
The growing threat from the north has led to more South Koreans calling for their own nuclear weapons. A Gallup Korea poll conducted after the Sept. 3 test found 60 percent of respondents supported nuclear weapons for the south.
But in an interview with CNN on Thursday, South Korean President Moon Jae-in ruled out the idea: "To respond to North Korea by having our own nuclear weapons will not maintain peace on the Korean Peninsula and could lead to a nuclear arms race in northeast Asia."
In response to the Sept. 3 nuclear test, the United Nations on Monday unanimously agreed on its toughest sanctions against North Korea to date, setting limits on its oil imports and banning its textile exports. North Korea condemned the sanctions, and warned that the United States would "suffer the greatest pain" it has ever experienced for leading the effort to ratchet up economic pressures on the reclusive nation.
On Thursday, North Korea issued another threat, this time targeting both Japan and the U.S. In a statement issued by North Korea's official news agency, Pyongyang said it would use nuclear weapons to "sink" Japan and "reduce the U.S. mainland to ashes and darkness."
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