North Korea's nuclear test caused collapse, study says

North Korea's nuclear test in September last year was so strong that it caused an on-site collapse at the test site, geologists in China claim.

Using high-quality seismic data to pinpoint the location of several tremors that followed the test, the researchers determined that one event 8.5 minutes after the nuclear test was in fact the cavity caused by the blast collapsing.

The nuclear test, North Korea's sixth, was the country's most powerful to date.

The findings come from a research team at the China University of Science and Technology, whose work was accepted for publication by the academic journal Geophysical Research Letters.

The study's lead author, Wen Lianxing, warned that if North Korea were to use the same area for another test it could cause an "environmental catastrophe."

"North Korea's past tests have altered the tectonic stress in the region to the extent that previously inactive tectonic faults in the region have reached their state of critical failure. Any further disturbance from a future test could generate earthquakes that may be damaging by their own force or crack the nuclear test sites of the past or the present," Wen said in a statement.

The international community and Beijing in particular have long been concerned that an accident or radiation leak at the site could have consequences across the border.

Punggye-ri is fewer than 100 miles (160 kilometers) away from China, and residents living along the border said they felt the powerful tremor caused by September's nuclear test.

North Korean leader Kim Jong Un said last week that he was ordering the test site to be shuttered, declaring that North Korea no longer needed to test nuclear weapons. Though the move was viewed by some as a good will gesture ahead of his historic summit Friday with South Korean President Moon Jae-in, others said it showed a level of confidence in North Korea's nuclear program.

"They're basically talking the talk of advanced nuclear powers. Advanced nuclear weapons states, they don't have to conduct tests anymore because they've reached that level," said Duyeon Kim, a visiting senior fellow at the Korean Peninsula Future Forum and columnist at the Bulletin of the Atomic Scientists.

Details about the happenings at Punggye-ri are nearly possible to corroborate independently.

North Korea is closed off to most foreigners and rarely releases details about its secretive nuclear program, and never if they are negative.

But reports of significant issues at the Punggye-ri test site due to the sixth nuclear test have trickled out anyway.

Satellite imagery supplied to the Arms Control Wonk blog showed the explosion from the nuclear test visibly displaced the mountain.

And Japan's TV Asahi reported last year that a collapse at the tunnel site shortly after the test possibly killed more than 200 people, citing unnamed sources close to North Korea. CNN and other western media outlets were unable to confirm those reports.

The site that reportedly collapsed is just one part of the sprawling Punggye-ri complex that's believed to include a series of tunnels, some of which burrow below other mountains.

Citing commercial satellite imagery, the North Korea monitoring website 38 North observed late last year what it described as "significant tunneling operations" at a portal that has yet to be used.

The site also reported some recent unusual activity at the site, but concluded it was too early to say if it was related to current political developments.