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

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.

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