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Eight-and-a-half minutes after North Korea set off a nuclear bomb on September 3, a second burst of energy shook the mountain where the test had just occurred. More than a week later, researchers are still puzzling over what caused that extra release of seismic energy—and what it says about North Korea’s nuclear-testing site, or the risks of a larger radiation leak. Monitoring stations in South Korea have already picked up minute levels of radiation from the test.

A number of theories have emerged to explain the second event, ranging from a tunnel collapse or a landslide to a splintering of the rock inside Mount Mantap, the testing site. But seismologists can’t agree and say that they may not get enough evidence to pin down the cause.

“This is an interesting mystery at this point,” says Göran Ekström, a seismologist at Columbia University in New York City.  

The nature of the first seismic signal is clearer because it matches the profile of a bomb blast. The US Geological Survey (USGS) determined the magnitude of the seismic event associated with the nuclear explosion at 6.3, whereas the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) in Vienna calculated it at 6.1 on the basis of a separate analysis. The explosion was many times the size of past North Korean tests and was the largest seismic signal from a nuclear test ever detected by the international network of seismic monitoring stations used by the CTBTO.

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