N Korea's 2017 bomb test set off quakes
The North Korean underground nuclear bomb test last year set off a series of aftershocks over a period of eight months, a study has found.
The shocks, which occurred on a previously unmapped nearby fault, are a window into both the physics of nuclear explosions, and how natural earthquakes can be triggered, according to researchers from Columbia University in the US.
The underground test — which took place on September 3 last year —was North Korea's sixth, and by far largest yet, yielding some 250 kilotonnes, or about 17 times the size of the bomb that destroyed Hiroshima.
Many experts believe the device was a hydrogen bomb — if true, a significant advance from cruder atomic devices the regime previously exploded. The explosion itself produced a magnitude 6.3 earthquake.
This was followed 8.5 minutes later by a magnitude 4 quake, apparently created when an area above the test site on the country's Mt Mantap collapsed into an underground cavity occupied by the bomb.
The test and collapse were picked up by seismometers around the world and widely reported at the time.
Later, seismic stations run by China, South Korea and the US picked up 10 smaller shocks, all apparently scattered within 5 or 10 kilometres around the test site.
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