The underground blast on Sunday caused a 6.3-magnitude earthquake, according to the US Geological Survey, and was followed a few minutes later by another with a magnitude of 4.1, leading to suggestions that the rock over the site had caved in, potentially releasing radioactive material into the atmosphere.
Pyongyang said the test was of an H-bomb that could be fitted onto a missile, heightening tensions over its weapons ambitions and prompting global condemnation.
The respected 38 North website, which is linked to Johns Hopkins University in the US, published satellite images taken Monday showing changes in the surface at the Punggye-ri test site where the ground had been lifted into the air by the tremors, and small landslides going into stream beds.
"These disturbances are more numerous and widespread than what we have seen from any of the five tests North Korea previously conducted," it said.
"There does not appear to be any evidence of a collapse crater, as might have been suggested from the post-test tremor," it added.
South Korea's defence minister has estimated the yield from the explosion at 50 kilotons, while 38 North said it was currently believed to be 100 kilotons or more. Japan has put it at 120 kilotons, eight times the size of the US device that destroyed Hiroshima in 1945.
Seoul's nuclear safety agency said Wednesday it had not detected traces of radioactive materials, such as xenon gas, in soil, water and air samples following the blast.
Background radiation in South Korea appeared not to have been affected by the test, it added.
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