The new 4.6 magnitude quake at a depth of zero kilometers could be due to a "collapse (cave in)" Chinese authorities said, without giving further details.
The first -- of 6.3 magnitude according to the US Geological Survey -- could be the North's sixth nuclear test, Seoul's military said, adding that it was detected near the North's Punggye-ri test site.
Japan said it had confirmed such a test.
USGS said the tremor was located 24 kilometers northeast of Sungjibaegam, in North Hamgyeong province. "It's an explosion rather than an earthquake," Jana Pursley, a USGS geophysicist, told AFP.
The second tremor was at the same location, according to latitude and longitude coordinates provided by the centre.
The apparent test came just hours after Pyongyang claimed to have developed a hydrogen bomb that could be loaded into the country's new intercontinental ballistic missile.
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