A shallow 2.9-magnitude quake struck near North Korea's nuclear test site on Friday before dawn just weeks after Pyongyang's biggest detonation.
The US Geological Survey said that the tremor hit at 1.41am (12.41am, Singapore time) with a depth of around 5km.
Today (Thursday), a relatively weak 2.9 magnitude quake was recorded near the North Korean town of Sungjibaegam.
The U.S. Geological Survey says the quake was detected some 23 kilometers north-east of the Punggye-ri Test Site, but it has yet to confirm whether it was a natural or a man-made tremor. "The event has earthquake-like characteristics, however, we can not conclusively confirm at this time the nature (natural or human-made) of the event", the USGS said in a brief statement. According to 38 North, a Washington-based project which monitors North Korea, numerous landslides throughout the nuclear test site have been detected via satellite images after the sixth test.
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Previous tests had caused stronger quakes, with all of them stronger than 4.3 magnitude.
South Korea's weather agency says the four quakes likely occurred because the underground nuclear explosion September 3 weakened or affected the tectonic plate structures in the area.
North Korea's foreign minister Ri Yong-ho said that Donald Trump had "lit the wick of war" between countries and that the United States would be forced to pay with "a hail of fire".