The US Geological Survey has noted seismic activity—a magnitude 5.1 earthquake—near a known North Korean nuclear test center. The last time this happened, it was thanks to the underground detonation of a nuclear device.
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North Korea has yet to issue a statement on the event, so for the time being, the USGS report is all there is to go on: a magnitude 5.1 earthquake was logged 12 miles northeast of Sungjibaegam at about 8:30 p.m. ET.
In 2013, the USGS recorded another 5.1 earthquake in the same place. That turned out to be caused by an underground nuclear test, so it’s not much of a leap to assume the same trigger for today’s event.