AP Photo/Lee Jin-man
During the last nuclear test in January 2016, people walk by a screen showing the news reporting about an earthquake near North Korea's nuclear facility, in Seoul, South Korea.
The quake occurred at the surface about 11 miles northeast of Sungjibaegam, North Korea, near the country's nuclear testing site.
Naturally occurring earthquakes typically occur beneath the surface. That's why this one is likely an artificial earthquake triggered by an explosion.
"Possible explosion, located near the location where North Korea has detonated nuclear explosions in the past," the USGS wrote on the incident page. "If this is indeed an explosion, the USGS National Earthquake Information Center cannot determine what type of explosion it may be, whether nuclear or any other possible type."
Last January, North Korea detonated what it called a 'miniature hydrogen bomb' (though experts doubt it was that large) in the same area. That test triggered what looked like a 5.1 magnitude earthquake.
Officials are still trying to confirm if this earthquake was indeed another nuclear bomb. If it is, it would be North Korea's fifth nuclear test.