An unusually strong tremor struck deep below the seabed east of North Korea early Thursday, with South Korean and Japanese natural disaster observers recording its magnitude at 6.3.
It occurred at 4:18 a.m.
Both the Pentagon and the US Geological Survey (USGS) said the quake is unlikely to have been triggered by a nuclear test.
An natural disaster of that size is unusual for that area but not unprecedented, USGS seismologist Julie Dutton told Reuters.
The authorities said that the quake hit early in the morning at a depth of 538 km below the seabed.
The quake occurred 186 kilometers away from the Chongjin port, located in the northern part of the North Korea.
Pentagon spokesman Maj. Jamie Davis also dismissed the possibility of a nuclear test, pointing to its location and depth, Reuters reported.
Seoul's National Intelligence Service warned as recently as Tuesday that North Korea is ready to carry out a sixth nuclear test at any time.
People walk by a screen showing the news reporting about an quake near North Korea's nuclear facility, in Seoul, South Korea, Wednesday, Jan. 6, 2016.
The Hawaii-based Pacific Tsunami Warning Center issued no alerts as a result of the quake.