An unusually strong tremor struck deep below the seabed east of North Korea early Thursday, with South Korean and Japanese earthquake observers recording its magnitude at 6.3.
While the U.S. Geological Survey placed its strength at 5.8, the agency's assessment still made it the strongest quake in the Sea of Japan off the North in more than two decades.
Given how rare major tremors are on the Korean Peninsula, earthquakes in and around the North are often viewed with suspicion because the reclusive state's past nuclear tests were first seen as seismic events.
But such a provocation was unlikely in this case, given the quake's depth and epicenter 192 kilometers (119 miles) southeast of Chongjin -- which also reduces the chance of any damage within North Korea according to experts.
"It occurred at 500 kilometers (310 miles) below the seabed. There's no way it was caused by a nuclear test. It's a natural earthquake," John Bellini, USGS National Earthquake Information Center geophysicist, was quoted by local news agency Yonhap.
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.
By Alex Jensen in Seoul