North Korea says it has conducted a successful hydrogen bomb test, a surprise announcement.
A South Korean weather agency saw signs of an "artificial quake" near a North Korean nuke test site.
The European Mediterranean Seismological Center detected unusual seismic activity in North Korea.
The agency measured the magnitude of the seismic activity at 5.1 on its website on Wednesday.
— Anderson Cooper 360° (@AC360) January 6, 2016
South Korean government officials didn't immediately respond to calls looking to determine whether a nuclear blast or natural earthquake had taken place in the North.
North Korea conducted its third test in February 2013.