No injuries or major damage has been reported, according to the Korea Meteorological Administration.
It is unclear whether a tsunami warning will be triggered.
South Korean tech giants said Wednesday no damage has been reported at production plants following the magnitude 5.4 natural disaster which struck the southeastern city of Pohang earlier in the day.
The epicenters of Korea's two strongest earthquakes are only 43 kilometers apart, and the KMA says it's analyzing their correlation.
South Korea has relatively little seismic activity, unlike neighbouring Japan.
Photos and video footage sent to local TV stations showed crumbled street walls and furniture violently shaking inside people's homes and people rushing out of buildings in panic.
Shattered storefronts and goods tumbling off store shelves were seen in images posted on social media, as well as cars smashed by fallen bricks and cracks in the ground.
South Korean officials are assessing the situation. An aftershock of 3.6 followed.
"All reactors, including those in Gyeongju, are operating normally without power shutdown or decreased power output", the KHNP said in a statement.
This is the second strongest quake to have ever been recorded in Korea, after a 5.8 magnitude natural disaster hit Gyeongju, south of Pohang, last year.