The epicentre of the quake on Sunday morning was near the coastal city of Pohang, some 170km southeast of Pyeongchang where the Winter Olympics are currently taking place.
There were no reports of casualties or damage.
The Korea Meteorological Administration said the quake was an aftershock from a magnitude 5.4 earthquake that struck Pohang in November, the Yonhap news agency reported.
Mobile phone users in the Olympic village with an appropriate app received an emergency alert in Korean.
