A damaged building is seen in Pohang, South Korea, on November 15, 2017.
South Korean Yonhap News Agency reported a 5.4 magnitude natural disaster which struck close to the city of Pohang on the southeastern coastline around 2:29 pm local time.
There have been no reports of human casualties so far.
North Korea and South Korea are located in the region known as the Ring of Fire - which extends around the Pacific Ocean and is a hotspot for quake activity.
This would be the strongest quake in South Korea since the 5.8-magnitude one which occurred near Gyeongju in September 2016.
Before the main tremor, earthquakes of 2.2 and 2.6 magnitude struck areas near Pohang city.
Residents in the capital, more than 300 kilometers (186 miles) away, felt tremors and said their buildings shook. Six aftershocks followed for about 30 minutes ranging from magnitude 2.4 to 3.6.
Korea Hydro and Nuclear Power said nuclear power plants in the country are operating normally.
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. Moon convened an emergency meeting with his secretaries.