South Korean Yonhap News Agency reported a 5.4 magnitude quake which struck close to the city of Pohang on the southeastern coastline around 2:29 pm local time.
It's the strongest quake in South Korea since a 5.8-magnitude occurred near the ancient city of Gyeongju, which is close to Pohang, in September 2016, Korea Meteorological Administration officials said.
The quake was felt across South Korea, including in the capital and on the southern island of Jeju, with reports there of buildings shaking and picture frames falling off walls, South Korean news agency Yonhap said.
The South Korean port city of Pohang is home to the headquarters of Posco - the country's top, and the world's fourth largest, steelmaker.
South Korea has relatively little seismic activity, compared with Japan to the east. No immediate damage was reported in the firm's steel mills.
Pohang is some 35km from the Wolseong Nuclear Power Plant, which has six operational reactors.
Buildings in Pohang, North Gyeongsang Province, are seen after an quake of magnitude 5.5 hit the area.
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