A 5.5 magnitude natural disaster struck the southeastern city of Pohang in North Gyeongsang Province on Wednesday, the state weather agency said.
According to the Korea Meteorological Administration, an aftershock of 5.4 was witnessed minutes later.
The tremors were felt all the way in the South Korean capital Seoul some 170 miles away. There has been no report of human casualties so far.
Residents in Pohang city just 5.6 miles from the epicentre of the quake were told to evacuate.More news: United Nations warns of sex trafficking, abuse among Rohingya refugees
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No injuries or major damage has been reported, according to the Korea Meteorological Administration.
But seismic activity is closely monitored because a spike in activity is often the first indication that North Korea has carried out a nuclear test.
The Pohang quake is the strongest Korea has experienced since 2016, after the magnitude 5.8 natural disaster in Gyeongju.
Local nuclear reactors were operating without disruption, Yonhap news agency said, citing officials at Korea Hydro and Nuclear Power.