On November 15, a magnitude 5.4 quake struck the city of Pohang, a southeastern coastal region of South Korea.
The quake was reported about 6 kilometers north of Pohang at 2:29 p.m. Wednesday, according to the Korea Meteorological Administration.
Residents in Seoul, which is more than 300 kilometers away from the epicentre, felt tremors, and said their buildings shook.
South Korea has relatively little seismic activity, unlike neighbouring Japan.
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. There have been no reports of any damage to them yet.
China sending envoy to North Korea
In a brief dispatch, the official Xinhua news agency said Song Tao, who heads the ruling Communist Party's external affairs department, would leave for North Korea on Friday.
Reports of shaking buildings and toppling furniture flooded in from Seoul to Jeju Island.
Images posted on social media showed shattered storefronts, goods tumbled off store shelves, cars smashed by fallen bricks and cracks on the ground.
Pohang is some 35km from the Wolseong Nuclear Power Plant, which has six operational reactors.
The quake shook South Korea's southeastern coast of Pohang.