Earlier, a 5.1-magnitude tremor struck the outskirts of Gyeongju city in Gyeongsang province at 7.44 p.m., with its epicentre located about 9 km southwest of the city.
The second tremor with a magnitude of 5.8 struck the nearby region, just 1.4 km away from the first epicentre, at 8.32 p.m., Xinhua news agency reported.
It was the country's biggest-ever quake, surpassing the previous strongest tremor that happened in January 1980.
The tremor was felt nationwide from adjacent cities of Ulsan and Busan to as far north as capital Seoul.
There has been no damage reported yet, the Ministry of Public Safety and Security was quoted as saying.
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