A series of quakes as big as magnitude 5.4 hit the southeastern port city of Pohang on Wednesday afternoon.
According to the Korea Meteorological Administration, an aftershock of 5.4 was witnessed minutes later.
Tremors were felt across the nation including Seoul and Busan, with some residents in the affected areas reporting damage.
The most powerful quake recorded in the south was a 5.8-magnitude tremor that hit the southeastern city of Gyeongju in September past year.
The quake was sensed across the country, including capital Seoul, some 270 km away from Pohang city, as well as the southern resort island of Jeju.
There have been no reports of human casualties so far.
A rare natural disaster of magnitude 5.4 rattled South Korea on Wednesday, its second most powerful quake on record, shaking buildings but causing no major damage or injuries.