There have been no reports of human casualties so far.
The 5.5 magnitude rocked the coast near Pohang in South Korea.
The seismic intensity was the second biggest in the country's history after the biggest ever quake of 5.8-magnitude hit the Gyeongju city, just south of the Pohang city, in September past year.
The outer wall of an elementary school in Pohang has fallen after a magnitude 5.5 natural disaster hit the southeastern city on November 15, 2017.
The quake occurred at around 2:29 p.m. some 9 kilometers north of Pohang, the Korea Meteorological Administration said.
Tremors were felt across the country but operations at nuclear reactors were not affected, the state-run nuclear operator Korea Hydro & Nuclear Power Co said in a statement.
Before the main tremor, earthquakes of 2.2 and 2.6 magnitude struck areas near Pohang city. A third aftershock with a 3.6 magnitude was recorded later, the weather agency said. The quake was felt across the nation, including Seoul about 370 km away from the port city.
South Korean media showed crumbled walls piled on parked cars, broken windows from some buildings and elementary school students taking shelter on a playground.
South Korean officials are assessing the situation.