A damaged building is seen in Pohang, South Korea, on November 15, 2017.
The 5.5 magnitude rocked the coast near Pohang in South Korea.
No casualties have been reported and it is still unclear whether a tsunami warning will be issued.
Nuclear reactors in the region operated normally when the nation's strongest-ever quake at magnitude 5.8 hit Gyeongju in September 2016. This is the second strongest quake to have ever been recorded in Korea, after a 5.8 magnitude natural disaster hit Gyeongju, south of Pohang, last year.
The weather service revised the magnitude from an initial 5.5 to 5.4 after a rigorous analysis.
Before the main tremor, earthquakes of 2.2 and 2.6 magnitude struck areas near Pohang city.
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Around 2:50pm (local time) a third aftershock with a magnitude of 3.6 was detected about 7 kilometers north of Pohang. Electric lamp hanging from the ceiling was shaken, with books falling from a bookshelf. Students were evacuated from school buildings and people stood on the streets after dashing out of office buildings and apartments.
Nuclear power plants were being normally operated, according to the Korea Hydro & Nuclear Power, an operator of the country's nuclear reactors.
Residents in the capital, more than 300 kilometers (186 miles) away, felt tremors and said their buildings shook.
South Korean President Moon Jae-In, on his way to Seoul after attending the Asean summit in Manila, is set to hold an emergency meeting upon arrival, his office said.
Following the quake, Paik Un-gyu, minister of trade, industry and energy, convened an emergency meeting via conference call to check major energy facilities, including power plants, pipelines and petroleum reserves, and ordered energy firm chiefs to take necessary measures if needed.