An quake measuring 5.4 on the Richter scale jolted South Korea on Wednesday.
While the US Geological Survey says it was centered off the coast about 9.3 kilometers northwest of Pohang, state-run Korea Meteorological Administration said the epicenter was inside Pohang. There has been no report of human casualties so far.
South Korea has relatively little seismic activity, unlike neighbouring Japan.
South Korean officials are assessing the situation.
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Before the main tremor, earthquakes of 2.2 and 2.6 magnitude struck areas near Pohang city.
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.
Photos and video footage sent to local TV stations showed crumbled street walls and furniture violently shaking inside people's homes and people rushing out of buildings in panic.
Pohang is some 35km from the Wolseong Nuclear Power Plant, which has six operational reactors.