A rare 5.4-magnitude natural disaster hit South Korea's southeast Wednesday afternoon, the second most powerful quake on record, in a country that seldom experiences significant tremors.
But seismic activity is closely monitored because a spike in activity is often the first indication that North Korea has carried out a nuclear test.
No casualties have been reported and it is still unclear whether a tsunami warning will be issued.
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.
The quake was felt across South Korea, including in the capital and on the southern island of Jeju, with reports there of buildings shaking and picture frames falling off walls, South Korean news agency Yonhap said.
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The Gyeongju National Museum said the quake was measured to be magnitude 4.4 inside the museum, adding that there was no damage done to the artifacts.
South Korean officials are assessing the situation.
The nuclear power plant Singori 3 in Ulsan is believed to remain unaffected by the quake.
According to the Korea Meteorological Administration, an aftershock of 5.4 was witnessed minutes later.