A magnitude 5.5 natural disaster has struck off South Korea's southeastern coast, about 6km (4 miles) north of the southeastern port city of Pohang, Yonhap news agency reported.
As many fear more quakes that may take place on the crucial day, Prime Minister Lee Nak-Yeon ordered officials to deploy "all government resources possibly available" to ensure that Thursday's test is held smoothly. Witnesses stated that the natural disaster could be felt in Seoul, more than 300 kilometers away, including at Gwanghwamun.
The Korea Meteorological Administration reported a second tremor, with a magnitude of 5.4, about nine kilometers from Pohang.
The Pohang natural disaster is the second most powerful tremor to occur in South Korea following a magnitude five-point-eight quake that struck in Gyeongju, North Gyeongsang Province, on September 12th previous year.
There were no immediate reports of any casualties or injuries.
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.
Around 2:50pm (local time) a third aftershock with a magnitude of 3.6 was detected about 7 kilometers north of Pohang.
South Korea was hit by a major 5.4 magnitude quake earlier today. We'll have the latest on that and more in our later newscast.