The quake, the second most powerful recorded in South Korea, struck at the shallow depth of nine kilometres (six miles) near the industrial city of Pohang at around 2:30 pm (0530 GMT), the Korea Meteorological Administration said.
Seven people were injured, Seoul’s public administration ministry said, but they warned that number could rise because the quake was shallow.
The Korean peninsula is rarely troubled by significant quakes.
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.
Wednesday’s quake was felt across the country, including in the capital Seoul.
It came a day ahead of the all-important college entrance exam during which the whole nation falls silent to help teenagers focus on the annual event, whose result could define their future in the hyper-competitive society.
The Ministry of Education announced late Wednesday that the exam would be postponed by a week because damage had been caused to some schools, Yonhap news agency reported, adding it was the first time a natural disaster had forced such a reschedule of the crucial exams.
South Koreans take significant steps to ensure their young students are not distracted during the annual exams.
Seoul’s financial markets and many businesses open late in the day to clear traffic for test-takers, while airport landings and takeoffs are suspended for 30 minutes during the main language listening test.
Photos and video footage sent to local TV stations showed crumbled street walls, furniture violently shaking inside people’s homes and people rushing out of buildings in panic.
Shattered storefronts and goods tumbling off store shelves were seen in images posted on social media, as well as cars smashed by fallen bricks and cracks in the ground.
Emergency centres nationwide were flooded with thousands of calls seeking information, while Kakao Talk — the South’s top mobile messenger application — reported service disruption due to heavy traffic.
Pohang, the main city closest to the epicentre, is home to the headquarters of Posco, the world’s fourth largest steelmaker. No immediate damage was reported in the firm’s steel mills.
Wednesday’s quake was followed by multiple aftershocks including a 4.3-magnitude tremor that hit about two hours later.
Local nuclear reactors were operating without disruption, Yonhap news agency said, citing officials at Korea Hydro and Nuclear Power.
The most powerful quake recorded in the South was a 5.8-magnitude tremor that hit the southeastern city of Gyeongju in September last year.