It's the strongest quake in South Korea since a 5.8-magnitude occurred near the ancient city of Gyeongju, which is close to Pohang, in September 2016, Korea Meteorological Administration officials said.
A 5.4-magnitude quake struck off South Korea's southeastern coast Wednesday afternoon, but no casualties were immediately reported.
Seven people were injured, Seoul's public administration ministry said, but they warned that number could rise because the quake was shallow.
The agency is looking into the possibility that the latest quake may have been caused by the activities of an eastern branch of Yangsan Fault, the most prominent fault plane in the southeastern part of the Korean Peninsula.
Analysts were monitoring "unusual seismic activity" in North Korea, the head of the nuclear test monitoring agency CTBTO said.
No immediate damage was reported in the firm's steel mills.
Magnitude 5.4 earthquake strikes city of Pohang + 'Dispatch' photos of earthquake aftermath
The university entrance exam is taken very seriously.
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.
South Korean media showed crumbled walls piled on parked cars, broken windows from some buildings and elementary school students taking shelter on a playground.
Wednesday's quake was followed by multiple aftershocks including a 4.3-magnitude tremor that hit about two hours later.
"So far, there have been no reports of casualties or damage to the nuclear power plants south of Pohang, " the Korea Times reported, citing local utilities.
Hate Crimes Rose 5 Percent in 2016, FBI Says
Hate incidents increased from 203 in 2015 to 285 in 2016. "Hate crimes demand priority attention due to their special impact". Anti-Jewish hate crimes, anti-Islamic hate crimes, and anti-Catholic hate crimes all saw various increases in 2016.