Experts were monitoring Pyonggang during September after Kim Jong-un triggered an quake with one of his tests, Express reported. No casualties have been reported yet.
The quake was centered off the coast about 9.3 kilometers (5.8 miles) northwest of the port city of Pohang, according to the U.S. Geological Survey.
The residents of Pohang were told to evacuate, the agency said, adding that reports emerged about buildings shaking following the tremor.
An natural disaster measuring 5.4 on the Richter scale jolted South Korea on Wednesday.
A number of windows were broken and walls crumbled beneath structures close to the central point of the quake.
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The 5.4-magnitude quake struck at 2:29 pm (0529 GMT) north of Pohang, the Korea Meteorological Administration (KMA) said on it website.
"All reactors, including those in Gyeongju, are operating normally without power shutdown or decreased power output", the KHNP said in a statement.
Devin, like you said the 5.4 magnitude quake hit the northern region of Korea's coastal city of Pohang at around half-past-two this afternoon.
South Korea has relatively little seismic activity, compared with Japan to the east. There have been no reports of any damage to them yet. Six aftershocks followed for about 30 minutes ranging from magnitude 2.4 to 3.6.