Experts were monitoring Pyonggang during September after Kim Jong-un triggered an quake with one of his tests, Express reported.
There have been no reports of human casualties so far.
The tremors were felt all the way in the South Korean capital Seoul some 170 miles away.
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South Korea's top steel maker POSCO, based in Pohang, said it experienced no damage from the quake so far.
The 5.4-magnitude quake struck at 2:29 pm (0529 GMT) north of Pohang, the Korea Meteorological Administration (KMA) said on it website.
The agency sent out an quake alert via a mobile network which asked some of the local residents in the affected area to evacuate immediately.
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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.
According to TV footage, some buildings were externally damaged and windows were broken.
South Korea has relatively little seismic activity, unlike neighboring Japan.
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Pohang is some 35km from the Wolseong Nuclear Power Plant, which has six operational reactors. A third aftershock with a 3.6 magnitude was recorded later, the weather agency said.