Experts were monitoring Pyonggang during September after Kim Jong-un triggered an natural disaster with one of his tests, Express reported.
Tremors were felt in the South Korean capital Seoul, around 270km from Pohang, Yonhap added.
The initial quake was recorded at a depth of 10km, USGS said.
North Korea and South Korea are located in the region known as the Ring of Fire - which extends around the Pacific Ocean and is a hotspot for natural disaster activity. No casualties have been reported yet.
The agency sent out an natural disaster alert via a mobile network which asked some of the local residents in the affected area to evacuate immediately.
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Residents in Pohang city just 5.6 miles from the epicentre of the quake were told to evacuate.
South Korean media showed crumbled walls piled on parked cars, broken windows from some buildings and elementary school students taking shelter on a playground. There have been no reports of any damage to them yet.
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.
Local nuclear reactors were operating without disruption, Yonhap news agency said, citing officials at Korea Hydro and Nuclear Power.