Initial reports on state media say three people have died.
Posts on social media in the region said people felt the quake across the Gulf in Dubai, Abu Dhabi and Bahrain.
Iran’s nuclear programme has roused concern among major powers that Tehran wants to build nuclear weapons – a charge Iran strongly denies.
The USGS said the quake struck at 11:52 GMT at a depth of 10km (6.2 miles), some 90km from Bushehr city, near to the power plant.
Iran’s seismological centre in Bushehr province, linked to Tehran University, registered a 6.1 magnitude quake close to the town of Kaki, some 60 miles south of Bushehr.