The British Geological Survey said the tremor had a 4.4 magnitude, with a depth of 7.4km, and the epicentre was about 20km north of Swansea.
The quake happened just after 2.30pm on Saturday.
A spokesperson told Sky News it was “reasonably-sized” and that the effects had been felt as far afield as Devon and Birmingham.
“This is the largest earthquake in mainland UK since the 5.2 magnitude Market Rasen earthquake in 2008,” he said.
Twitter users described their surprise as the aftershock was felt over a hundred miles away.
Tina Anton wrote on Twitter: “Just had a tremor as in earthquake in Cornwall! I lived in Greece and experienced loads there but never expected one in the southwest of England.”
Tim Johnson also posted: “Anyone else just feel an earthquake then? The whole Hilton Liverpool building just shook violently.”
This is the biggest event in the area since the 5.2 magnitude earthquake in 1906. pic.twitter.com/zq9JEIttQI
— BGS (@BritGeoSurvey) February 17, 2018
Jason Mohammad said: “Anyone else? Felt it in Salford #earthquake”
People closer-by described their shock when the tremor first hit.
“We just felt the earthquake, felt like a huge lorry going passed and shaking the house!” Hannah Jenkins posted on Twitter.
Other people described it feeling “like a piledriver hit the wall” and “sitting on a washing machine going full pelt”.
James Leonard wrote on Twitter: “Wow was that an earthquake in Swansea my house just shook.”
Mark Wyatt, a third year student at Cardiff University, was entering his house in the city centre when he heard a “weird, deep rumbling sound”.
The 21-year-old said: “We heard the noises before we felt the rumble, and then the house shook for a couple of seconds before it stopped.”
Sky’s Kay Burley felt the effects of the tremor near a lake in the Cotswolds: “The dogs were laying down then suddenly they jumped up and ran to my side, and then I noticed quite a significant ripple on the water. Then the shades on the table-lamp started to rattle.
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“It lasted for a few seconds really, nothing more than that. The dogs are not happy at all though,” she said.
South West Police called it a “minor” tremor and said there was no need for people to contact them “unless you have something to report” – such as injuries or damages.