It is thought to be the biggest such quake in the islands in nearly 90 years and lasted several seconds.
The Guernsey Met. Office says that the source of the tremor was 25 to 30 miles south of Guernsey and hit at 12.54pm.
It was initially estimated at a depth of between two and seven miles.
The magnitude was this afternoon confirmed as 4.2 with a depth of 7 miles and its location given as 49 09 N 2 25 W, which is approximately halfway between Les Roches Douvres Lighthouse and Corbiere Lighthouse in Jersey.
There were reports across both islands of a loud bang and some buildings shaking but no reports of any serious damage.
A full report will appear in tomorrow's Guernsey Press.