On Friday 14 December 2012, slightly after 11 pm GMT, the British Geological Survey recorded a Magnitude 3.0 Earthquake at a depth of 9 km beneath West Sussex, England, roughly 13 km north of Chichester. This is too small to have caused any damage or casualties, but is exceptionally large for southern England and was felt as far away as Hindhead in Surry and Hove in East Sussex.
The location of the 14 December Earthquake. Google Maps.
The precise cause of Earthquakes in southern England is unclear. Britain is being pushed to the east (along with the rest of Eurasia) by the expansion of the Atlantic Ocean and to the north by the impact of Africa into Europe from the south. There are also lesser areas of spreading beneath the North Sea, the Rhine Valley and the Bay of Biscay, all of which exert pressure on British rocks. However it is seldom possible to say that any one of these factors is the cause of a particular quake, and most are probably the result of a number of different influences.
Britain is also subject to quakes caused by glacial rebound; until about 10 000 years ago much of the north of the country was covered by a thick layer of snow and ice, which pushed the rocks of the lithosphere down into the underlying mantle. This ice is now gone, and the rocks are slowly springing back into place, causing the occasional Earthquake in the process. However this is more of an influence in the north and west of the country, where quakes are correspondingly more common, and it is unclear to what extent this effects Sussex or the southern English coast.
Chichester has been troubled by Earthquakes in the past; in 1833-4, which resulted in the loss of one life; one of only three Earthquake-related deaths in England since 1800.
Witness accounts of Earthquakes can help geologists to understand these events. If you felt this quake, or live in the area but did not (which is also useful information), then you can inform the BGS here.
See also Monmouth Beach, Lyme, closed due to danger of landslips, Woman killed by rockfall on Dorset Beach, An Earthquake off the coast of Margate, 8 April 2012, The Portsmouth Earthquake of 14 July 2011 and Earthquakes on Sciency Thoughts YouTube.
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