On Thursday 19 July 2012, slightly before 3.45 pm British Summertime (slightly before 2.45 pm GMT), the British Geological Survey recorded an Earthquake 9 km beneath Loch Indaal on the Isle of Islay, off the west coast of Scotland, measuring 1.7 on the Richter Scale. Roughly 33½ hours later, slightly before 0.15 am British Summertime on Saturday 21 July (slightly before 12.45 pm on 20 July, GMT), a second quake was recorded beneath the Loch, this time at a depth of 6 km and measuring 1.3 on the Richter Scale.
The location of the July 2012 tremors. BGS.
Earthquakes this small are highly unlikely to cause any damage or injuries, and often people will fail to notice them at all, or not realize that they are Earthquakes, though on this occasion both were reportedly felt by local people. It is possible the inhabitants of Islay are more likely to notice such events than people in other parts of the country because they have more of them; as a rough rule of thumb the further north and west you go in the UK, the more Earthquakes you will encounter, and the west coast of Scotland is generally the most quake-prone part of the country.
The exact cause of quakes in the UK is often hard to determine; as the country is not close to any major source of tectonic movement, but rather is subject to minor geological stresses from a number of different sources. Earthquakes in Scotland are most often attributed to glacial rebound; until about 10 000 years ago much of the north of the UK was covered by a thick layer of ice, which pushed the rocks of the British lithosphere down into the underlying mantle; these rocks have been slowly rising back up ever since the ice melted, causing occasional Earthquakes in the process.
The UK is also subject to tectonic stresses caused by the expansion of the Atlantic Ocean, which is slowly pushing the entire of Eurasia to the east, and from the northward motion of Africa, which is pushing into Europe from the south. In addition there are smaller spreading centers beneath the North Sea, the Rhine Valley and the Bay of Biscay, all of which exert tectonic stresses upon Britain.
If you felt either of these quakes (or were in the area but didn't, which is also useful information) then you can report it to the British Geological Survey here. Witness statements help geologists understand how shock-waves travel through rock, which in turn helps them to build up a picture of the structures beneath the surface.
See also Earthquake off the coast of County Mayo, Ireland, Earthquake on the Isle of Islay, Magnitude 6.2 Earthquake north of Norway, Earthquake in South Yorkshire and Earthquakes on Sciency Thoughts YouTube.
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