Guernsey

Guernsey is a British Crown dependency in the English Channel, approximately 50 kilometres from the Normandy coast of France and 30 kilometres west of the Cotentin Peninsula. The island is underlain by ancient Precambrian granite and gneiss — some of the oldest rocks in the British Isles — that have been geologically stable for hundreds of millions of years. The island sits firmly within the stable northwest European platform, far from any active plate boundary, and is one of the quietest seismic environments in Europe.

Historical records from Guernsey and the nearby Channel Islands document occasional felt earthquakes, all of very low magnitude and originating from distant sources. The most significant seismic event affecting the region was a moderate earthquake on 23 February 1927 centred approximately 100 kilometres south of the Channel Islands in the Bay of Biscay area, which was felt on Guernsey and in southern England. Very infrequent minor events are also associated with ancient fault structures beneath the English Channel — remnants of Variscan and Caledonian orogenic sutures that were active hundreds of millions of years ago but are now almost entirely locked.

The [[tsunami]] hazard at Guernsey is negligible from local geological sources. A very large Atlantic earthquake or submarine landslide could theoretically generate a small wave, but the English Channel geometry would significantly attenuate any such wave before it reached the island. Seismic monitoring in the Channel Islands region is conducted by the British Geological Survey as part of the UK national network.