This is an extract from the Museum of Wales website
"Where do earthquakes occur in Wales?There are several long-active fault systems in Wales. Once faults form, they create weak zones in the crust that can be reactivated time and time again. For example, the fault system running parallel to the Menai Strait between Anglesey and Bangor in north Wales are known to have been active over 500 million years ago and have also been the sites of more recent earthquakes.
The Menai Strait area is the most seismically active area in Wales and one of the most active in the UK. The last major earthquake occurred here in 1984, but many others are known from historical records (including 1827, 1842, 1852, 1874, 1879, 1903). Although many events have been recorded in south Wales, from Pembroke to Newport, only the Swansea area shows consistent activity, with significant earthquakes occurring in 1727, 1775, 1832, 1868 and 1906.
Largest earthquake in Wales for 100 yearsThe 1984, magnitude 5.4, Menai Strait earthquake was the largest onshore UK event for over 100 years. The epicentre was located in northern Llyn, north Wales and the quake originated at a depth of approximately 22 km in the Earth's crust"
The above extract shows that seismic activity in Wales is a real and present concern, the 1984 quake being similar in size to the recent earthquake in New Zealand. All of this means that there is a very urgent need to consider the risks in the current position at Wylfa, where the double trip of the 400KW Supergrid lines, as has happened 6 times already, could if combined with an earthquake have devastating consequences here in North Wales. The exposure relates to the double trip on the Grid combined with an event that could knock out the electrical back up systems, with a reactor fault transient. At present National Grid and Horizon do not seem to be considering a subsea or undergrounding cable from Anglesey into Deeside, with the 2009 ENSG report indicating that they propose to run overhead lines, susceptible to the very real dangers of a double trip which has already happened as the past records show. This poses an unacceptable risk to the local population in North Wales, who have very real experience of the consequences of Nuclear fallout as a result of the Chernobyl disaster. It is now that the lessons from Fukushima should be learnt here in the UK, particularly on Anglesey which has known fault line. DECC needs to look with care at the proposals from both Horizon and National Grid to ensure that those risk factors are urgently addressed and consideration given to subsea cable to Deeside, as a matter of nuclear safety.