The British Geological Survey says there was a magnitude 5.4 earthquake in the area in 1984.
It describes that year's event as "significant".
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The Welsh Government will later launch new plans to reduce the number of children living in poverty in Wales.
Plaid Cymru are to launch their campaign this morning ahead of May's general election.
A study by Swansea University has found a fifth of students in the UK have considered working in the sex industry to fund their studies.