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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Andrea Byrne presents the first episode of the new series Helimeds which follows the Welsh flying medics.
Variable cloud and clear spells overnight with isolated showers mainly in the north.
Becoming drier and brighter from the northwest during the day, but with the risk of scattered, locally heavy, showers developing later.