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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Fire crews have been responding to reports of flash flooding in Carmarthenshire. Some homes have been affected.
Medication designed for animals was taken from the building in Abertillery on Thursday night.
More than a thousand riders will be taking part in a series of events in and around the town.