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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Gareth Bale urged Wales to continue "riding the wave" after clinching their spot in the last eight of Euro 2016.
Wales have qualified for the Euro 2016 quarter finals after beating Northern Ireland 1-0 in Paris.
A rally has been held in Cardiff following the EU referendum to show support to migrant communities.