The intra-plate earthquake on the Lleyn Peninsula, North Wales afforded a rare opportunity to obtain accurate focal parameters for the main shock and the aftershock sequence. The focal mechanism does not correlate with movement on any of the large faults showing surface expressions in the area but does indicate a NW–SE oriented maximum compressional stress direction, in agreement with measurements made in other parts of Britain. Aftershock activity was concentrated around the focus of the main shock at depths of between 21 and 24 km. The depth indicates an unusually deep transition between brittle and ductile zones that is possibly a consequence of low heat flow in the region.