What are the largest two instrumental onshore earthquakes?
The 19 July 1984 Lleyn event of North Wales, with a magnitude of 5.4 ML, was the largest onshore earthquake this century in the UK and was felt over an area of around 240,000 square kilometres. The earthquake occurred in the lower crust at a depth of approximately 22 km and was followed by many aftershocks. Detailed mapping of the aftershock distribution highlighted a plane orientated WNW-ESE and dipping steeply NNE. This represents the fault plane and corresponds well with one of the planes of the mainshock focal mechanism. There is, however, no surface fault or feature which corresponds to this plane.
The maximum intensity in the epicentral area was 6 EMS (European macroseismic scale) and damage consisted of widespread cracks in plaster and falls of some chimneys and weak plaster. High intensities of 5 and 6 EMS reported from Liverpool appear to be due to the state of repair of some of the buildings.
The 2 April 1990 Bishop's Castle earthquake in the Welsh Borders, with a magnitude of 5.1 ML, was the second largest onshore earthquake in recent years and was felt over an area of approximately 140,000 square kilometres. It occurred at a depth of 14 km and the maximum intensity in the epicentral area was 6 EMS. Damage was minor, including cracks and fall of parts of chimney and plaster and was limited to the epicentral area, north to Wrexham and especially Shrewsbury. Only six aftershocks followed the mainshock, suggesting an almost total release of strain energy following the mainshock.
