Synopsis

The earthquake of 26 December 1979, with an epicentre north of Carlisle, near Longtown, Cumbria, was one of the most significant British earthquakes of the second half of the 20th century. It had a magnitude of 4.7 ML (local magnitude) and was felt over an area of around 84 000 km² at intensity 3 EMS (European Macroseismic Scale), covering most of central Scotland, the Borders, Cumbria and the NE of England. It was the mainshock of a sequence of around 90 events recorded by the British Geological Survey (BGS), then the Institute of Geological Sciences (IGS), UK seismic monitoring network. BGS undertook a macroseismic (felt effect) survey for the mainshock, with around 4000 usable responses received, and also for the two largest aftershocks, which occurred on 1 January 1980 and 13 December 1980, both with magnitude 3.8 ML. The results of these surveys have not been published until now. The highest intensities were reached around the Carlisle and Longtown areas, where 6 EMS was assigned from reports describing chimney stack and roof damage, with debris falling into the street, cracks in walls and other similar effects. Macroseismic estimates of the parameters of the earthquake agree well with the instrumental parameters so far as epicentre and magnitude are concerned, but there is a significant discrepancy with respect to depth. Also, although the size of the felt area is consistent with what is expected from average UK intensity attenuation, there is a marked directionality to the energy release, resulting in the earthquake being much more perceptible to the north than to the south.