Historic Earthquakes
United Kingdom
1979 December 26 03:57:04 UTC
Magnitude 4.5
Location 54.90N 2.68W
Depth 10 kilometers
by Graham Neilson,
Natural Environment Research Council,
Institute of Geological Sciences,
Edinburgh, Scotland
At 3:57 a.m. on the morning after Christmas Day
1979, a large area of northern England and southern
Scotland was shaken by an earthquake. In the
mesoseismal area, a few miles north of Carlisle, chimneys
toppled into the streets or fell through roofs, and
people ran in panic into the open. Over a much wider
area, including the densely populated industrial area
of central Scotland, people were awakened, and
ornaments and other small objects were disturbed. At
distances over 120 miles from the epicenter, only a
few people noted the tremor. Northern Ireland and the
Isle of Man also experienced the tremor but reportedly
at no very great intensity. The disturbed area covered
approximately 45,000 square miles.
Minutes after the earthquake struck, police forces in the disturbed area were deluged with telephone calls from the anxious public. Seismologists at the Institute of Geological Sciences (IGS) were soon at work making a preliminary analysis of the seismograms obtained from the Worldwide Standardized Seismograph Network station Eskdalemuir and from the large network of seismometers, LOWNET, situated in the Lowland Valley of central Scotland. It was soon apparent that the epicenter lay close to four small shocks that had been detected on LOWNET in early October 1979. An epicenter location, with fairly small errors, of about 16 miles was determined for the December 26 shock at 55.00°N., 2.88°W.
Assessment of magnitude (M) proved difficult because the seismograms of both Eskdalemuir and LOWNET were so close to the epicenter that they were saturated. It was not until the following day that the first reliable magnitude measurement of MS=5.0 was made by Robin Adams of the International Seismological Centre at Newbury in the south of England using a seismogram recorded there. Shortly afterwards, the European and Mediterranean Seismological Centre (CSEM) in Strasbourg, France, confirmed this value. However, by comparing the felt area of the December 26 shock with previous strongly felt British earthquakes of known magnitude, a magnitude figure of between 4.5 and 5.0 was estimated.
By 10 a.m. on December 27, a reasonably detailed picture of the earthquake had emerged, and this information was given to the press, radio, and television networks to help allay people's fears. In turn, the media responded by giving reports of where the earthquake was felt. Questionnaires were circulated, and a form was placed in a number of national and local newspapers for the public to fill in and return to IGS. These data will give a detailed picture of the shock's effects throughout the shaken area.
As press interest in the earthquake was satisfied, the task of complementing the preliminary data set got under way. Magnetic tape recordings from the IGS stations at Hereford, Leeds, Stoke-on-Trent, Nottingham, and Leicester were sent to Edinburg for analysis. Within a few days, station readings and seismograms from other British stations started to arrive in Edinburg together with data from other European stations. Not only the seismograms of the main shock but also those of the aftershock sequence were interpreted, and locations and magnitudes for these events were determined. The strongest of these, to date, occurred on December 27 at 07 h 23m GMT (ML=2.0) and 23 h 05 m GMT (ML=2.5) and on January 1, 1980 (ML=3.5). The latter event was felt over about 5,000 square miles.
The last comparable earthquake to occur in Britain was in 1926 and had an epicenter near Hereford. The degree of alarm felt by the public for the 1979 earthquake may thus largely be ascribed to the rarity of shocks of this magnitude in Britain. A few tremors had been recorded from the Carlisle area prior to the December 26 earthquake and were in keeping with the historic seismic record in northwest England and southern Scotland. Since 1786, 10 earthquakes are on record as originating in the Carlisle area, the largest being M=4.4 (estimated) shock on August 11, 1786. There is no obvious tectonic feature within the complex geology of this region that corresponds to the focal depth of 19 miles for the Carlisle earthquake. When all the data from this earthquake series are collected and analyzed, it is hoped that a clearer picture of the tectonic mechanism may emerge. The macroseismic data, among other things, will be used to derive an intensity attenuation law.
Although an unnerving experience for people sleeping after their Christmas celebrations, the Carlisle earthquake will provide British seismologists with a rare opportunity for a rich field study for some time.
Abridged from Earthquake Information Bulletin, March-April 1980, Volume 12, Number 2.
