Introduction
The British Geological Survey (BGS) has been charged with the task of operating and further developing a uniform network of seismograph stations throughout the UK in order to acquire standardised data on a long-term basis. The project is supported by a group of organisations under the chairmanship of the Office for the Deputy Prime Minister (ODPM formerly known as DTLR) with major financial input from the Natural Environment Research Council (NERC). The aims of the BGS Seismic Monitoring and Information Service are to develop and maintain a national database of seismic activity in the UK for use in seismic hazard assessment, and to provide near-immediate responses to the occurrence, or reported occurrence, of significant events to its customers and sponsors. A 24-hr on-call service is maintained for this purpose. Almost every week, seismic events are reported to be felt somewhere in the UK. A number of these prove to be sonic booms or are otherwise spurious, but a large proportion are natural or mining?induced earthquakes often felt at intensities which cause concern and, occasionally, some damage. In an average year, some 200 earthquakes are detected and located by BGS with around 15% being felt by people.
Within the 146-station, high sensitivity monitoring network, 18 strong motion instruments have been integrated. Data from all sensors is available for analysis and interpretation by BGS scientists in Edinburgh, in near real time, through radio and telephone links. The high sensitivity network has been expanded to cover the whole country since the 1970's and achieves a detection threshold for magnitude 2.0 earthquakes throughout the land area even in high noise conditions. All earthquakes of magnitude 2.5 and above have been captured since 1979. For more information about earthquakes, visit www.earthquakes.bgs.ac.uk.
Note:
The Magnitude column is the definitive magnitude of the earthquake and is used to create the symbol size.
Maximum EMS intensity. 2+ indicates felt, no macroseismic details. 3+, 4+ etc indicates felt at 3 or 4, but no survey carried out. 3, 4, 5 etc describes the maximum EMS intensity produced by the event.
ML : Richter local magnitude of the event (size of the earthquake).
Mgmc : Macroseismic magnitude determined from felt area of historical earthquakes.
Notes on the completeness of the historical and Instrumental Datasets
The historical catalogue has been compiled, in general, from macroseismic observations (ie felt effects). Before 1700, only earthquakes with magnitudes of 4.0 ML or greater are included. After 1700, all known events with magnitudes of 3.0 ML or greater are included together with some other, smaller ones. Accuracies of magnitude, location, and origin time vary with the quality of information available for this period as they do for instrumental measurements in the post 1970 period. In that case, variations are largely a function of the seismograph station coverage, which has been improving up to the present day.
The instrumental catalogue contains all detected earthquakes; however, the completeness improves as the seismograph monitoring networks were expanded over the years. As a guide, the completeness threshold was approximately magnitude 4.0 ML in 1970, improving to 2.5 ML in 1979 for mainland UK.