UK earthquake monitoring 2002/2003

The aims of the 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. 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 of the Deputy Prime Minister (ODPM) with major financial input from the Natural Environment Research Council (NERC). This Customer Group is listed in Annex A. In the 14th year of the project (April 2002 to March 2003), three subnetworks were upgraded with the installation of data loggers running under the QNX operating system, and a strong motion instrument was installed at Hartland in North Devon. The increasing number of acceleration records being captured by strong motion instruments, is feeding into a better understanding of attenuation and seismic hazard in the UK. Some 235 earthquakes were located by the monitoring network in 2002, with 87 of them having magnitudes of 2.0 ML or greater (Annex B). A total of 42 events in this magnitude category were reported as felt along with 6 smaller ones. Nine strong-motion records were captured from six of the nineteen sites now equipped with strong motion instruments. The largest earthquake in the reporting year, with a magnitude of 4.7 ML, occurred near Dudley on 22 September. It was felt up to 337 km away and over an area of 126,000 km2 (Isoseismal 3 EMS) and reached a maximum intensity of 5 on the European Macroseismic Scale (EMS, Annex H). A peak ground acceleration of 153 mms -2 was recorded on the three-component accelerometer at Keyworth, a distance of 83 km from the epicentre. The focal mechanism indicates strike slip movement along near vertical fault planes striking either NNE-SSW or WNW-ESE. The following month, an earthquake sequence commenced near Manchester with 117 events located, 37 of which were felt by the local population. The sequence caused widespread alarm in the greater Manchester area. The largest offshore earthquake occurred in the central North Sea on 12 October 2002 with a magnitude of 3.5 ML, approximately 70 km east of the Shetland Islands. In addition to earthquakes, BGS frequently receives reports of seismic events felt and heard, which on investigation prove to be sonic booms, spurious or in coalfield areas, where much of the activity is probably induced by mining. During the reporting period, data from six sonic events were processed and reported upon following public concern or media attention. All significant felt events and some others were reported rapidly to the Customer Group through seismic alerts sent by e-mail. The alerts were also published on the Internet (http://www.earthquakes.bgs.ac.uk). Monthly seismic bulletins were issued six weeks in arrears and, following revision, were compiled into an annual bulletin (Simpson, 2003). In all these reporting areas, scheduled targets have been met or surpassed. Maintenance and protection of historical archives, another primary goal of the project, has continued and has been enhanced by donations of the Soil Mechanics UK data, from a study in the early 1980’s and the British Association for the Advancement of Science Seismological Committee archives. The environmental monitoring stations at Eskdalemuir and Hartland observatories recorded a variety of parameters throughout the year and the data are now accessible on-line through an Internet connection.