British Geological Survey Technical Report WL/92/29 Global Seismology Series Bulletin of British earthquakes 1991 T Turbitt (editor) Contributors G D Ford, D D Galloway, P C Marrow, D W Redmayne, J A Richards, M E A Ritchie, B A Simpson, A B Walker and F Wright Bibliographic reference Turbitt, T (editor). 1992. Bulletin of British earthquakes 1991. British Geological Survey Technical Report WL/92/29. Issued October 1992 0 NERC copyright 1992 Edinburgh British Geological Survey 1992 BRITISH GEOLOGICAL SURVEY The full range of Survey publications is available Keyworth, Nottingham NG 12 5GG through the Sales Desks at Keyworth and at .a 0602 363100 Telex 378173 BGSKEX G Murchison House, Edinburgh, and in the BGS Fax0602363200 London Information Office in the Natural History Museum Earth Galleries. The adjacent bookshop Murchison House, West Mains Road, Edinburgh stocks the more popular books for sale over the EH93LA counter. 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INTRODUCTION 1.1 The Bulletin Seismic phase data, location details and magnitudes are presented for all earthquakes detected and located by BGS during 1991. The land areas of Great Britain and Northern Ireland and their coastal waters are covered within the limits of the detection capabilities of the seismograph network. The seismicity of the UK since 1969 is illustrated using data extracted from the previous catalogues of Burton and Neilson (1980) and Turbitt (1984 - 1991). 1.2 Summary of 1991 seismicity The largest earthquakes of the year, onshore, occurred at Newton, Powys, on 16 June (magnitude 2.8 ML), and at Balquhidder, Central Scotland, on 4 August (2.8 ML). Both were felt in the immediate area with intensity 3 MSK. Offshore the largest earthquake occurred in the Northern North Sea on 25 April with a magnitude of 4.2 ML. The epicentre was near the Alwyn oil fields but no felt reports were received. On 14 December a magnitude 3.6 ML earthquake was felt in the area around Boulogne in France. It is of significance to UK seismic hazard due to its position only 30 km from the south coast of England. Sixty five small events were detected in the coalfield areas of Fife; two were reported felt. In the coalfields of Derbyshire, Yorkshire, Nottinghamshire and Staffordshire over 100 events were detected, 13 of which were felt. A small event (0.8 ML) near Camborne in Cornwall at the site of an active tin-mine, was felt on 20 March. Earthquakes were also felt near Ardentinny, Strathclyde (16 June, 2.0 ML); Gelligaer, Mid Glamorgan (9 January, 1.2 ML) and Stirling, Central Scotland (31 October, 1.1 ML). A magnitude 2.2 ML earthquake at Loch Nevis, Highland on 27 September, was followed by 15 small aftershocks. Small swarms of activity occurred at Crianlarich, Central Scotland; Gleneagles, Tayside; Milngavie, Strathclyde and Dumfries, Dumfries and Galloway. Eight aftershocks of the 1984 Lleyn Peninsula earthquake were detected. 1 2. CATALOGUE FORMAT 2.1 Tables Hypocentral parameters, for each earthquake, are tabulated under the headings: Date - day, month, year Time - Hours, minutes, seconds of origin time Lat - Latitude, positive North Lon - Longitude, positive East KmE - Grid reference, easting from National Grid origin near the Scilly Isles. KmN - Grid reference, northing DeP - Hypocentral depth in km, blank indicates depth unknown. Note that depths for events of quality C, D and possibly B, are unreliable due to the large errors involved. Mag - Richter local magnitude Locality - A geographical indication of the epicentral area, usually the nearest town followed by the region. Int - Maximum felt intensity on the MSK scale (Medvedev et al, 1964), when known. + indicates that an event was reported felt at the intensity given but no survey was initiated to determine the maximum intensity. Comments and felt areas, where appropriate, are included on the next line. No - Total number of P and S readings used in the event location DM - Epicentral distance in kilometres to the closest station Gap - Largest azimuthal separation in degrees between stations RMS - Root mean square error of arrival time residuals in seconds ERH - Standard error of the epicentre in kilometres ERZ - Standard error of the focal depth in kilometres Q - Solution quality of the hypocentre averaged from QS and QD (below). A, excellent; B, good; C, fair; D, poor. SQD - S is quality factor ascribed to RMS, D is quality ascribed to number and distribution of stations. 2 Data on the earthquakes and seismograph stations operated in 1991 are arranged as follows: TABLE 1 is a chronological listing of all earthquakes in and near the UK for which a reliable epicentral location could be obtained. TABLE 2 is a listing of the events in Table 1 arranged in order of decreasing latitude to facilitate identification of earthquakes in selected regions. TABLE 3 is a chronological listing of events which, although detected by the seismograph network, had arrival patterns too weak to permit the computation of reliable locations. An indication of the estimated epicentre is given but errors could be very large. Also included are felt sonic events and unusual man made events such as aircraft crashes. These events are not in Tables 1 or 2. TABLE 4 is an alphabetical listing of the geographic coordinates of seismograph stations operated in 1991 by BGS, DIAS, and Leeds University. TABLE 5 lists the arrival times of phases for the events in Table 1 at each station, together with amplitude information used for magnitude calculation. TABLE 6 is the crustal seismic velocity model used for event location. 2.2 Figures FIGURE 1: the detection threshold of the network of seismograph stations in Table 4 for average background noise conditions where the detection criterion is signal received above 4 nanometres at 10 Hz on 3 stations. FIGURE 2: the variation of epicentral location errors within the UK area for a magnitude 2.0 ML earthquake. FIGURE 3: the epicentral location map of all the events in 1991 that are listed in Table 1. FIGURE 4: the locations of earthquakes in the UK of magnitude 2.5 ML and above from 1979 to 1991. FIGURE 5: the locations of earthquakes in the UK of magnitude 3.5 ML and above from 1969 to 1991. FIGURE 6: the locations of earthquakes in the North Sea area in 1991 detected by BGS instruments. 3 3. THE BGS UK SEISMOGRAPH NETWORK 3.1 Instrumentation A typical seismic network consists of up to seven 'outstation' vertical seismometers radio-linked over distances of up to 100 km to a central site where the data, along with that from a local 3-component set of two horizontal and one vertical seismometers, are recorded on magnetic tape by a Geostore recorder. Tapes are dispatched, usually once per week, to Edinburgh for analysis. A more detailed description of the system is given by Browitt et al (1985) and the response of the system is described by Turbitt and Stewart (1982). At some locations, on-line paper chart recorders display three channels to permit rapid investigation of reported felt tremors. Small computers operate at selected sites to detect earthquakes from a network. These data can be accessed by telephone line to give quasi-real-time, accurate, information. At other stations, low-gain vertical seismometers extend the dynamic range of the system to stronger motions and low frequency microphones are used to aid the discrimination of sonic booms. The improvements in geographic coverage of the UK with the installation of more seismic networks is described in Turbitt (1985), and more recent developments are described by Browitt and Turbitt (1992). 3.2 Detection Threshold The detection capabilities of a network depend upon station distribution, instrument sensitivity and background noise levels. For the BGS UK network the lower limit of sensitivity is governed by the background noise level. The contours in Figure 1 illustrate the lower threshold magnitude for an earthquake to exceed 4 nanometres at 10 Hz on at least three seismographs. Noise sources such as wind, waves, traffic and livestock vary considerably with time (about 0.5 to 15 nanometres, typically at 10 Hz) causing the magnitude thresholds to increase or decrease.