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The study of British earthquakesThe study of British earthquakes has in the past been somewhat neglected compared to some other countries, not necessarily those with more active seismicity. In the UK, historically, investigation of earthquakes has generally been in the hands of self-appointed investigators of semi-amateur status. Prior to 1889, such investigations were one-off affairs.Up to the 1970s, the most recent publication attempting to survey the whole history of British earthquakes was still that of Davison (1924) even though this was now 50 years out of date.Modern instrumental monitoring of British earthquakes began around 1970 with the establishment of LOWNET by the Global Seismology Group of BGS (then IGS) which has subsequently expanded to the present country-wide monitoring network (Browitt 1990, Walker and Browitt 1994).The distribution of British Earthquakes It is clear from this map that the spatial distribution of earthquakes is neither uniform nor random. For example, in Scotland most earthquakes are concentrated on the west coast, between Ullapool and Dunoon, with the addition of centres of activity near the Great Glen at Inverness and Glen Spean, and a small area around Comrie, Perthshire, and extending south to Stirling and Glasgow. The Outer Hebrides, the extreme north and most of the east of Scotland are virtually devoid of earthquakes. For the north-west of Scotland the absence of early written records, the small population, and the recent lack of recording instruments means that there may be a data gap; this is discussed further in Musson (1994b) in the context of an apparent event in 1925, possibly near Ullapool, with magnitude probably about 3.5, for which there are no first-hand reports. However, many other parts of Scotland, especially south of the Highland line, are quite well-documented, at least since 1600, and therefore the lack of earthquakes is genuine.Further south a similar irregularity is seen. If one draws a quadrilateral from Penzance to Holyhead to Carlisle to Doncaster, most English and Welsh earthquakes will be included within it. The northeast of England seems to be very quiet; almost aseismic. The southeast has a higher rate of activity, with a number of earthquakes which seem to be "one-off" occurrences. The most notable example of these is the 1884 Colchester earthquake, a magnitude 4.6 event which was the most damaging British earthquake in at least the last 400 years, and yet which occurred in an area (Essex) otherwise more or less devoid of earthquakes from the earliest historical period up to the present day (Musson et al 1990). There are also important centres of activity near Chichester and Dover. The former produced a swarm-like series of small, high-intensity earthquakes in the 1830s and was active again in 1963 and 1970.Offshore, there is significant activity in the English Channel and off the coast of Humberside. Because only the larger events in these places are likely to be felt onshore, the catalogue in the pre-instrumental period is probably under-representative of the true rate of earthquake activity in these zones. Even after the introduction of seismometers, offshore earthquakes may still have gone unnoticed on account of the distance to the nearest instruments. The Central Grabens of the North Sea are now known to be active features, only because of the improvements in instrumental monitoring over the last fifteen years (Marrow 1992, Musson et al 1993).The whole of Ireland is practically free of earthquakes. This is clearly a real phenomenon and not a product of reporting - as Ware, as early as the 17th century, remarks in describing an earthquake (probably Welsh) felt in Dublin in 1534, "... qui casus adeo rarus est in Hibernia, ut quando contingit, inter prodigia habeatur" [loosely, which is such a rare thing in Ireland that when it happens it is considered a wonder] (Ware 1662).Certain centres can be identified as showing typical patterns of activity. For example, the Caernarvon area of north-west Wales is one of the most seismically active places in the whole UK. Both large and small earthquakes, usually accompanied by many aftershocks, occur at regular intervals. The most recent of these larger events was the earthquake of 19 July 1984 (5.4 ), which was one of the largest ever UK earthquakes to have an epicentre on land and had a very protracted aftershock sequence (Turbitt et al 1985). Two further felt earthquakes have occurred there since, on 29 July 1992 (3.5) and 10 February 1994 (2.9 ). It is tempting to ascribe several early earthquakes of unknown epicentre (eg that of 20 February 1247) to this area just because it seems to be such a favoured site for large earthquakes.In South Wales, on the other hand, although a line of epicentres of significant events can be traced from Pembroke (an earthquake in 1892) to Newport (active in 1974), only the Swansea area shows consistent recurrence, with significant earthquakes occurring in 1727, 1775, 1832, 1868 and 1906. (Given this periodicity it may be that a further earthquake in this area is due in the near future.) The Hereford-Shropshire area has also produced large earthquakes in 1863, 1896, 1926 and 1990, but none of these share a common epicentre.The area of the Dover Straits is particularly significant because of the occurrence there of two of the largest British earthquakes in 1382 and 1580 (as described above). Since 1580 the only earthquakes there have been much smaller, raising the question of whether there is a danger of another 1580-style earthquake in the near future. The area may be structurally continuous with a zone of activity running east through Belgium, in which case it could be argued that stress in this area since 1580 has been released further east. This does not rule out another 1580-type earthquake in the future, but it is impossible to estimate how soon it might occur.In the north of England seismic activity occurs over a more or less continuous area from Leicester to Carlisle. The most prominent centres of repeating activity here are the upper end of Wensleydale (with significant earthquakes in 1768, 1780, 1871, 1933 and 1970) and to a lesser extent the Skipton area.The distribution of British earthquakes in timeIt has long been realised that larger earthquakes occur less frequently than smaller earthquakes, the relationship being exponential, ie roughly ten times as many earthquakes larger than 4 occur in a particular time period than earthquakes larger than magnitude 5.The average recurrence the UK may expect can be described as follows:an earthquake of 3.7 or larger every 1 yearan earthquake of 4.7 or larger every 10 yearsan earthquake of 5.6 or larger every 100 years.