English earthquakes, Disasters in England, Tidal waves, Historic building damage
A country generally not associated with major earthquakes. This webpage summarises some of the data on UK earthquakes, inclusive documented damage to buildings and even a tidal wave!
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While the UK is nowhere near in the same league as high seismicity areas such as California, Taiwan and Japan, it nevertheless has a moderate rate of seismicity, sufficiently high to pose a potential hazard to sensitive installations such as, nuclear power stations, dams and chemical plants. Consideration also has had to be made for other large construction projects such as the Channel Tunnel.
Historically rare, but the larger UK earthquakes have caused significant damage to older buildings with poor foundations and structure. Tidal waves flooding towns and sinking ships have even been recorded associated with UK earthquakes.
The scale of magnitude commonly used (inclusive TV news coverage) to express the seismic energy released by each earthquake is called the Richter Scale. Here are the typical effects of earthquakes in various magnitude ranges:
Thus, in the UK earthquakes of the magnitude 5 are rare. The 1580 earthquake described below is one of the largest known with an estimated magnitude of between 5.3 and 5.9. The minor property damage recorded in this earthquake is in no way compares to the scale of the recent (1991) earthquake in India which had a magnitude of 7.9.
In seismology a scale of seismic intensity is a way of measuring or rating the effects of an earthquake at different sites. The Modified Mercalli Intensity Scale is commonly used in the United States by seismologists seeking information on the severity of earthquake effects. Intensity ratings are expressed as Roman numerals between I at the low end and XII at the high end.
The Intensity Scale differs from the Richter Magnitude Scale in that the effects of any one earthquake vary greatly from place to place, so there may be many Intensity values (e.g.: IV, VII) measured from one earthquake. Each earthquake, on the other hand, should have just one Magnitude, although the several methods of estimating it will yield slightly different values (e.g.: 6.1, 6.3).
The oldest compilations of data were made in the late 19th and early 20th century. These provide the basic data for research, but they also tend to contain errors; confusing the events, dates and places. Great care is therefore needed with the interpretation of this data.
Neilson, Musson and Burton (1984) studied 101 earthquakes occurring between 1727 and 1979, using contemporary reports from press cuttings, books, letters and academic papers. Their findings, included two events in the Dover Strait (1776 and 1950), and one in Belgium (1938). Relatively few earthquakes were large enough to have been recorded simultaneously in England, France and Belguim.
A study undertaken during the design of the Channel Tunnel showed that in the southeast of England the largest recorded earthquake (1580) may have had a magnitude of 5.3 to 5.9 and an epicentre beneath the Channel. This event had an estimated focal depth of 20 to 25 kilometres associated with the Kent-Artois shear zone.
This showed that only two major earthquakes (by UK standards) are recorded to have affected the Dover Strait. These events occurred such a long time ago, however (1382 and 1580), that their interpretation is difficult. The findings of Neilson, Musson and Burton who established less significant 1776 and 1950 earthquakes in the Dover Strait ponts towards a return period of about 200 years in this area.
The earthquake at 3p.m. on 21 May 1382
This is the lesser known of the two major events, although some quite detailed records of its effects are preserved. These consist of approximately 80 references, of which about 30 are contemporary accounts. The maximum intensities appear to have been attained in Kent and Flanders. The records in Flanders are much more vague than in Kent, and the focal features can only be determined to a large extent by interpretation.
Various contemporary chronicles reported effects in England, such as severe shaking of trees, houses, churches, towers and castles, and panic among the population. The accounts of the archdiocese of Canterbury and papers attributed to Archbishop Courtenay describe the repairs needed after the earthquake. Significant damage was caused to St Augustine's Abbey and to Christ Church (collapse of the belfry, damage to the presbytery, priory and cloister). All Saints, to the west of Canterbury, needed repairs to the choir and strengthening to the (up to 1 metre thick) walls. The treasury accounts of the manors of Hollingbourne and Loose similarly record the costs for various repairs. At Saltwood a prisoner was able to escape in the confusion caused by the earthquake.
In London a sitting of Parliament and a trial of heretics at Blackheath were interrupted by the earthquake. Damage to St Paul's Cathedral, repaired in 1387, was attributed mainly to the 'terrible earthquake'.
In Flanders several buildings collapsed in Bruges, severe damage occurred to buildings in Liege and chimneys were brought down in Ghent.
The earthquake at 6p.m. on 6 April 1580
This better known event has been studied by a number of researchers, and its focus has been attributed to a wide area from London to the North Sea. It has been suggested that it can be correlated with the major earthquake of 1931 between Holland and Suffolk, but most researchers would now accept its most likely focus as being in the Channel. There are about 130 records, mostly contemporary, for this event.
Sections of wall fell in Dover, including the loss of a piece of the cliff and castle wall. At Sandwich people reported hearing a loud noise, which seemed to come from somewhere in the Channel. A gable end fell from the north wing of St Peter's church, four arches cracked in St Mary's church and part of a chimney fell down. Damage also occurred at Saltwood Castle, Sutton, Postling and Lydden churches. The tower at St Peter's in Broadstairs still bears a large crack which has been attributed to this event.
In Calais, clearly the worst affected town in France, the tremors lasted for about a quarter of an hour with damage caused both by the earthquake and by a tidal wave which engulfed the town and surrounding countryside. Several people and a large number of cattle were drowned. Part of the town wall and several houses collapsed, killing and injuring several people and animals. The watchtower split and half of it fell, the keeper and his wife having to be rescued from the remaining half
A great sea swell arose in the Channel sinking 25 to 30 British, French and Flemish vessels. A passenger on a boat from Dover reported that his vessel had touched the sea bed five times and that the sea had risen into the air more than 15 m higher than his vessel. About 12 hours later, on 7 April, between 4and 5 o'clock, some 30 houses fell down near Dover and a second deluge was reported to have drowned 120 people. There is a report of 120 vessels being lost off the English coast, with a further 15 near to Mont St Michel.
Boulogne is said to have been flooded and buildings are said to have shaken like leaves. Wine casks rolled off their stands, furniture was overturned and tables were lifted into the air. The belfry of Notre-Dame-de Lorette and several buildings at Lille were brought down. Stones and copings fell from buildings in Arras, Douai and Bethune.
Further to the south at Pontoise the cathedral of Notre Dame suffered breakage of windows and the falling of stones from the vaulting. Several churches and buildings were damaged at Rouen. Churches and houses at Beauvais were shaken and the bishopric clock struck several times as though sounding the alarm. The quake is said to have lasted for half a quarter of an hour.
Further afield stones fell from the cathedral at Ely. Part of Stratford Castle in Essex collapsed. About half dozen chimney stacks came down in London, a pinnacle on Westminster Abbey was damaged and stones falling from the roof of Christ's Church Hospital killed two children, The top of the bell tower of St Margaret's church at Stoke Golding, Leicester collapsed.
Cracks occurred in the castle keep at Airaines in Picardy. Many people were killed in St Amand-les-Eaux. Chimneys fell and damage was caused to the ridges and gables of houses in Mons. A tower collapsed in Sichem. Chimneys fell and wails were cracked in Ghent. Several people were killed and injured by falling chimneys and ridge tiles at Oudenaarde in Flanders. Peasants working in the fields reported hearing a loud rumbling noise moving from west to east and saw the ground roll in 3 or 4 successive waves. After shocks were reported at several locations in east Kenton 6-7 April and 1-2 May 1580, but not apparently on the continent.
The adoption of an epicentre in the Channel is supported by the location of the maximum intensities, the after shocks and the tidal wave. However it does not account for some continental intensities, apparently of the same order as those in Kent but located far from the Channel and the North Sea coasts. Wherever the focus of the earthquake actually was, it is seen as having a deep focus, probably in the lower crust, associated with the reactivation of a crustal fracture, probably within the Kent-Artois shear zone. This great depth justifies the reported effects a long way from the epicentre.
The study of British earthquakes
The 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
The figure below shows a map of earthquakes in the UK, taken from Musson (1994).
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 time
It 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 year
an earthquake of 4.7 or larger every 10 years
an earthquake of 5.6 or larger every 100 years.
Data, text and figures sources:
from seismic risk assessment in Engineering Geology of the Channel Tunnel
and from the BGS website plus other sources.