British Earthquakes Research Articles

Chapter 2 Seismic hazard

Abstract It is often thought that earthquakes do not occur in the UK; however, the seismicity of the UK is usually classified as low-to-moderate. On average, a magnitude 3.2 M w moment magnitude or larger earthquake occurs once per year, and 4.2 M w or larger every 10 years. The latter is capable of causing non-structural damage to property. The damage caused by British earthquakes is generally not life-threatening, and no-one has been killed in a British earthquake (at the time of writing, May 2013) since 1940. Damage is caused by shaking, not by ground rupture, so the discovery of a fault surface trace at a construction site is not something to be worried about as far as seismic hazard is concerned. For most ordinary construction in the UK, earthquake hazard can be safely discounted; this is not the case with high-consequence facilities such as dams, bridges and nuclear power plants.

Historical development of the British and Scandinavian earthquake archives

The study of historical earthquakes itself has an interesting history, and Nick Ambraseys figures highly in it. This historiographic tribute relates the pivotal role that he played in the development of the British and Scandinavian Earthquake Archives. Maintaining his academic distance from direct hazard consulting, he nevertheless exerted a significant influence on the scientific conduct of hazard analyses. From the collection of instrumental and macroseismic data to the assignment of magnitudes, the modus operandi which he recommended was followed in pioneering seismic hazard studies in Britain, the North Sea and Norway, during the 1980s. Because of the confidentiality of seismic hazard reports produced for the UK nuclear industry, there is little record in the open seismological literature of the contribution of Nick Ambraseys. As a civil engineer, he had an exceptional interest in the earthquake evidence that could be gleaned from historical documents. An illustrative reference is given to the discovery of fresh evidence from an Aegean earthquake that shows how even the most knowledgeable of earthquake historians can be surprised.

A critical history of British earthquakes

This paper reviews the history of the study of historical British earthquakes. The publication of compendia of British earthquakes goes back as early as the late 16th Century. A boost to the study of earthquakes in Britain was given in the mid 18th Century as a result of two events occurring in London in 1750 (analogous to the general increase in earthquakes in Europe five years later after the 1755 Lisbon earthquake). The 19th Century saw a number of significant studies, culminating in the work of Davison, whose book-length catalogue was published finally in 1924. After that appears a gap, until interest in the subject was renewed in the mid 1970s. The expansion of the U.K. nuclear programme in the 1980s led to a series of large-scale investigations of historical British earthquakes, all based almost completely on primary historical data and conducted to high standards. The catalogue published by BGS in 1994 is a synthesis of these studies, and presents a parametric catalogue in which historical earthquakes are assessed from intensity data points based on primary source material. Since 1994, revisions to parameters have been minor and new events discovered have been restricted to a few small events.

Seismogenesis and state of stress in the UK

In this paper I present a compilation of focal mechanisms for earthquakes with magnitudes greater than 3.0 M L in the British Isles that can be used to help constrain our understanding of seismicity and it's driving forces in the British Isles. The fault plane solutions consist of both previously published mechanisms for significant British earthquakes, and new solutions calculated from regional and local data for more recent and smaller earth- quakes that were previously unpublished. Focal mechanisms for earthquakes in the UK are dominantly strike-slip with northwest-southeast compression and northeast-southwest tension,or reverse, with northwest-southeast com- pression. In many cases there is also an oblique component to the slip. P and T axes from individual solutions are relatively well constrained in azimuth, though less so in dip, with P-axes orientation for most events clustering between north and north-west, indicating sub-horizontal compression. However, some spatial variation in P- and T-axes orientation is also apparent, with near north/northeast compression and east-west extension in north- west Scotland, changing to northwest-southeast compression in England and Wales. I estimate a best-fitting stress tensor, under the assumption of uniform stress using two different inversion methods for both the entire focal mechanism data set and two different subsets of the data. The results from the two different datasets suggest that there is a significant difference in the stress state between northwest Scotland and England and Wales. Calculated σ 1 directions for England and Wales are northwest-southeast, consistent both with existing stress data and expected stresses from first order plate motions. By contrast, the inversion results for northwest Scotland show near east-west extension with possible σ 1 and σ 2 directions lying in a north south band, and that the magnitudes of σ 1 and σ 2 are similar. The relative magnitude of the principal stresses, R, determined for England and Wales suggests that the intermediate stress σ 2 is close to the average value of σ 1 and σ 3 . The clear difference in the stress inversion results between northwest Scotland and England and Wales suggests that the principal stress directions expected from first order plate motions have been modified in Scotland by local stress conditions due to glacio-isostatic adjustment.

Rotational Earthquake Effects in the United Kingdom

The United Kingdom is an area of low to moderate seismicity, and damaging earthquakes are uncommon. However, even in the limited record of damage from historical British earthquakes, a number of instances can be found of rotational effects on parts of structures, primarily chimneys or the tops of spires. We have assembled all the instances we know of from the United Kingdom record and present them here with illustrations and extracts from the original reports. It is not possible to determine whether these are the effects of true rotational motion or the effects of translatory shaking. Interestingly, this problem was considered in some detail by field investigators as long ago as the 1880s.

Observations from the Folkestone, U.K., Earthquake of 28 April 2007

Earthquakes in Britain are usually minor with respect to damage. However, the Folkestone earthquake ( ML 4.3, Mw 4.0), which occurred on the southeastern coast of England (Figure 1) on 28 April 2007 at 07:18 UTC (08:18 BST) caused significant damage. One person was injured by falling masonry, and the Shepway District Council invoked emergency procedures to deal with the effects of the earthquake. This is the first time that emergency procedures have been invoked for a British earthquake. Approximately 60,000 homes lost power for 85 minutes due to tripping of two high-voltage transformers. The epicenter of the earthquake, latitude 51.102°N and longitude 1.169°E (Ottemoller et al. forthcoming), locates very close to Folkestone (Figures 1A–C). Ottemoller et al. (forthcoming) determine a depth of around 6 ± 3 km from United Kingdom (UK) and European short-period and broadband data. The focal mechanism, shown in Figure 1C, indicates normal faulting with a significant component of strike-slip on either a north-northwest to south-southeast trending or east-northeast to west-southwest trending plane (Ottemoller et al. forthcoming). The earthquake was followed by nine after-shocks ( ML 1.0–1.8), which occurred up to 5 May 2007. None of these were felt. A compendium of observations from the Folkestone earthquake is presented here. This includes analysis of the first “strong-motion” record from a British earthquake, a description of the damage, an investigation of how the distribution of damage may relate to the distribution of unconsolidated deposits, and the results of the macroseismic survey for the earthquake. Assignment of maximum intensity, and its spatial extent, provoked a lively debate among the authors, which is also documented here. There is a distinct east-west divide in terms of the distribution of seismicity in the UK. Most activity is concentrated in western Scotland, northwestern England, Wales, the Midlands and Cornwall (Figure 1A …

British earthquakes

British earthquakes

On the Perceptibility of Earthquakes

A method is presented to derive a first order approximation of the number of people likely to feel any earthquake, assuming a uniform population distribution. The “most perceptible” earthquake is a function of the frequency of earthquake occurrence (there are more small earthquakes) and effect (large ones will be felt by more people). The method is demonstrated taking the UK as a test case. The trade-off between the two trends seems to produce a peak value of about 4.5 ML for the earthquake that the typical inhabitant of the UK is most likely to have felt. It is also found that a UK citizen who lives for 70 years has a roughly 42% chance of experiencing a British earthquake at some point in their life. In practice, of course, the issue is complicated both by irregular population distribution and to some extent regional irregularities in the magnitude-frequency distribution for the UK. The method is easily adaptable to other countries; its relevance is chiefly in the generation of statistics of interest to the public at large, thus aiding public understanding of science.

Seismic imaging of a hot upwelling beneath the British Isles

The Iceland plume has had an important influence on vertical motions in the North Atlantic. The convecting mantle in this region contains a large-scale low-velocity seismic anomaly, which correlates with a long-wavelength gravity high and bathymetric feature. This suggests that an arm of plume material has extended, or is extending, from Iceland, in a direction perpendicular to the Mid-Atlantic Ridge. Here we present the results of a detailed teleseismic traveltime study that reveals the high-resolution morphology of this low-velocity anomaly beneath the British Isles. Our images provide insights into the nature of plume-lithosphere interactions. The low-velocity anomaly imaged in this study correlates geographically with a region of high gravity anomalies and high topography that was associated with Paleogene magmatism and phases of epeirogenic uplift during the Cenozoic Era. There is evidence that the distribution of British earthquakes is also related to the low-velocity anomaly. The low-velocity anomaly is interpreted to represent hot material from the original Iceland plume head that became trapped beneath thinned regions of lithosphere ca. 60 Ma.

Intensity attenuation in the U.K.

Intensity attenuation is relatively little studied compared with the attenuation of peak ground acceleration, due to the fact that the PGA can be used for engineering design, while intensity cannot. However, intensity has other uses, including the estimation of effects (including damage) of future earthquakes, and hence, at least in a general way, the study of earthquake risk. Knowledge of intensity attenuation is also useful in calibrating hazard models against historical experience. In this study, the attenuation of intensity in the U.K. is thoroughly evaluated from a data set comprising 727 isoseismals from 326 British earthquakes, including both modern and historical events. Best results are obtained by restricting the data set to events contributing at least two isoseismals. The preferred equation is $$ I = 3.31 + 1.28{\rm ML} - 1.22\ln\!R $$ where I is intensity (European Macroseismic Scale), ML is local magnitude, and R is hypocentral distance. The sigma (uncertainty) value is 0.46. Some sample applications of this formula are demonstrated.

The Cenozoic uplift and earthquake belt of mainland Britain as a response to an underlying hot, low-density upper mantle

A belt of hot, low-density uppermost mantle underlying mainland Britain down to at least 200 km depth, revealed by seismic tomography, may be the prime cause of the Cenozoic uplift and exhumation. We use finite-element modelling to demonstrate how isostatic uplift can occur in response to such a low-density hotspot beneath continental crust. To explain the narrow width of the uplift of northern Britain, the lower crust must be ductile (power-law rheology assumed) and the asymmetrical uplift may be bounded at least on the west side by a pre-existing fault or faults of appropriate polarity. Faulting has probably been reverse under NW–SE regional compression since the onset of the Cenozoic. With the assistance of continuing denudation, inferred gross Cenozoic exhumation of up to 3000 m can be explained. British earthquakes concentrate along a similar north–south belt, with the strongest events in the west. We suggest that the earthquakes result from the continuing tectonics associated with the hot upper mantle, the uplift it causes, and the weakened crust. The underlying low-density region gives rise to tensional loading stress in all directions and bending stresses are associated with the upper-crustal flexuring accompanying uplift. These large stresses supplement NW–SE regional compression. Available earthquake mechanisms are approximately consistent with this stress environment.

Fatalities in British earthquakes

Fatalities in British earthquakes

The felt effects of the Carlisle earthquake of 26 December 1979

Synopsis The earthquake of 26 December 1979, with an epicentre north of Carlisle, near Longtown, Cumbria, was one of the most significant British earthquakes of the second half of the 20th century. It had a magnitude of 4.7 ML (local magnitude) and was felt over an area of around 84 000 km 2 at intensity 3 EMS (European Macroseismic Scale), covering most of central Scotland, the Borders, Cumbria and the NE of England. It was the mainshock of a sequence of around 90 events recorded by the British Geological Survey (BGS), then the Institute of Geological Sciences (IGS), UK seismic monitoring network. BGS undertook a macroseismic (felt effect) survey for the mainshock, with around 4000 usable responses received, and also for the two largest aftershocks, which occurred on 1 January 1980 and 13 December 1980, both with magnitude 3.8 ML. The results of these surveys have not been published until now. The highest intensities were reached around the Carlisle and Longtown areas, where 6 EMS was assigned from reports describing chimney stack and roof damage, with debris falling into the street, cracks in walls and other similar effects. Macroseismic estimates of the parameters of the earthquake agree well with the instrumental parameters so far as epicentre and magnitude are concerned, but there is a significant discrepancy with respect to depth. Also, although the size of the felt area is consistent with what is expected from average UK intensity attenuation, there is a marked directionality to the energy release, resulting in the earthquake being much more perceptible to the north than to the south.

New method of estimating regional stress orientations: application to focal mechanism data of recent British earthquakes

SUMMARY It is demonstrated how a recently devised graphical method for estimating palaeostresses from exposed striated fault planes can be adapted for use with earthquake focal mechanism data. Provided the data set includes one or more earthquakes for which the fault plane can be identified, the principal stress directions can be determined more precisely than by using the alternative Right Dihedra method. Using the new so-called Right Trihedra method stress directions are found which are compatible with recent British earthquakes. The method can also be used, in the case of some focal mechanisms, to resolve the ambiguity regarding which focal plane corresponds to the fault plane.

The Colchester forgeries: faking photographic evidence of earthquake damage

ABSTRACTThe photographic archive of the 22 April 1884 Colchester earthquake, the most damaging British earthquake in the last 400 years, is wellknown. Evidence has come to light that at least two of these photographs are partly or wholly faked. This shows that not even the camera can necessarily be relied upon to accurately portray the extent of damage from historical earthquakes.

The use of newspaper data in historical earthquake studies

Studies of historical British earthquakes are an essential component of assessing seismic hazard in the U.K.; such studies rely heavily on macroseismic data obtained from printed newspapers. This paper discusses the ways in which newspapers have reported British earthquakes and the nature and limitations of the data that may be acquired from this source. Historical changes in the press as a source of macroseismic data are discussed. It is suggested that an understanding of the background and nature of newspaper data is an important component in the process of revaluating historical earthquakes from such data.

The great British earthquake of 1984 seen from afar

The Lleyn Peninsular earthquake which occurred at 0656 on 1984 July 19 was recorded at three medium aperture seismological array stations located at teleseismic distances. From these recordings an estimate is made of the hypocentre, origin time, magnitude and fault-plane solution of the earthquake. The fault-plane solution was determined using the Pearce algorithm and indicates that the focal mechanism is predominantly strike-slip. The fault-plane solution was used to generate synthetic seismograms for comparison with the observed to confirm the nature of the source and in particular the depth of focus which was estimated to be 20.5 km. It is concluded that the determination of the earthquake parameters using only three teleseismic seismograms is in good agreement with the results obtained from an analysis of 45 local and regional seismological stations.

British Earthquakes, University of East Anglia, 18–19 April 1985

British Earthquakes, University of East Anglia, 18–19 April 1985

The essex earthquake of 1884

AbstractThe largest British earthquake to occur in several hundred years took place in Essex, in the south east of England, in 1884. From contemporary accounts and a recent visit to the area affected by the earthquake, the intensity of the earthquake was assessed. It is estimated that the peak accelerations generated may have been as much as 0.1 g. This would be of consequence to some modern structures, designed without earthquake provisions.

Historical Records of British Earthquakes

Historical Records of British Earthquakes