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EARTHQUAKE QUESTIONS AND ANSWERS
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An earthquake is the sudden release of strain energy in the Earth's crust resulting in waves of shaking that radiate outwards from the earthquake source. When stresses in the crust exceed the strength of the rock, it breaks along lines of weakness, either a pre-existing or new fault plane. The point where an earthquake starts is termed the focus or hypocentre and may be many kilometres deep within the earth. The point at the surface directly above the focus is called the earthquake epicentre.
Anywhere! However, they are unevenly distributed over the earth, with the majority occurring at the boundaries of the major crustal plates. These plate boundaries are of three types: destructive, where the plates collide; constructive, where the plates move apart; and conservative plate boundaries, like the San Andreas Fault, where the plates slide past each other. Earthquakes also occur, less frequently, within the plates and far from the plate boundaries, as in eastern USA, Australia and the United Kingdom.
Around 75% of the world's seismic energy is released at the edge of the Pacific, where the thinner Pacific plate is forced beneath thicker continental crust along 'subduction zones'. This 40,000 km band of seismicity stretches up the west coasts of South and Central America and from the Northern USA to Alaska, the Aleutians, Japan, China, the Philippines, Indonesia and Australasia.
Around 15% of the total seismic energy is released where the Eurasian and African plates are colliding, forming a band of seismicity which stretches from Burma, westwards to the Himalayas to the Caucasus and the Mediterranean.
One of the largest earthquakes ever was the Chile event of 22 May 1960 with moment magnitude of 9.5 Mw. Other large earthquakes include Lisbon, 1 November 1755, magnitude 8.7 Ms; Assam, 12 June 1897, magnitude 8.7 Ms; Alaska, 28 March 1964, moment magnitude 9.2 Mw. Although the magnitude scale is open ended, the strength of the crustal rocks prior to fracturing limits the upper magnitude of earthquakes.
NUMBER OF EARTHQUAKES PER YEAR MAGNITUDE 7.0 OR GREATER
Total 1900-1989 = 1822 events = 20 per year
Statistics were compiled from the Earthquake Data Base System of the US Geological Survey, National Earthquake Information Centre, Golden CO, USA.
YES, between 200 and 300 earthquakes are detected and located in the UK, by
the British Geological Survey annually. Although distant from the nearest plate
boundary, the Mid-Atlantic Ridge, earthquakes occur as crustal stresses within
the tectonic plates are relieved by movement occurring on pre-existing fault planes.
The risk from these earthquakes is not insignificant and must be considered when
engineering for sensitive installations.
FREQUENCY OF OCCURRENCE OF MAINLAND UK EARTHQUAKES (BASED ON OBSERVATIONS
BETWEEN 1979 AND 1994)
FREQUENCY OF OCCURRENCE OF NORTH SEA EARTHQUAKES (BASED ON OBSERVATIONS
BETWEEN 1979 AND 1994)
The UK is a region of low seismicity, by global standards, and long-term examination of both the instrumental and historical data is required for seismic risk assessment. Statistical tables of the occurrence frequency are produced, enabling more accurate calculations of seismic risk, that is, the expected amount of damage which may occur in a given period of time. This is a combination of seismic hazard, the level of ground motion which is expected due to seismic activity, and seismic vulnerability, the amount of damage experienced by a structure due to a given level of ground motion. These factors are considered when engineering for sensitive installations and appropriate precautions can then be taken to prevent damage.
The North Sea earthquake of 7 June 1931, with a magnitude of 6.1ML and with an epicentre offshore in the Dogger Bank area (120 km NE of Great Yarmouth), is the largest known earthquake in the UK. The felt area encompassed most of Britain, E of Ireland, the Netherlands, Belgium, N France, parts of NW Germany, Denmark and SW Norway. Damage in Britain was reported from 71 different places, with the strongest effects at Filey, where the top of a church spire was rotated. Bridlington, Beverley and Hull were also affected, with most of the damage affecting chimneys and plaster. A factory roof is reported to have collapsed at Staines (Surrey) and rocks or cliff collapse occurred at Flamborough Head and Mundesley, Norfolk. The earthquake was reported felt by a number of vessels in the North Sea and a woman in Hull died of a heart attack, apparently as a result of the earthquake.
The 19 July 1984 Lleyn event of North Wales, with a magnitude of 5.4 ML, was the largest onshore earthquake this century in the UK and was felt over an area of around 240,000 square kilometres. The earthquake occurred in the lower crust at a depth of approximately 22 km and was followed by many aftershocks. Detailed mapping of the aftershock distribution highlighted a plane orientated WNW-ESE and dipping steeply NNE. This represents the fault plane and corresponds well with one of the planes of the mainshock focal mechanism. There is, however, no surface fault or feature which corresponds to this plane.
The maximum intensity in the epicentral area was 6 EMS (European macroseismic scale) and damage consisted of widespread cracks in plaster and falls of some chimneys and weak plaster. High intensities of 5 and 6 EMS reported from Liverpool appear to be due to the state of repair of some of the buildings.
The 2 April 1990 Bishop's Castle earthquake in the Welsh Borders, with a magnitude of 5.1 ML, was the second largest onshore earthquake in recent years and was felt over an area of approximately 140,000 square kilometres. It occurred at a depth of 14 km and the maximum intensity in the epicentral area was 6 EMS. Damage was minor, including cracks and fall of parts of chimney and plaster and was limited to the epicentral area, north to Wrexham and especially Shrewsbury. Only six aftershocks followed the mainshock, suggesting an almost total release of strain energy following the mainshock.
The Colchester earthquake of 1884, with a magnitude of 4.6 ML, was the most damaging earthquake in the UK for several centuries. There was considerable damage to churches, including the top of a spire falling, falling masonry from roofs, falling turrets and parapets. The maximum intensity in the epicentral area was 8 EMS. Damage to residential properties included shattering of brick walls, and chimney falls, often through roofs.
YES. Seismicity distribution for mainland and offshore UK is neither random nor uniform in density, with more frequent and larger events occurring on the west coast. In Scotland, most of this activity is concentrated between Ullapool and Dunoon with centres near the Great Glen and clusters of activity at Comrie. North Wales, especially around Caernarvon and the Lleyn Peninsula, and the Welsh border area also show higher levels of seismicity. The NE of Scotland and the SE of England are, in contrast, areas of low seismicity, although examination of the historical record shows that NE Scotland, around Inverness, and SE England were both active. Areas like Aberdeen and Caithness have, however, always been quiet. Offshore, in the North Sea, there is a clear correlation of epicentres with the major structures, the Viking and Central grabens, the Norwegian coastal region and with the NE Atlantic passive margin. The master basin-bounding faults are, therefore, currently active. The stuctural highs in this region are, in contrast, relatively aseismic, for example the West Shetland Platform and the Mid North Sea High.
Earthquakes occur in the crust where deformation is by brittle fracture. Beyond a 'transition zone' earthquakes are no longer possible and plastic deformation occurs. Onshore UK seismicity generally occurs within the seismogenic zone, to mid-crustal depths. However, the activity on the Lleyn Peninsula following the 1984 mainshock and subsequent aftershock series occurred at depths of around 22 km in the lower crust. Other well constrained deep activity has occurred in the Welsh Borders around Newtown, suggesting a lowering of this brittle-ductile transition zone. In contrast, shallower than average focal depths of around 6 km are obtained for Cornwall where radiogenic granites are responsible for the highest heat flow in the UK. Variation in the cut-off depth for crustal seismicity is thought to be due to a combination of heat flow and chemical/mineralogical differences (decreased quartz levels) in the crustal rocks.
In areas of high seismicity and dense monitoring, for example along the San Andreas fault complex, major faults can be mapped at the surface and often correlated with specific earthquakes. Occasionally major earthquakes can occur on previously unknown 'blind' faults with no surface representation, as with the 1994 Northridge earthquake. It is more difficult, if not impossible, to identify the causative faults in areas of low seismicity. The length of the fault involved in generating small magnitude events need only be of the order of a few hundred metres and the faults generally show no related surface features. Location errors for the calculated hypocentre also need to be considered. These vary according to the magnitude of the event, and the station density. If the earthquake occurred offshore or near the coast, there is an asymmetrical distribution of the monitoring stations and correspondingly much greater location errors.
Onshore, surface geological maps are highly detailed for the UK showing an abundance of mapped faults. For a given epicentre, if surface fault density is high and location errors are large, the error 'circle' can encompass many possible causative faults. The causative fault may be listric in nature, shallowing with depth, and extrapolation between the focus at depth and any surface feature vertically above would not be relevant. Deeper earthquakes in the mid-lower crust may occur on faults that have no connection to the surface and, therefore, no related surface feature.
Two of the main tools for obtaining further hypocentral parameters are focal mechanism studies and spectral analysis. The former, involves mapping the pattern of dilatations and compressional P-wave first arrivals which plot in 4 quadrants, separated by a pair of focal planes, one of which represents the fault plane. The focal mechanism provides information on the type of fault movement and the local stress regime operational. Spectral analysis of the recorded ground motion involves plotting the spectral level against the frequency for the seismic wave spectrum and provides an indication of the size of the radius of the circular fault plane, the seismic moment and moment magnitude (Mw).
YES. Eleven people are known to have died as the result of British earthquakes.
Six were killed by falling stones, two fell from upper floors, two died of shock
and one committed suicide. Details are summarised below:
Magnitudes are ML (Richter local magnitude); where estimated from macroseismics, in some cases, they are only given as approximate values.
It is a measure of earthquake size and is determined from the logarithm of the maximum displacement or amplitude of the earthquake signal as seen on the seismogram, with a correction for the distance between the focus and the seismometer. This is necessary as the closer the seismometer is to the earthquake, the larger the amplitude on the seismogram, irrespective of the size or magnitude of the event. Since the measurement can be made from P, S or surface waves, several different scales exist, all of which are logarithmic because of the large range of earthquake energies (for example a magnitude 6 ML is 30 times larger, in terms of energy than a magnitude 5 ML). The Richter local magnitude (ML) is defined to be used for 'local' earthquakes up to 600 km away, and is the magnitude scale used by BGS when locating UK earthquakes.
Surface wave magnitude (Ms) is based on the maximum amplitude of the surface wave having a period of 20 + 2 s. It is used for observations near the earthquake epicentre where the surface wave is larger than the body wave. This scale applies to any epicentral distance or type of seismograph.
Body wave magnitude (mb) is calculated from the body waves (P,PP,S) and are usually used at larger distance from the earthquake epicentre (P-wave attenuation is less than surface waves, with distance). It can be used for any earthquake of any depth.
Moment magnitude (Mw) is considered the best scale to use for larger earthquakes as the Ms saturates at about magnitude 8. Moment magnitude is measured over the broad range of frequencies present in the earthquake wave spectrum rather than the single frequency sample that the other magnitude scales use.
For comparison purposes, a magnitude 5 ML earthquake is equivalent to the explosion of 1,000 tons of TNT whereas a magnitude 6 ML earthquake is the energy equivalent of 30,000 tons of TNT or a 30 kilotonne nuclear explosion.
The Richter magnitude scale (ML), described above is the best known magnitude scale. Charles Richter developed it in the 1930s for use on earthquakes in southern California, using high-frequency data from nearby or 'local' stations. It is also the scale used by BGS to describe UK earthquakes when using our network of 140 monitoring local stations. Other magnitude scales include body-wave magnitude (mb), and surface wave magnitude (Ms). One of these three scales is generally used, depending on the frequency range and type of signal. Values for the magnitude of a given event may, therefore, vary according to the monitoring agency and preferred scale used. Although moment magnitude (Mw) is considered the most reliable measure of earthquake size, especially for the largest events, it is more difficult to routinely calculate and requires analysis of the frequency spectra of the earthquake.
Magnitude is a measure of earthquake size and remains unchanged with distance from the earthquake. Intensity, however, describes the degree of shaking caused by an earthquake at a given place and decreases with distance from the earthquake epicentre. We can, therefore talk about a magnitude 5.4 ML event with intensity of 6 EMS in the epicentral area, on the Lleyn Peninsula, but intensity 3 EMS at Carlisle. Magnitude measurement requires instrumental monitoring for its calculation, however, assigning an intensity requires a sample of the felt responses of the population. This is then graded according to the EMS intensity scale. For example, Intensity 1, Not felt, 2, Scarcely perceptible, 3, weak, felt by a few, up to 12 assigned for total devastation. Study of intensity and the production of isoseismal maps, contouring areas of equal intensity, is particularly important for the study of earthquakes which occurred prior to instrumental monitoring.
NO. There is no evidence that earthquakes are becoming more frequent, we are simply recording larger numbers, especially of small earthquakes. The number of larger events remains stable. As extensive world-wide monitoring networks continue to expand, more events are located each year. The table below details USGS data for the frequency of earthquakes since 1900:
FREQUENCY OF OCCURRENCE OF EARTHQUAKES
BASED ON OBSERVATIONS SINCE 1900
Although it is known that most global earthquakes will concentrate at the plate boundaries, there is no reliable method of accurately predicting the time, place and magnitude of an earthquake. Most current research is concerned with minimising the risk associated with earthquakes, by assessing the combination of seismic hazard and the vulnerability of a given area. Many seismic countries, however, have research programs based on identifying possible precursors to major earthquakes. This includes the study of dilatancy, how rocks crack and expand under the increased stress associated with the earthquake. Some major earthquakes, but not all, are heralded by the occurrence of foreshocks. which can be detected by dense local monitoring networks. Other instruments can measure changes in the levels of radon gas, electrical and magnetic properties, velocity changes of seismic waves and changes in topography. Long term monitoring and examination by these sensors is required as some or all of these factors may change due to the opening of cracks prior to the earthquake.
All attempts to predict earthquakes have, however, been generally considered as failures and it is unlikely that accurate prediction will occur in the near future. Efforts will, instead, be channelled into hazard mitigation. Earthquakes are difficult or impossible to predict because of their inherent random element and their near-chaotic behaviour