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EARTHQUAKE QUESTIONS AND ANSWERS   

Select from the list below:

What is an earthquake?
Where do earthquakes occur?
Which countries have the largest and most frequent earthquakes?
What is the biggest earthquake that has ever happened?
How many global earthquakes occur each year?
Do earthquakes occur in Britain?
Why do we monitor UK earthquakes?
What is the biggest earthquake Britain has ever had?
What are the largest two instrumental, onshore earthquakes?
What is the most damaging British earthquake?
Is there any pattern to UK seismic activity?
What limits the depth of earthquakes in Britain?
Can we identify the fault which triggered a British earthquake?
Have earthquakes caused deaths in Britain?
What is earthquake magnitude?
Why do we need more than one earthquake magnitude scale?
What is the difference between magnitude and intensity?
Are earthquakes on the increase?
Can earthquakes be predicted?
Glossary of terms used in seismology

What is an earthquake?

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.

 

Where do earthquakes occur?

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.

 

Which countries have the largest and most frequent earthquakes?

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.

 

What is the biggest earthquake that has ever happened?

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.

 

How many global earthquakes occur each year ?

NUMBER OF EARTHQUAKES PER YEAR MAGNITUDE 7.0 OR GREATER

(1900-1989)

YEAR NUMBER YEAR NUMBER YEAR NUMBER
1900 13 1930 13 1960 22
1901 14 1931 26 1961 18
1902 8 1932 13 1962 15
1903 10 1933 14 1963 20
1904 16 1934 22 1964 15
1905 26 1935 24 1965 22
1906 32 1936 21 1966 19
1907 27 1937 22 1967 16
1908 18 1938 26 1968 30
1909 32 1939 21 1969 27
1910 36 1940 23 1970 29
1911 24 1941 24 1971 23
1912 22 1942 27 1972 20
1913 23 1943 41 1973 16
1914 22 1944 31 1974 21
1915 18 1945 27 1975 21
1916 25 1946 35 1976 25
1917 21 1947 26 1977 16
1918 21 1948 28 1978 18
1919 14 1949 36 1979 15
1920 8 1950 39 1980 18
1921 11 1951 21 1981 14
1922 14 1952 17 1982 10
1923 23 1953 22 1983 15
1924 18 1954 17 1984 8
1925 17 1955 19 1985 15
1926 19 1956 15 1986 6
1927 20 1957 14 1987 11
1928 22 1958 10 1988 8
1929 19 1959 15 1989 7

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.

 

 

Do earthquakes occur in Britain?

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)

Magnitude(ML) Average
5.0 and higher every 8 yrs
4 - 4.9 every 2 yrs
3 - 3.9 3/yr
2 - 2.9 26/yr
1 - 1.9 140/yr

 

FREQUENCY OF OCCURRENCE OF NORTH SEA EARTHQUAKES (BASED ON OBSERVATIONS BETWEEN 1979 AND 1994)

Magnitude (ML) Average
4.0 and higher 1/yr
3.0 - 3.9 4/yr
2.0 - 2.9 25/yr

Why do we monitor UK earthquakes?

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.

What is the biggest earthquake Britain has ever had?

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.

What are the largest two instrumental, onshore earthquakes?

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.

What is the most damaging British earthquake?

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.

Is there any pattern to UK seismicity?

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.

What limits the depth of Earthquakes in Britain?

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.

Can we identify the fault which triggered a British earthquake?

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).

Have earthquakes caused deaths in Britain?

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:

Date Epicentre Magnitude Number of deaths, Place, Cause
6 Apr 1580 Dover Straits >6 2, London, falling masonry
15 Jul 1757 Penzance 4.5 1, Penryn, fell out window
7 Sep 1801 Comrie 4.5 2, near Edinburgh, falling masonry
18 Sep 1833 Chichester 3 1, Cocking, falling rock
22 Apr 1884 Colchester 4.5 1, Wivenhoe, shock, (uncertain)

1, Manningtree, suicide

1 Feb 1915 Conisbrough < 3 1, Conisbrough, Falling rock
7 Jun 1931 North Sea 5.6 1, Hull, Shock?
12 Dec 1940 Porthmadog 4.7 1, Criccieth, Fell downstairs

Magnitudes are ML (Richter local magnitude); where estimated from macroseismics, in some cases, they are only given as approximate values.

What is earthquake magnitude?

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.

Why do we need more than one earthquake magnitude scale?

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.

What is the difference between magnitude and intensity?

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.

Are earthquakes on the increase?

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

Descriptor Magnitude Average Annually
Great 8 and higher 1
Major 7 - 7.9 18
Strong 6 - 6.9 120
Moderate 5 - 5.9 800
Light 4 - 4.9 6,200 (estimated)
Minor 3 - 3.9 49,000 (estimated)
Very Minor 2 - 3.0 about 1,000 per day
Very Minor 1 - 2 about 8,000 per day

Can earthquakes be predicted?

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

Common Terms Used in Seismology (the study of earthquakes).

Aftershock An earthquake which follows a larger earthquake or main shock and originates at or near the focus of the larger earthquake. Generally, major earthquakes are followed by a larger number of aftershocks, decreasing in frequency with time.
Amplitude
The maximum height of a wave crest or depth of a trough.
Array
An ordered arrangement of seismometers or geophones, the data from which feeds into a central receiver.
Arrival
The appearance of a seismic wave on the seismic record.
Arrival time
The time at which a particular wave phase arrives at a detector.
Aseismic area
An area that is almost free of earthquakes.
Body wave
A seismic wave that travels through the interior of the earth and is not related to a boundary surface.
Crust
The outer layer of the Earth's surface.
Earthquake
Shaking of the earth caused by a sudden movement of rock beneath its surface.
Earthquake swarm
A series of minor earthquakes, none of which may be identified as the main shock, occurring in a limited area and time.
Elastic wave
Rock is an elastic material that when strained by normal external forces can return to its original state. When the strength of the rock is exceeded, the rock ruptures, generating elastic seismic or earthquake waves.
Epicentre
That point on the Earth's surface directly above the hypocentre of an earthquake.
Fault
A weak area in the Earth's crust where two sides of a fracture or fracture zone move relative to each other.
First arrival
The first recorded signal on a seismogram is the direction of the first P-wave, where upward ground motion is compressional and downward motion is dilatational.
Focus
The point where earthquake rupture or fault movement originates.
Foreshock
A small earthquake that may precede a larger earthquake or main shock and that originates at or near the focus of the larger event.
Frequency
The frequency of a wave (Hz) is the number of wave cycles per second.
Hypocentre
The calculated location of the focus of an earthquake.
Induced seismicity
Non-natural events induced by man's activity. These include mining induced events, events caused by loading of dams or pumping of water in geothermal areas.
Intensity
A measure of the effects of an earthquake at a particular place on humans and (or) structures. The intensity at a point depends not only upon the strength of the earthquake (magnitude) but also upon the distance from the earthquake to the epicentre and the local geology at that point.
Isoseismal line
A line enclosing points on the Earth's surface at which earthquake intensity is the same. It is usually elliptical in shape
Love wave
A major type of surface wave having a horizontal motion that is shear or transverse to the direction of propagation. It is named after A.E.H. Love, the English mathematician who discovered it.
Magnitude
A measure of the strength of an earthquake. There are several scales depending on which part of the seismogram is examined. These include Richter local magnitude (ML), Body wave magnitude (mb) and surface wave magnitude (Ms). Moment magnitude (Mw) is calculated from spectral analysis.
Mantle
The layer that lies between the crust and the core of the earth.
Microseism
A motion in the Earth that is unrelated to an earthquake. It is caused by a variety of natural and artificial agents, for example wave action, wind, traffic and industrial noise.
MSK
MSK intensity is the intensity scale used in Europe before the introduction of the EMS scale. It is a 12-grade scale ranging from not felt to complete devastation.
P wave
The first and faster of the body waves which moves by a series of compressions, similar to a sound wave. They can travel through both solid and liquid.
Phase
The onset of a displacement on a seismogram indicating the arrival of the different types of seismic wave.
Plate
One of the segments which make up the Earth's crust. The plates are continuously moving relative to each other.
Plate boundary
The place where two or more plates in the Earth's crust meet.
Prediction
Predicting the time, place and magnitude of an earthquake.
Rayleigh wave
A type of surface wave having a retrograde, elliptical motion at the free surface. It is named after Lord Rayleigh, the English physicist who predicted its existence.
Reflected wave
A wave that has turned back from a boundary or discontinuity in the earth's crust.
Refraction
The change in direction of a wave on reaching a boundary of different density and velocity.
Richter scale
A popular name for the local magnitude scale (See Magnitude).
S wave
The second arrival on a seismogram, the S wave, is slower than the P-wave. It is a shear wave and cannot travel through liquids.
Seismogram
A record of an earthquake or ground vibration. The wave trace is made up of P-waves, S-waves and surface waves, the pattern of onsets of the first two arrivals help to determine the location. The seismogram can be either a paper record or a digital record that is analysed by computer.
Seismograph
An instrument that registers the occurrence of an earthquake and the time it occurred as a written record.
Seismologist
A scientist who studies earthquakes.
Seismometer
An instrument that not only measures the time of the arrival of earthquake waves, but also allows the exact motion of the ground to be computed from the record.
Seismoscope
An instrument that registers the occurrence of an earthquake, but not the time.
Signal-to-noise ratio
The comparison between the amplitude of the seismic signal and the amplitude of noise caused by seismic unrest and (or) the seismic instruments.
Subduction zone
An elongated region along which a crustal plate descends relative to another crustal block, for example, the descent of the Pacific plate beneath the South American plate.
Surface waves
Seismic waves with motion restricted to near the ground surface (Love and Rayleigh)
Teleseism
An earthquake that is distant from the recording station.
Travel time
The time required for a wave train to travel from its source to a point of observation.
Tsunamis
A huge sea wave caused by earthquakes. (Referred to by many as a tidal wave.)
Volcanic earthquake
Earthquakes associated with volcanic activity.
Wavelength
The distance between two successive crests or troughs of a wave.

 

 
(Edited from www.gsrg.nmh.ac.uk/~phoh/faq1.htm)
 
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