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TECTONICS | Earthquakes☆
TECTONICS | Earthquakes☆ GJH McCall{, Cirencester, Gloucester, UK ã 2013 Elsevier Inc. All rights reserved.
Introduction The Nature of Earthquakes The Importance of Seismological Records The Global Distribution of Earthquakes Earthquakes as a Major Hazard: Tectonic, Volcanic, and Man-Made Earthquakes Tectonic Earthquakes Secondary effects Mitigation Research into earthquakes Volcanic Earthquakes Man-Made Earthquakes Moonquakes and Seisms on Other Planets Examples of Recent Global Earthquakes Sumatra-Andaman earthquake Pakistan 2005 Pakistan 2008 L’Aquila, Italy, 2009 Haiti 2010 Christchurch, N.Z., 2011 Tohoku 2012
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Introduction Earthquakes have many and diverse relationships with other Earth processes, and their study has a wide range of possible applications. Earthquakes will be considered here under the following headings: 1. 2. 3. 4.
The nature of earthquakes, The importance of seismological records, The global distribution of earthquakes, and Earthquakes as a hazard – tectonic, volcanic, and man-made earthquakes.
The Nature of Earthquakes An earthquake is a sudden movement of the Earth’s surface, caused by a release of strain built up over long periods on faults. The rocks are elastic and can store energy in the same way as a compressed spring. Earthquakes are focused on faults in the rock mass. Most have foci within the crust but a few, in plate boundary zones and beneath stable cratonic areas (where they are related to events in nearby subduction zones), have foci at great depths, down to about 700 km, in the mantle; beyond this depth the rock mass is insufficiently rigid to rupture. The very deep earthquakes are not well understood. Most earth- quakes have foci less than 30 km deep. Not all the built-up strain is relieved by earthquakes; much of it is relieved by continual small adjustments, a process of creep. However, where friction prevents such accommodation, the strain builds up until something has to give, and there is a sudden rupture of the weakest part of the solid rock, the forces being accommodated by sudden dislocation of the rocks on either side of the fault plane (Figure 1). This process can happen on all three types of fault: normal, reverse, and transcurrent. The point directly above the focus is the called the epicenter; here, the effects of the earthquake will be greatest. If the focus is shallow, the effects will be greater than if it is deep – the 1960 Agadir earthquake, Morocco, was not of great magnitude, but it was very shallow and the epicenter was right under the city. The magnitude is a measure of the amount of energy released by the earthquake. It is calculated from the size or amplitude of the waves traced by the pen of a seismograph, an instrument that picks up the waves at some distance from the epicenter and records them in the form of a wavy trace on a rotating drum (Figure 2) coupled to a clock. The principle of the seismograph is illustrated by ☆
Change History: June 2013. GJH McCall added the section ‘Examples of Recent Global Earthquakes’.
{
Deceased.
Reference Module in Earth Systems and Environmental Sciences
http://dx.doi.org/10.1016/B978-0-12-409548-9.03017-7
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Figure 1 Block diagram showing the relationship between an earthquake focus, epicenter, and fault.
Figure 2 Record on three seismographs of the Spitak 1988 main shock (magnitude 6.9). Reproduced with permission from Rommer and Ambraseys (1989). Earthquake Engineering and Dynamics. Chichester: John Wiley & Sons. © John Wiley & Sons Limited.
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a chandelier that swung in the great Lisbon earthquake of 1755 – a freely pivoting horizontal strut is attached to an upright support (Figure 3). A heavy mass at the strut end is attached to the pen, which traces a continuous line on the paper. Most of the time there are no ground movements, so the trace is horizontal. All seismographs have to be standardized so that valid comparisons can be made between their traces. Waves weaken as they travel outwards from the earthquake focus, so allowance has to be made for distance between the focus and the instrument (this can be calculated by measuring the difference between the arrival times of P and S waves). Three seismographs are required to measure north–south and east–west horizontal movements and vertical movements. The Worldwide Standardized Seismograph Network was established in 1962. All the instruments are standardized as if they were situated 100 km away from the focus. Waves produced by earthquakes spread through the Earth (Figure 4). They comprise body waves and surface waves. Body waves travel through the Earth and are of two types, primary (P) and secondary (S) waves. P waves can pass through both solid and molten material within the Earth’s interior; they travel fastest and are the first to arrive at a given location, and they are also the first to be felt by the man in the street. They are longitudinal or compressional waves, vibrating forwards and backwards in the direction of travel. They travel at about 6 km s 1 through continental crust and 8 km s 1 through oceanic upper mantle. They may produce booming sound waves in the atmosphere. S waves travel about half as fast as P waves (3.6 km s 1 and 4.7 km s 1 in continental crust and oceanic upper mantle, respectively); they cannot pass through fluids and thus do not penetrate the liquid outer core. They are shear or transverse waves: as they pass through the rock they move particles both from side to side and up and down, at right angles to the direction of travel. Two kinds of surface waves, which travel just below the surface, are called Love and Rayleigh waves. They arrive shortly after the body waves. Love waves travel faster than Rayleigh waves and push the rock particles sideways, at right angles to the direction of travel. Like S waves, they shear buildings and constructions sideways, causing immense damage, but have no vertical motion. The slowest waves, Rayleigh waves, push particles upwards and backwards; the particles move in the vertical plane, following an elliptical path as the wave passes by. Charles Richter in 1935, working in California, devised the Richter Scale of magnitude, in which the absolute strength at the focus can be calculated on a logarithmic scale: a rise of one unit of magnitude represents a tenfold increase in absolute strength (i.e., a magnitude 5 earthquake is 10 times a strong as a magnitude 4 earthquake). The difference in energy release is even greater – an increase of one unit of magnitude represents 30–32 times as much energy being released. Theoretically, earthquakes with magnitudes of more than 10 could occur, but the greatest magnitude so far measured for any earthquake is about 9.5. The Richter scale is given in Table 1. It has been superseded as a scale for measuring the comparative intensity at the focus by the moment magnitude scale, but the principles are the same – the moment magnitude scale allows more refined methods of comparison.
Figure 3 The components of a seismograph designed to record vertical ground movement.
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Figure 4 The way in which earthquake waves spread through the globe and are reflected at boundaries, returning to the surface. Measurement of the speed of such return is used to delineate the materials of the inner Earth according to density and physical state. Reproduced from Van Andel, T.J. (1994). New Views on an Old Planet. Cambridge: Cambridge University Press.
Table 1 Magnitude 0–1.9 2–2.9 3–3.9 4–4.9 5–5.9 6–6.9 7–7.9 8–8.9
The Richter scale Qualitative description
Average number per year
Average intensity equivalent close to epicenter
Minor Light Moderate Strong Major Great
700 000 300 000 40 000 6200 800 120 18 1 every 10–20 years
I–V; recorded but not felt I–V; recorded but not felt I–V; felt by some I–V; felt by many V–VII; slight damage VII; damaging IX–IX; destructive XII; widely devastating
The Importance of Seismological Records We cannot directly study the rocks of the crust below the limits of borehole drilling (a few kilometers), though ancient rock systems do expose sections of the ancient deep crust (as in the Kapuskasing Belt, Ontario, Canada) and perhaps even the crust–mantle contact (as in Oman and Western Newfoundland). The behavior of earthquake waves, however, provides us with invaluable evidence about the nature of the lower crust, mantle, and core because the velocities of P and S waves are functions of the density of the material through which they pass. Knowledge of rock density can tell us much about the physical state of the materials deep within the Earth, and the behavior of S waves tells us that the outer core is molten. In Figure 5, the different densities of common rock materials are plotted against the P-wave velocity. It is fair to say that earthquake waves form the basis of our knowledge of the mantle and core. Artificially produced seisms can also be picked up by seismographs, and, thus, nuclear explosions can be globally monitored. The explosion in the submarine Kursk in 2000 was picked up by distant seismograph stations in Africa, and this provided valuable evidence of what happened. Tomographic methods have recently been developed, producing three-dimensional images of the deep Earth, including subducted slabs of crust, using a technique akin to the use of tomography in medicine. Earthquakes occur in sequences: slight foreshocks may give warning of a major earthquake, and aftershocks occur for long after the main shock. Foreshocks and aftershocks are generally of lower intensity than the main shock, but sometimes very large shocks
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Figure 5 The different densities of common rock types plotted against P-wave velocity.
Figure 6 Surface trace of the Meckering 1968 earthquake (magnitude 6.9) in Western Australia. Reproduced from Everingham, I. (1968). Preliminary report on the 14th October 1968 earthquake at Meckering, Western Australia. Record 1968/142. Canberra: Bureau of Mineral Resources, Geology and Geophysics.
occur, as in the 1999 Izmit earthquake in Turkey. In the 1988 Spitak earthquake (magnitude 6.9) an aftershock of magnitude 6.2 occurred 4 min after the main shock. Earthquakes may produce a trace of the rupture on the land surface, dislocating the land for many kilometers. In Figure 6 the trace produced across wheat fields by the Meckering 1968 (magnitude 6.9) earthquake is shown. Such traces are invaluable in studying the sense of the movement and displacement.
The Global Distribution of Earthquakes Earthquakes do not occur to the same extent all over the globe. The major events are largely concentrated at the boundaries of tectonic plates, and the concentration and magnitude are greater in zones of plate convergence (subduction and collision) than in zones of plate divergence (mid-ocean ridges and rift valleys). This distribution is clearly shown in Figure 7.
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Figure 7 The global distribution of earthquakes that occurred in 1994. Reproduced from US National Earthquake Information Center.
Not all earthquakes occur on plate boundaries, however: the destructive Killari earthquake in India in 1993 occurred within a stable cratonic area. The immensely destructive Lisbon earthquake of 1755 was also nowhere near a plate boundary.
Earthquakes as a Major Hazard: Tectonic, Volcanic, and Man-Made Earthquakes The most damaging earthquakes are not necessarily of high magnitude. The 1994 Kobe earthquake, one of the most destructive and costly in living memory (55 000 houses destroyed as well as freeway, rail, and port installations), had its epicenter 20 km from the city and had a magnitude of only 6.8. The 1960 Agadir earthquake had a magnitude of only 5.8, but the focus was shallow and right beneath the city. Thus, magnitude, though a valuable absolute measurement, tells us little about the degree of damage and the loss of property and life, even at the epicenter. The nature of the subsurface rocks can have a significant effect, especially if shock-induced liquefaction occurs, and thus we need another measurement scale. The Mercalli intensity scale measures the relative intensity of the effects felt at a specific site (the intensity will commonly decrease away from the epicenter, but secondary effects such as subsurface variation and shock liquefaction complicate this relationship). In Europe, a modification of the Mercalli scale, the MSK scale (named after Medvedev, Sponheuer, and Karnik), is now used. This scale is given in Table 2.
Tectonic Earthquakes Earthquakes may be divided into tectonic earthquakes, volcanic earthquakes, and man-made earthquakes. In considering the natural-hazard aspect, it is the tectonic earthquakes that are by far the most destructive natural hazards. This hazard largely affects urban populations, and human design and construction has a unique role in mitigating this hazard. The actual physical process of ground motion presents little threat to humans in the open: most casualties (other than the casualties of secondary tsunamis) occur inside buildings that partially or totally collapse. The correct design of buildings and constructions such as bridges and viaducts can thus greatly mitigate the damage and casualties resulting from an earthquake. The vulnerability of a building to earthquake damage varies according to many factors. Vertical ground motion is the principal damaging component causing collapse, burial of people, and death. Lateral ground motion breaks or deforms power lines, pipelines, water pipes and sewers, roadways, railways (Figure 8), and bridges. Quite small lateral offsets can be very damaging. In the Mexico City earthquake of 1985, much damage was caused by adjacent high rise buildings swaying with different wave motions and knocking each other down. It is noticeable that in Beijing, an earthquake-prone city, the high-rise buildings are widely spaced, with intervening areas of low-rise buildings, so that they cannot interact in this way. In the case of Kobe, the sixth floor of one high-rise building pancaked (Figure 9). The lower floors were built of steel-encased reinforced concrete and the upper floors of pure reinforced concrete; the junction on the sixth floor acted as an element of weakness.
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Table 2
The modified Mercalli (MSK) intensity scale
Intensity
Effects
I II III
Felt rarely. Sometimes dizzyness and nausea. Birds and animals uneasy. Trees, structures, liquids sway. Felt indoors by a few, especially on upper floors. Delicately suspended objects swing. Felt indoors, especially on upper floors by several people. Usually rapid vibration as if a lightly loaded lorry passing.Hanging objects and standing motor cars rock slightly. Felt indoors by many and outside by a few. Some awakened. No-one usually frightened. Sensation of heavy object striking building. Vibration as of heavy lorries passing. Crockery, windows, doors rattle. Walls and frames creak. Hanging objects and standing motor cars sway. Felt indoors by almost everyone, outdoors by most people. Many awakened, a few frightened and run outdoors. Buildings tremble. Crockery and windows sometimes break. Pictures and doors clatter. Small objects move. Some liquids spilt. Clocks stopped. Trees shaken. Animals anxious. Felt by all indoors and outdoors. Many frightened, some alarmed. All awakened. People, trees, bushes shaken. Liquids set in motion. Small bells ringing. Crockery broken. Plaster cracks and falls. Books and vases fall over. Some furniture moved. Domestic animals try to escape. Minor landslides on steep slopes. All frightened, run out of doors, general alarm. Some people thrown to ground. Trees shaken quite strongly. Waves and mud stirred up in lakes. Sandbanks collapse. Large bells ring. Suspended objects quiver. Much damage to badly constructed buildings and old walls. Slight damage to well-built buildings. Chimneys crack. Much plaster, tiles, loose bricks fall. Heavy furniture overturned. Concrete ditches damaged. Alarm approaches panic. Vehicle drivers disturbed. Trees broken and shaken. Sand and mud spurt from the ground. Marked changes to springs and wells. Much damage to ordinary and older buildings. Walls, pillars, chimneys, towers, statues, gravestones crack and fall. Very heavy furniture overturned. General panic. Ground cracked open (10 cm). Much damage to structures built to withstand earthquakes. Frequent partial or total collapse of other buildings. In reservoirs, underground pipes broken. Buildings dislodged from foundations, rock falls. Widely cracked ground, fissures up to 1 m wide. Frequent river bank and coastal landslides and shifted sands. Water levels change. Water thrown onto riversides. Serious damage to dams, embankments, bridges. Severe damage to well-built wooden structures. Masonry structures destroyed along with their foundations. Rails bent. Open cracks or waves on roads. Pipes torn apart. Widespread serious ground disturbance, broad fissures, landslips, landslides. Muddy water spurts upward. Tsunamis develop. Severe damage to all wooden structures. Great damage to dams. Few masonry structures remain upright. Pillars of bridges and viaducts wrecked. All pipelines wrecked. Rails badly bent. Total damage to all constructions. Great disturbance to ground with many shearing cracks. Many landslides on slopes, rockfalls common, rock masses dislocated, water channels altered and dammed. Ground surface waves like water and ground remains undulating. Objects thrown into the air.
IV
V
VI
VII
VIII
IX X
XI
XII
Though earthquakes are mainly an urban hazard, catastrophic earthquakes may strike village populations where low-rise housing is substandard – as in the case of the Cairo earthquake in 1992 and the Killari, central India, earthquake in 1993. In the case of the Cairo earthquake, poorly constructed extra storeys had been added to moderate-rise housing. In Killari, stone-built lowrise houses were poorly constructed (Figure 10). In the catastrophic Bam earthquake of December 2003, the mud bricks of the lowrise dwellings crumbled and collapsed, leaving few air spaces to allow buried victims to breathe; another factor responsible for the scale of the fatalities was the fact that all the dwellings in southern Iran have basements cooled by wind towers designed for the sweltering summer heat, and many victims would have been asleep in them. Some of the most devastating historic and recent earthquakes are listed in Table 3. Where cities are situated in plate-boundary zones the effects are most disastrous. The San Francisco earthquake of 1906 (Figure 11) provides an example of this. The Kobe earthquake of 1995, in which the financial loss was US $200 billion, occurred in a city that experiences a tremor every few days. An analysis of the locations of 100 of the largest cities in the world, which accommodate 10% of the global population, shows that they can expect to experience an earthquake of intensity VI or more on the MSK scale within 50 years. The earthquake hazard extends beyond high-risk cities such as those sited on plate boundaries. Entire countries may be at quite low risk, yet have some vulnerability. The UK is a low-risk country, and earthquakes of more than magnitude 5.5 are extremely unlikely (Figure 12). Charles Davidson published a list of 1191 recorded shocks in Britain between AD 974 and AD 1924. In Lincoln in 1185, “great stones were rent; houses of stone fell; the metropolitan church of Lincoln was rent from top to bottom” and there is a similar report from the cathedral city of Wells in 1248. Two apprentices were killed in London in 1580 as a result of an earthquake in the Dover Straits. The Colchester earthquake in 1884 (magnitude 4.7) peaked in intensity near the epicenter (between Pelden and Langenhoe; Figure 13) at MSK VIII and caused widespread damage, which was compensated by a Mansion House Fund that paid out £9000 (equivalent to £500 000 today). The Roermond earthquake in the Netherlands in 1998 (magnitude 5.8) caused £30–40 million of damage, despite the single fatality. It is predicted that a magnitude 5.7 earthquake focused at a depth of 5 km directly under Manchester would cause havoc. The increasing size of conurbations and cities increases their vulnerability: the Colchester area would suffer much more nowadays from a comparable earthquake to the 1884 event, because the population and industry are now much denser than at that time.
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Figure 8 A railway track in the western USA twisted and shortened by lateral motion during an earthquake.
Figure 9 High-rise buildings in Kobe after the 1995 earthquake, showing the sixth floor pancaked by vertical motion. Reproduced from Esper, P. and Tachibana, E. (1998). The Kobe earthquake. In: Maund, J.G. and Eddleston, M. (eds.) Geohazards in Engineering Geology, pp 105–116. Engineering Geology Special Publication 15. London: Geological Society.
Earthquakes can occur in areas that are not considered to be at risk. The Spitak earthquake in Armenia in 1988 is such a case. The region was not considered to be at high risk, and a nuclear power station was planned for the Spitak area. This earthquake caused the whole process of earthquake risk assessment in the Soviet Union to be revised. The New Madrid, Missouri, earthquakes of 1811–1812 are even more surprising. There is a reliable historical record of three earthquakes spaced over 2 months with magnitudes 8.2, 8.1, and 8.3. They rang the bells of Boston and rattled Quebec and provide a remarkable example of major interplate seismicity.
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Figure 10 (A) Damage to a low-rise poorly constructed stone building of the type affected by the Killari, India, 1991 earthquake (photograph National Geophysical Data Center USA). (B) Improved training in building similar low-rise buildings in the Yemen. Reproduced from Degg, M.R. (1995). Earthquakes, volcanoes and tsunamis: tectonic hazards in the built environment of southern Europe. Built Environment 21, 94–113. London: Courtesy of the Geological Society.
Secondary effects The secondary effects of earthquakes can be as destructive and lethal as the primary effects, or more so. Tsunami The so-called tidal wave generated by earthquakes is probably the most lethal secondary effect: during the twentieth century earthquakes in Chile caused fatalities in Hawaii and Japan. The effects of tsunamis may be felt thousands of kilometers from the earthquake epicenter, but can be mitigated by systematic warning systems. Fire Tokyo in 1923, San Francisco in 1906, and Kobe in 1995 all suffered from the secondary effects of fire. This was exacerbated by the fact that water supplies were cut off. In Tokyo there was a firestorm. In San Francisco, 70% of the damage was due to fire. Liquefaction of sands, silts, and clays Another important secondary effect is that thixotropic sands and silts, which liquefy on shock, greatly increase the damage: examples of this are the waterfront area in the Messina earthquake, Sicily, of 1908, in which 98% of the houses were ruined and 160 000 died; Mexico City in 1985, where the worst damage was in building developments founded on old lake deposits (the wave amplitude was magnified 8–50 times here); and Anchorage, Alaska, where a magnitude 8.4 earthquake with an epicenter 130 km away caused devastation in a housing development founded on the thixotropic Bootlegger clay formation (Figure 14). Here, the risk was well known but there was a lack of communication between the geologists and the planners. Landslides and rock falls Very damaging landslides or rock falls can be triggered by earthquakes and may occur sometime after the main shock or aftershocks. In Montana in 1985, 30 million tonnes of rock were set in motion, fatally burying 36 people at a camp site. In Peru in 1970, rock and ice slides triggered by an earthquake killed 20 000 people.
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Table 3 Some important earthquakes in the last 2000 years (various sources): note that magnitudes are on various scales. An earthquake at Gujarat, India, in 2001, which killed more than 50 000 people has been omitted from the table Year AD
Place
Casualties
342 454 565 856 1170 1290 1456 1556 1716 1737 1755 1755 1759 1783 – 1786 1797 1822 1828 1896 1897 1906 1908 1915 1920 1923 1932 1933 1933 1935 1939
Antioch Sparta Antioch Corinth Sicily Chihli, China Naples Shensi, China (M 8.3?) Algiers Calcutta North Persia Lisbon (M 6.9?) Baalbek Calabria
40 000 20 000 30 000 45 000 15 000 100 000 60 000 830 000 20 000 300 000 40 000 60 000 20 000 50 000 41 000 22 000 34 000 28 000 1542 700 160 000 30 000 180 000 143 000 70 000 120 3000 60 000 30 000
1948 1960 1960 1964 1970 1971 1975 1976 1976
Quito Aleppo Honshu Sanriku, Japan Assam (M8.7) San Francisco (M 7.9?) Messina, Reggio (M 7.5) Avezzano, Italy (M 7.0) Kansu, China (M 8.5) Tokyo (M 8.2) Kansu, China (M 8.5) Long Beach, CA Sanriku, Japan Quetta (M 7.5) Concepcion, Chile (M 8.5) Erzincian, Turkey (M 8.0?) Soviet–Iran border Agadir (M 9.5?) Chile (M 9.5?) Anchorage (M 8.4) Peru (M 7.9) San Fernando, CA Haicheng, China (M 7.5) Guatemala (M 7.5) Tangshan, China (M 7.7)
1985 1988
Mexico City (M 8.1) Spitak (M 6.9)
19 000 60 000 10 000 114 80 000 64 1328 22 000 240 000 (some estimates are as high as 850 000) 10 000 30 000
1989 1989 1990 1991 1992
Loma Prieta, CA (M 7.1) Newcastle, Australia Northwest Iran (7.7) Killari, India Cairo (M 5.5–5.9)
63 10 40 000 10 000 <500
1994 1995 1999 2003
Northridge, CA (M 6.7) Kobe, Japan (M 7.2) Izmit, Turkey (M 7.6) Bam, Iran
60 5429 >17 000 45 000
1939
40 000
Estimated loss
6 major towns and all villages in 30 000 sq. miles leveled US$ 400 million 98% of houses ruined Vast landslides Firestorm killed 38 000, 25 000 houses destroyed US$ 50 million 8800 houses destroyed by tsunami
30 000 dwellings destroyed
58 600 houses destroyed Devastating rock and ice falls US$ 1 billion
Vast damage
US$ 4 billion US$ 14 billion; accompanied by landslides and rockfalls US$ 7 billion US$ 7billion US$ 8 billion Immense destruction of village housing 40 000 homeless; may have been due to construction of Aswan Dam US$ 20 billion US$ 200 billion Immense destruction Immense destruction of modern city and ancient citadel destroyed
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Figure 11 Gross displacement of a large building in the San Francisco 1906 earthquake.
Figure 12 The magnitudes and locations of earthquakes in Great Britain greater than magnitude 3 after 1700 and greater than magnitude 4 before 1700. Reproduced from Musson, R. (1996). British earthquakes and the seismicity of the UK. Geoscientist 16, 24–25.
Disease The risk of disease is a major concern after earthquakes, particularly in hot climates. Water supplies may be cut off and the populace may resort to using polluted supplies of water mixed with sewage and drainage effluents, which may be contaminated by corpses. Starvation Normal food supplies may be cut off and transport arteries may be blocked, so it is important to bring in food supplies immediately from the world outside.
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Figure 13 Intensity plot of the Great Colchester Earthquake of 1884 using the MSK scale. Reproduced from Musson, R., Neislon, G., and Burton, P.W. (1990) Microseismic Reports on Historic British Earthquakes XIV: 22 April 1984 Colchester. BGS Seismology Report W1/90/33. Edinburgh: British Geological Survey.
Figure 14 Destruction of the housing development at Turnagain Point above the Bootlegger Clay, Anchorage, 1994. The bluff moved 606 m towards the bay, and 75 homes were destroyed. Sand lenses in the clay lost strength (photograph National Geophysical Data Center, USA).
Exposure The 1988 Spitak earthquake in Armenia illustrates the problem of exposure. Many people lost their dwellings and were living in the open in very cold December climatic conditions. The need for tents, warm clothing, and blankets was urgent. Looting Looting is prevalent after earthquakes.
Mitigation The potential for mitigation of the earthquake hazard is limited. The main ways of mitigating the hazard are through good building and constructional design, planning development away from at-risk areas, and warning. However, warning is a very difficult matter. Research into earthquakes is at an interim stage, and the scientific community is at present by no means in consensus about the physical processes involved. There is the problem of how threatened populations react to warnings: if the event fails to occur, especially more than once, the population may not heed future warnings; alternatively, giving a warning to evacuate may engender panic. In countries with controlled political systems, such as China, warning and evacuation may be easier than in a democratic Western country. The Chinese did evacuate the city of Haicheng twice in 1974 and 1975, on 4 December and again on 4 February, based on seismology, community monitoring levels, radon gas in water, water temperature, tiltmeters, magnetometer readings, and
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patterns of animal behavior. An earthquake of magnitude 7.3 struck at 7.36 Am on 4 February. However, the great Tangshan earthquake of 1976 struck without any prediction or warning and killed at least 240 000 people (possibly many more). Millions of US dollars have been spent on research into earthquakes in California, and some improvement in prediction has been achieved, yet the existence of the Northridge Fault, the site of the 1994 earthquake, which killed 60 people and caused 20 billion dollars worth of damage, was not even known before the event. The best mitigation procedure would be to have international teams ready with emergency supplies and equipment, trained personnel, and sniffer dogs, at a distance from earthquake-prone regions, ready to been flown in by plane and helicopter.
Research into earthquakes The most important research into earthquakes has involved statistical, geographical, geological, theoretical, and mathematical studies of seismicity. An example is a study by Lya Tuliani in Russia, which addressed problems of geodynamics and seismology, tectonosphere layering, and lithostructure in seismically active regions in order to develop risk estimates. The procedure involved mathematical data processing. It was claimed that this study allowed highly accurate prediction of the coordinates of high-risk sites. This statistical, mathematical, and office-based approach contrasts with research in the USA (which involves actually drilling down to earthquake foci on faults), research into the Boothiel lineament, the site of the New Madrid earthquakes (which has revealed sand boils caused by the earthquake), and excavations in the alluvium of the rice paddies west of Beijing, China (where the actual earthquake fault of a seventeenth century event has been exposed cutting the clayey alluvium in open pits). Research has been carried out into much older earthquakes in Iran, based on the dislocation of qanats (horizontal wells). All these approaches and many more are invaluable, but the problem of predicting earthquakes is extremely complex and may never be completely solved.
Volcanic Earthquakes There can be a connection between major tectonic earthquakes and volcanic eruptions. In Chile in 1960, a major earthquake triggered the eruptions of several volcanoes, and in Sicily there is a record of Etna erupting a day or two before a major tectonic earthquake. However, the swarms of small seisms that usually precede volcanic eruptions (though there may be no such prelude) pose little threat to life and property. They do, however, provide valuable warnings of forthcoming eruption, and arrays of instruments are mounted on dangerous volcanoes for this purpose.
Man-Made Earthquakes Small seismic disturbances can be triggered by human activity. In the USA, the Boulder Dam and Lake Mead are constructed in a region that is highly strained; many small shocks have been correlated with changes in water depth. In Colorado, the injection of liquid wastes down boreholes has also been shown to trigger small seisms.
Moonquakes and Seisms on Other Planets Very small earthquakes do occur on the largely quiescent Moon when it undergoes maximum tidal stresses resulting from the attraction of the Earth and Sun. Similar stresses must operate on the Earth and cause minor seisms, but the effect is of no importance in such a dynamic body. Moonquakes and artificial seisms produced on the surfaces of other extraterrestrial bodies – Mars, Venus, and Mercury – can provide a valuable insight into their internal make up. A fascinating project would be to site an instrument from an unmanned spacecraft on Io, Jupiter’s volcanically active satellite, to obtain detail of its interior configuration – Io must be seismically active.
Examples of Recent Global Earthquakes Sumatra-Andaman earthquake This occurred off the SW coast of Sumatra on 26 December 2004. It was due to slip on the plate boundary between the Indian Ocean Plate and the Burma Plate and slippage occurred over a length of 1600 km and involved a vertical displacement of 15 m in addition to horizontal displacement. It had a moment magnitude of 9.1–9.3 and is energywise the third largest earthquake ever recorded. The hypocentre was in the Indian Ocean, just north of Simalue Island off the coast of Sumatra. The depth was 30 km. It cause devastation and great loss of life. There have been several large quakes since along this plate boundary, but none has generated a comparable tsunami.
Pakistan 2005 A major earthquake occurred on 8 October 2005, centered on Muzffarabad in Kashmir: this was of magnitude 7.2 on European plates. There were 74 698 deaths: the remote villages were difficult to reach by aid teams. It was followed 12 h later by another, comparable quake in magnitude and there were 978 aftershocks.
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Pakistan 2008 Another major earthquake occurred in Baluchistan on 29 October 2008, with an epicenter 60 km north of Quetta. The moment magnitude was 6.4. There were 30 000 deaths. This again was due to the interaction of the Indian and European pates.
L’Aquila, Italy, 2009 A powerful earthquake struck the town of L’Aquila in the Abruzzi, Italy and neighboring villages on 6 April 2009. More than 300 people died. The moment magnitude was 6.3, the epicenter L’Aquila and the depth 9 km. This is unique in that on 20 October 2012, six scientists were convicted of giving inexact, incomplete and contradictory information before the event (Nield, 2012). There is parallel with the Nevado del Ruiz eruption in Columbia in 1995 (Hall, 1992), where there was governmental interference on the issue of warnings.
Haiti 2010 A powerful earthquake on 12 June 2010, was caused by the North American plate moving past the Caribbean plate on a conservative plate margin – the two plates were moving in the same direction at different speeds. The magnitude was 7 on the Moment Magnitude scale. 316 000 people were killed, one million made homeless. 250 000 homes were destroyed and 30 000 other buildings. Many are still in temporary accommodation. Disease was rampant, especially cholera, in the rainy, tropical conditions and poor sanitation. The main prison was damaged, the inmates escaped and looting was another problem.
Christchurch, N.Z., 2011 A powerful earthquake struck New Zealand’s second city, on South Island, on 22 February 2011. The epicenter was 2 km west of Lyttleton, the port town, and 10 km SE of the city. It struck 6 months after another quake on the Darfield fault 40 km west of the city. There were 185 deaths and great destruction. The Pacific plate is being sub ducted below the Australian plate. The depth was 1–2 km and the movement strike-slip with some vertical component. The magnitude was 6.3 ML. There is some argument as to whether this was an aftershock of the earlier Canterbury earthquake which was of 7.1 magnitudes, but they seem to be regarded as separate events, with different epicenters and foci. There have been numerous aftershocks. The cathedral was severely damaged. Devastation was great, the city is still a demolition zone and reconstruction has yet to commence.
Tohoku 2012 This, the fourth largest earthquake ever recorded, energy-wise, was due to shallow thrust faulting on the plate boundary between the Pacific and Okhotsk plates: it involved a megathrust 300 km long by 200 km wide, part of the subduction zone at the boundary of the Pacific plate (Ritsema et al., 2012). 20 000 lives were lost, there were 300 000 refugees and a 20 km radius area was evacuated around the Fukushima Daishi nuclear reactor. The total cost is estimated at $300 billion. However, the main secondary effect was a huge tsunami. These ‘superquakes’ of the twenty-first century have revised our thinking, for both the Sumatra-Anadaman and Tohoku Earthquakes were larger than what scientists expected these fault systems to produce (Goldfinger et al., 2013). What a given fault system can produce remains poorly known.
Further Reading Bolt BA (1999) Earthquakes. New York: Freeman. Bommer JJ and Ambraseys NN (1989) The Spitak, Armenia, USSR earthquake of 7 December 1988: A summary engineering geology report. Earthquake Engineering and Structural Dynamics 18: 921–925. Chen Y, Tsoi KL, Chen F, et al. (1988) The Great Tangshan Earthquake of 1976. Oxford: Pergamon. Degg MR (1992) Some implications of the 1985 Maxican earthquake for hazard assessment. In: McCall GJH, Laming DJC, and Scott SC (eds.) Geohazards – Natural and Man-Made, pp. 93–114. London: Chapman and Hall. Degg MR (1995) Earthquakes, volcanoes and tsunamis: Tectonic hazards in the built environment of southern Europe. Built Environment 21: 94–113. Degg MR (1998) Hazard mitigation in the urban environment. In: Maund JG and Eddleston M (eds.) Geohazards in Engineering Geology, pp. 329–337. London: Geological Society, Engineering Geology Special Publication 15. Esper P and Tachibana E (1998) The Kobe earthquake. In: Maund JG and Eddleston M (eds.) Geohazards in Engineering Geology, pp. 105–116. London: Geological Society, Engineering Geology Special Publication 15. Everingham, I. (1968). Preliminary report on the 14th October 1968 earthquake at Meckering, Western Australia. Record 1968/142. Canberra: Bureau of Mineral Resources, Geology and Geophysics. Keller GR (2000) Seismic properties of rocks. In: Hancock PL and Skinner BJ (eds.) The Oxford Companion to the Earth. Oxford: Oxford University Press. McCall GJH (1996) Natural hazards. In: McCall GJH, de Mulder EFJ, and Marker BR (eds.) Urban Geoscience, pp. 81–125. Rotterdam: Balkema. McCall GJH (2000) The great Colchester earthquake of 1884 revisited. Geoscientist 10: 4–6. McCall GJH (2004) Remembering Bam. Geoscientist 14: 8–9. Menard HW (1974) Geology, Resources and Society. San Francisco: WH Freeman and Co.. Musson R (1996) British earthquakes and the seismicity of the UK. Geoscientist 16: 24–25. Musson, R., Neislon, G., and Burton, P. W. (1990) Microseismic Reports on Historic British Earthquakes XIV: 22 April 1984 Colchester. BGS Seismology Report W1/90/33. Edinburgh: British Geological Survey. Scarth A (1997) Savage Earth. London: HarperCollins. Tuliani LI (1999) Seismicity and Earthquake Risk: On the Basis of Thermodynamic and Rheological Parameters of the Tectonosphere. Moscow: Scientific World. Van Andel TJ (1994) New Views on an Old Planet. Cambridge: Cambridge University Press. Wong IG (2000) Earthquake mechanisms and plate tectonics. In: Hancock PL and Skinner BJ (eds.) The Oxford Companion to the Earth, pp. 287–289. Oxford: Oxford University Press.
Introduction The Nature of Earthquakes The Importance of Seismological Records The Global Distribution of Earthquakes Earthquakes as a Major Hazard: Tectonic, Volcanic, and Man-Made Earthquakes Tectonic Earthquakes Secondary effects Mitigation Research into earthquakes Volcanic Earthquakes Man-Made Earthquakes Moonquakes and Seisms on Other Planets Examples of Recent Global Earthquakes Sumatra-Andaman earthquake Pakistan 2005 Pakistan 2008 L’Aquila, Italy, 2009 Haiti 2010 Christchurch, N.Z., 2011 Tohoku 2012
1 1 4 5 6 6 9 12 13 13 13 13 13 13 13 14 14 14 14 14
Introduction Earthquakes have many and diverse relationships with other Earth processes, and their study has a wide range of possible applications. Earthquakes will be considered here under the following headings: 1. 2. 3. 4.
The nature of earthquakes, The importance of seismological records, The global distribution of earthquakes, and Earthquakes as a hazard – tectonic, volcanic, and man-made earthquakes.
The Nature of Earthquakes An earthquake is a sudden movement of the Earth’s surface, caused by a release of strain built up over long periods on faults. The rocks are elastic and can store energy in the same way as a compressed spring. Earthquakes are focused on faults in the rock mass. Most have foci within the crust but a few, in plate boundary zones and beneath stable cratonic areas (where they are related to events in nearby subduction zones), have foci at great depths, down to about 700 km, in the mantle; beyond this depth the rock mass is insufficiently rigid to rupture. The very deep earthquakes are not well understood. Most earth- quakes have foci less than 30 km deep. Not all the built-up strain is relieved by earthquakes; much of it is relieved by continual small adjustments, a process of creep. However, where friction prevents such accommodation, the strain builds up until something has to give, and there is a sudden rupture of the weakest part of the solid rock, the forces being accommodated by sudden dislocation of the rocks on either side of the fault plane (Figure 1). This process can happen on all three types of fault: normal, reverse, and transcurrent. The point directly above the focus is the called the epicenter; here, the effects of the earthquake will be greatest. If the focus is shallow, the effects will be greater than if it is deep – the 1960 Agadir earthquake, Morocco, was not of great magnitude, but it was very shallow and the epicenter was right under the city. The magnitude is a measure of the amount of energy released by the earthquake. It is calculated from the size or amplitude of the waves traced by the pen of a seismograph, an instrument that picks up the waves at some distance from the epicenter and records them in the form of a wavy trace on a rotating drum (Figure 2) coupled to a clock. The principle of the seismograph is illustrated by ☆
Change History: June 2013. GJH McCall added the section ‘Examples of Recent Global Earthquakes’.
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Reference Module in Earth Systems and Environmental Sciences
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Figure 1 Block diagram showing the relationship between an earthquake focus, epicenter, and fault.
Figure 2 Record on three seismographs of the Spitak 1988 main shock (magnitude 6.9). Reproduced with permission from Rommer and Ambraseys (1989). Earthquake Engineering and Dynamics. Chichester: John Wiley & Sons. © John Wiley & Sons Limited.
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a chandelier that swung in the great Lisbon earthquake of 1755 – a freely pivoting horizontal strut is attached to an upright support (Figure 3). A heavy mass at the strut end is attached to the pen, which traces a continuous line on the paper. Most of the time there are no ground movements, so the trace is horizontal. All seismographs have to be standardized so that valid comparisons can be made between their traces. Waves weaken as they travel outwards from the earthquake focus, so allowance has to be made for distance between the focus and the instrument (this can be calculated by measuring the difference between the arrival times of P and S waves). Three seismographs are required to measure north–south and east–west horizontal movements and vertical movements. The Worldwide Standardized Seismograph Network was established in 1962. All the instruments are standardized as if they were situated 100 km away from the focus. Waves produced by earthquakes spread through the Earth (Figure 4). They comprise body waves and surface waves. Body waves travel through the Earth and are of two types, primary (P) and secondary (S) waves. P waves can pass through both solid and molten material within the Earth’s interior; they travel fastest and are the first to arrive at a given location, and they are also the first to be felt by the man in the street. They are longitudinal or compressional waves, vibrating forwards and backwards in the direction of travel. They travel at about 6 km s 1 through continental crust and 8 km s 1 through oceanic upper mantle. They may produce booming sound waves in the atmosphere. S waves travel about half as fast as P waves (3.6 km s 1 and 4.7 km s 1 in continental crust and oceanic upper mantle, respectively); they cannot pass through fluids and thus do not penetrate the liquid outer core. They are shear or transverse waves: as they pass through the rock they move particles both from side to side and up and down, at right angles to the direction of travel. Two kinds of surface waves, which travel just below the surface, are called Love and Rayleigh waves. They arrive shortly after the body waves. Love waves travel faster than Rayleigh waves and push the rock particles sideways, at right angles to the direction of travel. Like S waves, they shear buildings and constructions sideways, causing immense damage, but have no vertical motion. The slowest waves, Rayleigh waves, push particles upwards and backwards; the particles move in the vertical plane, following an elliptical path as the wave passes by. Charles Richter in 1935, working in California, devised the Richter Scale of magnitude, in which the absolute strength at the focus can be calculated on a logarithmic scale: a rise of one unit of magnitude represents a tenfold increase in absolute strength (i.e., a magnitude 5 earthquake is 10 times a strong as a magnitude 4 earthquake). The difference in energy release is even greater – an increase of one unit of magnitude represents 30–32 times as much energy being released. Theoretically, earthquakes with magnitudes of more than 10 could occur, but the greatest magnitude so far measured for any earthquake is about 9.5. The Richter scale is given in Table 1. It has been superseded as a scale for measuring the comparative intensity at the focus by the moment magnitude scale, but the principles are the same – the moment magnitude scale allows more refined methods of comparison.
Figure 3 The components of a seismograph designed to record vertical ground movement.
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Figure 4 The way in which earthquake waves spread through the globe and are reflected at boundaries, returning to the surface. Measurement of the speed of such return is used to delineate the materials of the inner Earth according to density and physical state. Reproduced from Van Andel, T.J. (1994). New Views on an Old Planet. Cambridge: Cambridge University Press.
Table 1 Magnitude 0–1.9 2–2.9 3–3.9 4–4.9 5–5.9 6–6.9 7–7.9 8–8.9
The Richter scale Qualitative description
Average number per year
Average intensity equivalent close to epicenter
Minor Light Moderate Strong Major Great
700 000 300 000 40 000 6200 800 120 18 1 every 10–20 years
I–V; recorded but not felt I–V; recorded but not felt I–V; felt by some I–V; felt by many V–VII; slight damage VII; damaging IX–IX; destructive XII; widely devastating
The Importance of Seismological Records We cannot directly study the rocks of the crust below the limits of borehole drilling (a few kilometers), though ancient rock systems do expose sections of the ancient deep crust (as in the Kapuskasing Belt, Ontario, Canada) and perhaps even the crust–mantle contact (as in Oman and Western Newfoundland). The behavior of earthquake waves, however, provides us with invaluable evidence about the nature of the lower crust, mantle, and core because the velocities of P and S waves are functions of the density of the material through which they pass. Knowledge of rock density can tell us much about the physical state of the materials deep within the Earth, and the behavior of S waves tells us that the outer core is molten. In Figure 5, the different densities of common rock materials are plotted against the P-wave velocity. It is fair to say that earthquake waves form the basis of our knowledge of the mantle and core. Artificially produced seisms can also be picked up by seismographs, and, thus, nuclear explosions can be globally monitored. The explosion in the submarine Kursk in 2000 was picked up by distant seismograph stations in Africa, and this provided valuable evidence of what happened. Tomographic methods have recently been developed, producing three-dimensional images of the deep Earth, including subducted slabs of crust, using a technique akin to the use of tomography in medicine. Earthquakes occur in sequences: slight foreshocks may give warning of a major earthquake, and aftershocks occur for long after the main shock. Foreshocks and aftershocks are generally of lower intensity than the main shock, but sometimes very large shocks
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Figure 5 The different densities of common rock types plotted against P-wave velocity.
Figure 6 Surface trace of the Meckering 1968 earthquake (magnitude 6.9) in Western Australia. Reproduced from Everingham, I. (1968). Preliminary report on the 14th October 1968 earthquake at Meckering, Western Australia. Record 1968/142. Canberra: Bureau of Mineral Resources, Geology and Geophysics.
occur, as in the 1999 Izmit earthquake in Turkey. In the 1988 Spitak earthquake (magnitude 6.9) an aftershock of magnitude 6.2 occurred 4 min after the main shock. Earthquakes may produce a trace of the rupture on the land surface, dislocating the land for many kilometers. In Figure 6 the trace produced across wheat fields by the Meckering 1968 (magnitude 6.9) earthquake is shown. Such traces are invaluable in studying the sense of the movement and displacement.
The Global Distribution of Earthquakes Earthquakes do not occur to the same extent all over the globe. The major events are largely concentrated at the boundaries of tectonic plates, and the concentration and magnitude are greater in zones of plate convergence (subduction and collision) than in zones of plate divergence (mid-ocean ridges and rift valleys). This distribution is clearly shown in Figure 7.
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Figure 7 The global distribution of earthquakes that occurred in 1994. Reproduced from US National Earthquake Information Center.
Not all earthquakes occur on plate boundaries, however: the destructive Killari earthquake in India in 1993 occurred within a stable cratonic area. The immensely destructive Lisbon earthquake of 1755 was also nowhere near a plate boundary.
Earthquakes as a Major Hazard: Tectonic, Volcanic, and Man-Made Earthquakes The most damaging earthquakes are not necessarily of high magnitude. The 1994 Kobe earthquake, one of the most destructive and costly in living memory (55 000 houses destroyed as well as freeway, rail, and port installations), had its epicenter 20 km from the city and had a magnitude of only 6.8. The 1960 Agadir earthquake had a magnitude of only 5.8, but the focus was shallow and right beneath the city. Thus, magnitude, though a valuable absolute measurement, tells us little about the degree of damage and the loss of property and life, even at the epicenter. The nature of the subsurface rocks can have a significant effect, especially if shock-induced liquefaction occurs, and thus we need another measurement scale. The Mercalli intensity scale measures the relative intensity of the effects felt at a specific site (the intensity will commonly decrease away from the epicenter, but secondary effects such as subsurface variation and shock liquefaction complicate this relationship). In Europe, a modification of the Mercalli scale, the MSK scale (named after Medvedev, Sponheuer, and Karnik), is now used. This scale is given in Table 2.
Tectonic Earthquakes Earthquakes may be divided into tectonic earthquakes, volcanic earthquakes, and man-made earthquakes. In considering the natural-hazard aspect, it is the tectonic earthquakes that are by far the most destructive natural hazards. This hazard largely affects urban populations, and human design and construction has a unique role in mitigating this hazard. The actual physical process of ground motion presents little threat to humans in the open: most casualties (other than the casualties of secondary tsunamis) occur inside buildings that partially or totally collapse. The correct design of buildings and constructions such as bridges and viaducts can thus greatly mitigate the damage and casualties resulting from an earthquake. The vulnerability of a building to earthquake damage varies according to many factors. Vertical ground motion is the principal damaging component causing collapse, burial of people, and death. Lateral ground motion breaks or deforms power lines, pipelines, water pipes and sewers, roadways, railways (Figure 8), and bridges. Quite small lateral offsets can be very damaging. In the Mexico City earthquake of 1985, much damage was caused by adjacent high rise buildings swaying with different wave motions and knocking each other down. It is noticeable that in Beijing, an earthquake-prone city, the high-rise buildings are widely spaced, with intervening areas of low-rise buildings, so that they cannot interact in this way. In the case of Kobe, the sixth floor of one high-rise building pancaked (Figure 9). The lower floors were built of steel-encased reinforced concrete and the upper floors of pure reinforced concrete; the junction on the sixth floor acted as an element of weakness.
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Table 2
The modified Mercalli (MSK) intensity scale
Intensity
Effects
I II III
Felt rarely. Sometimes dizzyness and nausea. Birds and animals uneasy. Trees, structures, liquids sway. Felt indoors by a few, especially on upper floors. Delicately suspended objects swing. Felt indoors, especially on upper floors by several people. Usually rapid vibration as if a lightly loaded lorry passing.Hanging objects and standing motor cars rock slightly. Felt indoors by many and outside by a few. Some awakened. No-one usually frightened. Sensation of heavy object striking building. Vibration as of heavy lorries passing. Crockery, windows, doors rattle. Walls and frames creak. Hanging objects and standing motor cars sway. Felt indoors by almost everyone, outdoors by most people. Many awakened, a few frightened and run outdoors. Buildings tremble. Crockery and windows sometimes break. Pictures and doors clatter. Small objects move. Some liquids spilt. Clocks stopped. Trees shaken. Animals anxious. Felt by all indoors and outdoors. Many frightened, some alarmed. All awakened. People, trees, bushes shaken. Liquids set in motion. Small bells ringing. Crockery broken. Plaster cracks and falls. Books and vases fall over. Some furniture moved. Domestic animals try to escape. Minor landslides on steep slopes. All frightened, run out of doors, general alarm. Some people thrown to ground. Trees shaken quite strongly. Waves and mud stirred up in lakes. Sandbanks collapse. Large bells ring. Suspended objects quiver. Much damage to badly constructed buildings and old walls. Slight damage to well-built buildings. Chimneys crack. Much plaster, tiles, loose bricks fall. Heavy furniture overturned. Concrete ditches damaged. Alarm approaches panic. Vehicle drivers disturbed. Trees broken and shaken. Sand and mud spurt from the ground. Marked changes to springs and wells. Much damage to ordinary and older buildings. Walls, pillars, chimneys, towers, statues, gravestones crack and fall. Very heavy furniture overturned. General panic. Ground cracked open (10 cm). Much damage to structures built to withstand earthquakes. Frequent partial or total collapse of other buildings. In reservoirs, underground pipes broken. Buildings dislodged from foundations, rock falls. Widely cracked ground, fissures up to 1 m wide. Frequent river bank and coastal landslides and shifted sands. Water levels change. Water thrown onto riversides. Serious damage to dams, embankments, bridges. Severe damage to well-built wooden structures. Masonry structures destroyed along with their foundations. Rails bent. Open cracks or waves on roads. Pipes torn apart. Widespread serious ground disturbance, broad fissures, landslips, landslides. Muddy water spurts upward. Tsunamis develop. Severe damage to all wooden structures. Great damage to dams. Few masonry structures remain upright. Pillars of bridges and viaducts wrecked. All pipelines wrecked. Rails badly bent. Total damage to all constructions. Great disturbance to ground with many shearing cracks. Many landslides on slopes, rockfalls common, rock masses dislocated, water channels altered and dammed. Ground surface waves like water and ground remains undulating. Objects thrown into the air.
IV
V
VI
VII
VIII
IX X
XI
XII
Though earthquakes are mainly an urban hazard, catastrophic earthquakes may strike village populations where low-rise housing is substandard – as in the case of the Cairo earthquake in 1992 and the Killari, central India, earthquake in 1993. In the case of the Cairo earthquake, poorly constructed extra storeys had been added to moderate-rise housing. In Killari, stone-built lowrise houses were poorly constructed (Figure 10). In the catastrophic Bam earthquake of December 2003, the mud bricks of the lowrise dwellings crumbled and collapsed, leaving few air spaces to allow buried victims to breathe; another factor responsible for the scale of the fatalities was the fact that all the dwellings in southern Iran have basements cooled by wind towers designed for the sweltering summer heat, and many victims would have been asleep in them. Some of the most devastating historic and recent earthquakes are listed in Table 3. Where cities are situated in plate-boundary zones the effects are most disastrous. The San Francisco earthquake of 1906 (Figure 11) provides an example of this. The Kobe earthquake of 1995, in which the financial loss was US $200 billion, occurred in a city that experiences a tremor every few days. An analysis of the locations of 100 of the largest cities in the world, which accommodate 10% of the global population, shows that they can expect to experience an earthquake of intensity VI or more on the MSK scale within 50 years. The earthquake hazard extends beyond high-risk cities such as those sited on plate boundaries. Entire countries may be at quite low risk, yet have some vulnerability. The UK is a low-risk country, and earthquakes of more than magnitude 5.5 are extremely unlikely (Figure 12). Charles Davidson published a list of 1191 recorded shocks in Britain between AD 974 and AD 1924. In Lincoln in 1185, “great stones were rent; houses of stone fell; the metropolitan church of Lincoln was rent from top to bottom” and there is a similar report from the cathedral city of Wells in 1248. Two apprentices were killed in London in 1580 as a result of an earthquake in the Dover Straits. The Colchester earthquake in 1884 (magnitude 4.7) peaked in intensity near the epicenter (between Pelden and Langenhoe; Figure 13) at MSK VIII and caused widespread damage, which was compensated by a Mansion House Fund that paid out £9000 (equivalent to £500 000 today). The Roermond earthquake in the Netherlands in 1998 (magnitude 5.8) caused £30–40 million of damage, despite the single fatality. It is predicted that a magnitude 5.7 earthquake focused at a depth of 5 km directly under Manchester would cause havoc. The increasing size of conurbations and cities increases their vulnerability: the Colchester area would suffer much more nowadays from a comparable earthquake to the 1884 event, because the population and industry are now much denser than at that time.
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Figure 8 A railway track in the western USA twisted and shortened by lateral motion during an earthquake.
Figure 9 High-rise buildings in Kobe after the 1995 earthquake, showing the sixth floor pancaked by vertical motion. Reproduced from Esper, P. and Tachibana, E. (1998). The Kobe earthquake. In: Maund, J.G. and Eddleston, M. (eds.) Geohazards in Engineering Geology, pp 105–116. Engineering Geology Special Publication 15. London: Geological Society.
Earthquakes can occur in areas that are not considered to be at risk. The Spitak earthquake in Armenia in 1988 is such a case. The region was not considered to be at high risk, and a nuclear power station was planned for the Spitak area. This earthquake caused the whole process of earthquake risk assessment in the Soviet Union to be revised. The New Madrid, Missouri, earthquakes of 1811–1812 are even more surprising. There is a reliable historical record of three earthquakes spaced over 2 months with magnitudes 8.2, 8.1, and 8.3. They rang the bells of Boston and rattled Quebec and provide a remarkable example of major interplate seismicity.
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Figure 10 (A) Damage to a low-rise poorly constructed stone building of the type affected by the Killari, India, 1991 earthquake (photograph National Geophysical Data Center USA). (B) Improved training in building similar low-rise buildings in the Yemen. Reproduced from Degg, M.R. (1995). Earthquakes, volcanoes and tsunamis: tectonic hazards in the built environment of southern Europe. Built Environment 21, 94–113. London: Courtesy of the Geological Society.
Secondary effects The secondary effects of earthquakes can be as destructive and lethal as the primary effects, or more so. Tsunami The so-called tidal wave generated by earthquakes is probably the most lethal secondary effect: during the twentieth century earthquakes in Chile caused fatalities in Hawaii and Japan. The effects of tsunamis may be felt thousands of kilometers from the earthquake epicenter, but can be mitigated by systematic warning systems. Fire Tokyo in 1923, San Francisco in 1906, and Kobe in 1995 all suffered from the secondary effects of fire. This was exacerbated by the fact that water supplies were cut off. In Tokyo there was a firestorm. In San Francisco, 70% of the damage was due to fire. Liquefaction of sands, silts, and clays Another important secondary effect is that thixotropic sands and silts, which liquefy on shock, greatly increase the damage: examples of this are the waterfront area in the Messina earthquake, Sicily, of 1908, in which 98% of the houses were ruined and 160 000 died; Mexico City in 1985, where the worst damage was in building developments founded on old lake deposits (the wave amplitude was magnified 8–50 times here); and Anchorage, Alaska, where a magnitude 8.4 earthquake with an epicenter 130 km away caused devastation in a housing development founded on the thixotropic Bootlegger clay formation (Figure 14). Here, the risk was well known but there was a lack of communication between the geologists and the planners. Landslides and rock falls Very damaging landslides or rock falls can be triggered by earthquakes and may occur sometime after the main shock or aftershocks. In Montana in 1985, 30 million tonnes of rock were set in motion, fatally burying 36 people at a camp site. In Peru in 1970, rock and ice slides triggered by an earthquake killed 20 000 people.
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Table 3 Some important earthquakes in the last 2000 years (various sources): note that magnitudes are on various scales. An earthquake at Gujarat, India, in 2001, which killed more than 50 000 people has been omitted from the table Year AD
Place
Casualties
342 454 565 856 1170 1290 1456 1556 1716 1737 1755 1755 1759 1783 – 1786 1797 1822 1828 1896 1897 1906 1908 1915 1920 1923 1932 1933 1933 1935 1939
Antioch Sparta Antioch Corinth Sicily Chihli, China Naples Shensi, China (M 8.3?) Algiers Calcutta North Persia Lisbon (M 6.9?) Baalbek Calabria
40 000 20 000 30 000 45 000 15 000 100 000 60 000 830 000 20 000 300 000 40 000 60 000 20 000 50 000 41 000 22 000 34 000 28 000 1542 700 160 000 30 000 180 000 143 000 70 000 120 3000 60 000 30 000
1948 1960 1960 1964 1970 1971 1975 1976 1976
Quito Aleppo Honshu Sanriku, Japan Assam (M8.7) San Francisco (M 7.9?) Messina, Reggio (M 7.5) Avezzano, Italy (M 7.0) Kansu, China (M 8.5) Tokyo (M 8.2) Kansu, China (M 8.5) Long Beach, CA Sanriku, Japan Quetta (M 7.5) Concepcion, Chile (M 8.5) Erzincian, Turkey (M 8.0?) Soviet–Iran border Agadir (M 9.5?) Chile (M 9.5?) Anchorage (M 8.4) Peru (M 7.9) San Fernando, CA Haicheng, China (M 7.5) Guatemala (M 7.5) Tangshan, China (M 7.7)
1985 1988
Mexico City (M 8.1) Spitak (M 6.9)
19 000 60 000 10 000 114 80 000 64 1328 22 000 240 000 (some estimates are as high as 850 000) 10 000 30 000
1989 1989 1990 1991 1992
Loma Prieta, CA (M 7.1) Newcastle, Australia Northwest Iran (7.7) Killari, India Cairo (M 5.5–5.9)
63 10 40 000 10 000 <500
1994 1995 1999 2003
Northridge, CA (M 6.7) Kobe, Japan (M 7.2) Izmit, Turkey (M 7.6) Bam, Iran
60 5429 >17 000 45 000
1939
40 000
Estimated loss
6 major towns and all villages in 30 000 sq. miles leveled US$ 400 million 98% of houses ruined Vast landslides Firestorm killed 38 000, 25 000 houses destroyed US$ 50 million 8800 houses destroyed by tsunami
30 000 dwellings destroyed
58 600 houses destroyed Devastating rock and ice falls US$ 1 billion
Vast damage
US$ 4 billion US$ 14 billion; accompanied by landslides and rockfalls US$ 7 billion US$ 7billion US$ 8 billion Immense destruction of village housing 40 000 homeless; may have been due to construction of Aswan Dam US$ 20 billion US$ 200 billion Immense destruction Immense destruction of modern city and ancient citadel destroyed
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Figure 11 Gross displacement of a large building in the San Francisco 1906 earthquake.
Figure 12 The magnitudes and locations of earthquakes in Great Britain greater than magnitude 3 after 1700 and greater than magnitude 4 before 1700. Reproduced from Musson, R. (1996). British earthquakes and the seismicity of the UK. Geoscientist 16, 24–25.
Disease The risk of disease is a major concern after earthquakes, particularly in hot climates. Water supplies may be cut off and the populace may resort to using polluted supplies of water mixed with sewage and drainage effluents, which may be contaminated by corpses. Starvation Normal food supplies may be cut off and transport arteries may be blocked, so it is important to bring in food supplies immediately from the world outside.
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Figure 13 Intensity plot of the Great Colchester Earthquake of 1884 using the MSK scale. Reproduced from Musson, R., Neislon, G., and Burton, P.W. (1990) Microseismic Reports on Historic British Earthquakes XIV: 22 April 1984 Colchester. BGS Seismology Report W1/90/33. Edinburgh: British Geological Survey.
Figure 14 Destruction of the housing development at Turnagain Point above the Bootlegger Clay, Anchorage, 1994. The bluff moved 606 m towards the bay, and 75 homes were destroyed. Sand lenses in the clay lost strength (photograph National Geophysical Data Center, USA).
Exposure The 1988 Spitak earthquake in Armenia illustrates the problem of exposure. Many people lost their dwellings and were living in the open in very cold December climatic conditions. The need for tents, warm clothing, and blankets was urgent. Looting Looting is prevalent after earthquakes.
Mitigation The potential for mitigation of the earthquake hazard is limited. The main ways of mitigating the hazard are through good building and constructional design, planning development away from at-risk areas, and warning. However, warning is a very difficult matter. Research into earthquakes is at an interim stage, and the scientific community is at present by no means in consensus about the physical processes involved. There is the problem of how threatened populations react to warnings: if the event fails to occur, especially more than once, the population may not heed future warnings; alternatively, giving a warning to evacuate may engender panic. In countries with controlled political systems, such as China, warning and evacuation may be easier than in a democratic Western country. The Chinese did evacuate the city of Haicheng twice in 1974 and 1975, on 4 December and again on 4 February, based on seismology, community monitoring levels, radon gas in water, water temperature, tiltmeters, magnetometer readings, and
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patterns of animal behavior. An earthquake of magnitude 7.3 struck at 7.36 Am on 4 February. However, the great Tangshan earthquake of 1976 struck without any prediction or warning and killed at least 240 000 people (possibly many more). Millions of US dollars have been spent on research into earthquakes in California, and some improvement in prediction has been achieved, yet the existence of the Northridge Fault, the site of the 1994 earthquake, which killed 60 people and caused 20 billion dollars worth of damage, was not even known before the event. The best mitigation procedure would be to have international teams ready with emergency supplies and equipment, trained personnel, and sniffer dogs, at a distance from earthquake-prone regions, ready to been flown in by plane and helicopter.
Research into earthquakes The most important research into earthquakes has involved statistical, geographical, geological, theoretical, and mathematical studies of seismicity. An example is a study by Lya Tuliani in Russia, which addressed problems of geodynamics and seismology, tectonosphere layering, and lithostructure in seismically active regions in order to develop risk estimates. The procedure involved mathematical data processing. It was claimed that this study allowed highly accurate prediction of the coordinates of high-risk sites. This statistical, mathematical, and office-based approach contrasts with research in the USA (which involves actually drilling down to earthquake foci on faults), research into the Boothiel lineament, the site of the New Madrid earthquakes (which has revealed sand boils caused by the earthquake), and excavations in the alluvium of the rice paddies west of Beijing, China (where the actual earthquake fault of a seventeenth century event has been exposed cutting the clayey alluvium in open pits). Research has been carried out into much older earthquakes in Iran, based on the dislocation of qanats (horizontal wells). All these approaches and many more are invaluable, but the problem of predicting earthquakes is extremely complex and may never be completely solved.
Volcanic Earthquakes There can be a connection between major tectonic earthquakes and volcanic eruptions. In Chile in 1960, a major earthquake triggered the eruptions of several volcanoes, and in Sicily there is a record of Etna erupting a day or two before a major tectonic earthquake. However, the swarms of small seisms that usually precede volcanic eruptions (though there may be no such prelude) pose little threat to life and property. They do, however, provide valuable warnings of forthcoming eruption, and arrays of instruments are mounted on dangerous volcanoes for this purpose.
Man-Made Earthquakes Small seismic disturbances can be triggered by human activity. In the USA, the Boulder Dam and Lake Mead are constructed in a region that is highly strained; many small shocks have been correlated with changes in water depth. In Colorado, the injection of liquid wastes down boreholes has also been shown to trigger small seisms.
Moonquakes and Seisms on Other Planets Very small earthquakes do occur on the largely quiescent Moon when it undergoes maximum tidal stresses resulting from the attraction of the Earth and Sun. Similar stresses must operate on the Earth and cause minor seisms, but the effect is of no importance in such a dynamic body. Moonquakes and artificial seisms produced on the surfaces of other extraterrestrial bodies – Mars, Venus, and Mercury – can provide a valuable insight into their internal make up. A fascinating project would be to site an instrument from an unmanned spacecraft on Io, Jupiter’s volcanically active satellite, to obtain detail of its interior configuration – Io must be seismically active.
Examples of Recent Global Earthquakes Sumatra-Andaman earthquake This occurred off the SW coast of Sumatra on 26 December 2004. It was due to slip on the plate boundary between the Indian Ocean Plate and the Burma Plate and slippage occurred over a length of 1600 km and involved a vertical displacement of 15 m in addition to horizontal displacement. It had a moment magnitude of 9.1–9.3 and is energywise the third largest earthquake ever recorded. The hypocentre was in the Indian Ocean, just north of Simalue Island off the coast of Sumatra. The depth was 30 km. It cause devastation and great loss of life. There have been several large quakes since along this plate boundary, but none has generated a comparable tsunami.
Pakistan 2005 A major earthquake occurred on 8 October 2005, centered on Muzffarabad in Kashmir: this was of magnitude 7.2 on European plates. There were 74 698 deaths: the remote villages were difficult to reach by aid teams. It was followed 12 h later by another, comparable quake in magnitude and there were 978 aftershocks.
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TECTONICS | Earthquakes
Pakistan 2008 Another major earthquake occurred in Baluchistan on 29 October 2008, with an epicenter 60 km north of Quetta. The moment magnitude was 6.4. There were 30 000 deaths. This again was due to the interaction of the Indian and European pates.
L’Aquila, Italy, 2009 A powerful earthquake struck the town of L’Aquila in the Abruzzi, Italy and neighboring villages on 6 April 2009. More than 300 people died. The moment magnitude was 6.3, the epicenter L’Aquila and the depth 9 km. This is unique in that on 20 October 2012, six scientists were convicted of giving inexact, incomplete and contradictory information before the event (Nield, 2012). There is parallel with the Nevado del Ruiz eruption in Columbia in 1995 (Hall, 1992), where there was governmental interference on the issue of warnings.
Haiti 2010 A powerful earthquake on 12 June 2010, was caused by the North American plate moving past the Caribbean plate on a conservative plate margin – the two plates were moving in the same direction at different speeds. The magnitude was 7 on the Moment Magnitude scale. 316 000 people were killed, one million made homeless. 250 000 homes were destroyed and 30 000 other buildings. Many are still in temporary accommodation. Disease was rampant, especially cholera, in the rainy, tropical conditions and poor sanitation. The main prison was damaged, the inmates escaped and looting was another problem.
Christchurch, N.Z., 2011 A powerful earthquake struck New Zealand’s second city, on South Island, on 22 February 2011. The epicenter was 2 km west of Lyttleton, the port town, and 10 km SE of the city. It struck 6 months after another quake on the Darfield fault 40 km west of the city. There were 185 deaths and great destruction. The Pacific plate is being sub ducted below the Australian plate. The depth was 1–2 km and the movement strike-slip with some vertical component. The magnitude was 6.3 ML. There is some argument as to whether this was an aftershock of the earlier Canterbury earthquake which was of 7.1 magnitudes, but they seem to be regarded as separate events, with different epicenters and foci. There have been numerous aftershocks. The cathedral was severely damaged. Devastation was great, the city is still a demolition zone and reconstruction has yet to commence.
Tohoku 2012 This, the fourth largest earthquake ever recorded, energy-wise, was due to shallow thrust faulting on the plate boundary between the Pacific and Okhotsk plates: it involved a megathrust 300 km long by 200 km wide, part of the subduction zone at the boundary of the Pacific plate (Ritsema et al., 2012). 20 000 lives were lost, there were 300 000 refugees and a 20 km radius area was evacuated around the Fukushima Daishi nuclear reactor. The total cost is estimated at $300 billion. However, the main secondary effect was a huge tsunami. These ‘superquakes’ of the twenty-first century have revised our thinking, for both the Sumatra-Anadaman and Tohoku Earthquakes were larger than what scientists expected these fault systems to produce (Goldfinger et al., 2013). What a given fault system can produce remains poorly known.
Further Reading Bolt BA (1999) Earthquakes. New York: Freeman. Bommer JJ and Ambraseys NN (1989) The Spitak, Armenia, USSR earthquake of 7 December 1988: A summary engineering geology report. Earthquake Engineering and Structural Dynamics 18: 921–925. Chen Y, Tsoi KL, Chen F, et al. (1988) The Great Tangshan Earthquake of 1976. Oxford: Pergamon. Degg MR (1992) Some implications of the 1985 Maxican earthquake for hazard assessment. In: McCall GJH, Laming DJC, and Scott SC (eds.) Geohazards – Natural and Man-Made, pp. 93–114. London: Chapman and Hall. Degg MR (1995) Earthquakes, volcanoes and tsunamis: Tectonic hazards in the built environment of southern Europe. Built Environment 21: 94–113. Degg MR (1998) Hazard mitigation in the urban environment. In: Maund JG and Eddleston M (eds.) Geohazards in Engineering Geology, pp. 329–337. London: Geological Society, Engineering Geology Special Publication 15. Esper P and Tachibana E (1998) The Kobe earthquake. In: Maund JG and Eddleston M (eds.) Geohazards in Engineering Geology, pp. 105–116. London: Geological Society, Engineering Geology Special Publication 15. Everingham, I. (1968). Preliminary report on the 14th October 1968 earthquake at Meckering, Western Australia. Record 1968/142. Canberra: Bureau of Mineral Resources, Geology and Geophysics. Keller GR (2000) Seismic properties of rocks. In: Hancock PL and Skinner BJ (eds.) The Oxford Companion to the Earth. Oxford: Oxford University Press. McCall GJH (1996) Natural hazards. In: McCall GJH, de Mulder EFJ, and Marker BR (eds.) Urban Geoscience, pp. 81–125. Rotterdam: Balkema. McCall GJH (2000) The great Colchester earthquake of 1884 revisited. Geoscientist 10: 4–6. McCall GJH (2004) Remembering Bam. Geoscientist 14: 8–9. Menard HW (1974) Geology, Resources and Society. San Francisco: WH Freeman and Co.. Musson R (1996) British earthquakes and the seismicity of the UK. Geoscientist 16: 24–25. Musson, R., Neislon, G., and Burton, P. W. (1990) Microseismic Reports on Historic British Earthquakes XIV: 22 April 1984 Colchester. BGS Seismology Report W1/90/33. Edinburgh: British Geological Survey. Scarth A (1997) Savage Earth. London: HarperCollins. Tuliani LI (1999) Seismicity and Earthquake Risk: On the Basis of Thermodynamic and Rheological Parameters of the Tectonosphere. Moscow: Scientific World. Van Andel TJ (1994) New Views on an Old Planet. Cambridge: Cambridge University Press. Wong IG (2000) Earthquake mechanisms and plate tectonics. In: Hancock PL and Skinner BJ (eds.) The Oxford Companion to the Earth, pp. 287–289. Oxford: Oxford University Press.