ABOUT EARTHQUAKES


Earthqaukes are any sudden disturbance within the Earth manifested at the surface by a shaking of the ground. This shaking, which accounts for the destructiveness of an earthquake, is caused by the passage of elastic waves through the Earth's rocks. These seismic waves are produced when some form of stored energy, such as elastic strain, chemical energy, or gravitational energy, is released suddenly.

Few natural phenomena can wreak as much havoc as earthquakes. Over the centuries they have been responsible for millions of deaths and an incalculable amount of damage to property. While earthquakes have inspired dread and superstitious awe since ancient times, little was understood about them until the emergence of seismology at the beginning of the 20th century. Seismology, which involves the scientific study of all aspects of earthquakes, has yielded answers to such long-standing questions as why and how earthquakes occur. These matters are discussed in this article, as are the distribution, size, and effects of earthquakes.

General considerations

Principal types of seismic waves

Seismic waves generated by an earthquake source are commonly classified into three main types. The first two, the P and S waves, are propagated within the Earth, while the third, consisting of Love and Rayleigh waves, is propagated along its surface. The existence of these types of seismic waves was predicted during the 19th century, and modern investigators have found that there is a close correspondence between such theoretical calculations and seismographic measurements of the waves.

The P (or primary) waves travel through the body of the Earth at the highest speeds. They are longitudinal waves that can be transmitted by both solid and liquid materials in the Earth's interior. With P waves, the particles of the medium vibrate in a manner similar to sound waves, and the transmitting rocks are alternately compressed and expanded.

The other type of body wave, the S (or secondary) wave, travels only through solid material within the Earth. With S waves, the particle motion is transverse to the direction of travel and involves the shearing of the transmitting rock.

Because of their greater speed, the P waves are the first to reach any point on the Earth's surface. The first P-wave onset starts from the spot where an earthquake originates. This point, usually at some depth within the Earth, is called the focus, or hypocentre. The point immediately above the focus at the surface is known as the epicentre.

Love and Rayleigh waves are guided by the free surface of the Earth. They follow along after the P and S waves have passed through the body of the planet. Both Love and Rayleigh waves involve horizontal particle motion, but only the latter type has vertical ground displacements. As Love and Rayleigh waves travel, they disperse into long wave trains, and at substantial distances from the source they cause much of the shaking felt during earthquakes.

Properties of seismic waves

At all distances from the focus, the mechanical properties of the rocks, such as incompressibility, rigidity, and density, play a role in the speed with which the waves travel and the shape and duration of the wave trains. The layering of the rocks and the physical properties of surface soil also affect these characteristics of the waves. In most cases, elastic behaviour occurs in earthquakes, but the shaking of surface soils from the incident seismic waves sometimes results in nonelastic behaviour, including slumping (i.e., the downward and outward movement of unconsolidated material) and the liquefaction of sandy soil.

When a seismic wave encounters an interface or boundary that separates rocks of different elastic properties, it undergoes reflection and refraction. There is a special complication if a conversion between the wave types occurs at such a boundary: either an incident P or S wave can yield in general reflected P and S waves and refracted P and S waves. Boundaries between structural layers also give rise to diffracted and scattered waves. These additional waves are in part responsible for the complications observed in ground motion during earthquakes. Modern research is concerned with computing, from the theory of waves in complex structures, synthetic records of ground motion that are realistic in comparison with observed ground shaking.

The frequency range of seismic waves is large. Seismic waves may have frequencies from as high as the audible range (greater than 20 hertz [Hz]) to as low as the free oscillations of the whole Earth, with gravest period being 54 minutes (i.e., the Earth vibrates in various modes, and the mode with the lowest pitch takes 54 minutes to complete a single vibration; see below Long-period oscillations of the globe). Attenuation of the waves in rock imposes high-frequency limits, and in small to moderate earthquakes measured surface waves have frequencies extending from about one to 0.005 Hz.

The amplitude range of seismic waves is also great in most earthquakes. The displacements of the ground extend from 10-10 to 10-1 metres. In the greatest earthquakes, the ground amplitude of the predominant P waves may be several centimetres at periods of two to five seconds. Very close to the seismic sources of great earthquakes, investigators have measured large wave amplitudes with accelerations to the ground exceeding that of gravity at high frequencies and ground displacements of one metre at low frequencies.

Seismic instruments and systems

Ground motion in earthquakes and microseisms (small, often long-continuing oscillations of the ground that do not originate in earthquakes) are both recorded by seismographs. Most of these instruments are of the pendulum type. Still in use today are mechanical seismographs that have a pendulum of large mass (up to several tons) and that produce seismograms by scratching a line on smoked paper on a rotating drum. In more advanced instruments, seismograms are recorded by means of a ray of light from the mirror of a galvanometer through which passes an electric current generated by electromagnetic induction when the pendulum of the seismograph moves. Technological developments, notably in electronics, have given rise to high-precision pendulum seismometers and sensors of ground motion. In these instruments, the electric voltages produced by motions of the pendulum or the equivalent are passed through electronic circuitry to amplify the ground motion for more exact readings.

Generally speaking, seismographs are divided into three types: short period; long (or intermediate) period; and ultra-long period, or broad-band, instruments. Short-period instruments are used to record P- and S-body waves with high magnification of the ground motion. For this purpose, the seismograph response is shaped to peak at a period of about one second or less. The long- or intermediate-period instruments of the type used by the World-Wide Standard Seismographic Network (WWSSN; see below) have a response maximum at about 20 seconds. Again, in order to provide as much flexibility as possible for research work, the trend has been toward the operation of very-broad-band seismographs, often with digital representation of the signals. This is usually accomplished with very-long-period pendulums and electronic amplifiers that pass signals in the 0.005 to 50 Hz band.

When seismic waves close to their source are to be recorded, special design criteria are needed. Instrument sensitivity must ensure that the largest ground movements remain on scale. For most seismological and engineering purposes the wave frequency is high, and so the pendulum or its equivalent can be small. For comparison, displacement meters need a long free period and pendulum with consequent instability. Accelerometers that measure the rate at which the ground velocity is changing have an advantage for strong-motion recording, because they allow integration to be carried out to estimate ground velocity and displacement. The ground accelerations to be registered range up to twice gravity (2g). Recording such accelerations can be easily accomplished with short torsion suspensions or force-balance mass-spring systems.

Because many strong-motion instruments need to be placed at unattended sites in ordinary buildings for periods of months or years before a strong earthquake occurs, they usually record only when a trigger mechanism is actuated with the onset of motion. Solid-state memories are now used, particularly with digital recording instruments, making it possible to preserve the first few seconds before the trigger starts the permanent recording. In the past, recordings were usually made on film strips for up to a few minutes' duration. In present-day equipment, digitized signals are stored directly on magnetic cassette tape or on a memory chip. In most cases absolute timing has not been provided on strong-motion records but only accurate relative time marks; the present trend, however, is to provide Universal Time (the local mean time of the prime meridian) by means of special radio receivers or small crystal clocks.

The prediction of strong ground motion and response of engineered structures in earthquakes depends critically on measurements of the spatial variability of earthquake intensities near the seismic wave source. In an effort to secure such measurements, special arrays of strong-motion seismographs are being installed in areas of high seismicity around the world. Large-aperture seismic arrays (linear dimension on the order of one to 10 kilometres) of strong-motion accelerometers can now be used to improve estimations of speed, direction of propagation, and type of seismic wave components. Like an array of radio telescopes, a seismic array allows wave correlations for consecutive time and frequency intervals so that variations in shaking over small-to-moderate distances can be measured.

Finally, because 70 percent of the Earth's surface is covered by water, there is a need for ocean-bottom seismometers to augment the global land-based system of recording stations. Research is under way to determine the feasibility of extensive long-term recording by instruments on the seafloor. Japan already has a semipermanent seismograph system of this type. The system was placed on the seafloor off the Pacific coast of central Honshu in 1978 by means of a cable.

Because of the mechanical difficulties of maintaining permanent ocean-bottom instrumentation, different systems have been considered. These include instruments that are placed in an ocean-bottom package; signals from the instruments are either transmitted to the ocean surface for retransmission by auxiliary apparatus or transmitted via cable to a shore-based station. Another system is designed to release automatically its recording component, allowing it to float to the surface for later recovery.

The use of ocean-bottom seismographs should yield much improved global coverage of seismic waves and provide important information on the seismicity of oceanic regions. Ocean-bottom seismographs will enable investigators to determine the details of the crustal structure of the seafloor and, because of the relative thinness of the oceanic crust, should make it possible for them to collect clear seismic information about the upper mantle. Such systems are also expected to provide new data on focal mechanism, on the origin and propagation of microseisms, and on the nature of ocean-continent margins.

Effects of earthquakes

Primary effects

Earthquakes have varied effects, including changes in geologic features, damage to man-made structures, and impact on human and animal life.

Geomorphological changes are often caused by an earthquake: e.g., movements--either vertical or horizontal--along geological fault traces; the raising, lowering, and tilting of the ground surface with related effects on the flow of groundwater; liquefaction of sandy ground; landslides; and mudflows. The investigation of topographical changes is aided by geodetic measurements, which are made systematically in a number of countries seriously affected by earthquakes.

Earthquakes can do significant damage to buildings, bridges, pipelines, railways, embankments, and other man-made structures. The type and extent of damage inflicted are related to the strength of the ground motions and to the behaviour of the foundation soils.

In the most intensely damaged region, called the meizoseismal area, the effects of a severe earthquake are usually complicated and depend on the topography and the nature of the surface materials; they are often severer on soft alluvium and unconsolidated sediments than on hard rock. At distances of more than 100 kilometres (62 miles) from the source, the main damage is caused by surface waves. In mines there is frequently little damage below depths of a few hundred metres even though the surface immediately above is considerably affected.

Further effects of interest are the occurrence of earthquake sounds and lights. The sounds are generally low-pitched and have been likened to the noise of an underground train passing through a station. The occurrence of such sounds implies the existence of significant short periods in the P waves in the ground (a wave period is the length of time between the arrival of successive crests in a wave train). Occasionally luminous flashes, streamers, and balls are seen in the night sky during earthquakes. These lights have been attributed to electric induction in the air along the earthquake source.

Intensity scales

The level of violence of seismic shaking varies considerably over the affected area. This intensity is not capable of simple quantitative definition and, particularly before seismographs capable of accurate measurement of ground motion were developed, the shaking was estimated by reference to intensity scales that describe the effects in qualitative terms. Subsequently, the divisions in these scales have been associated with accelerations of the local ground shaking. Intensity depends, however, in a complicated way not only on ground accelerations but also on the periods and other features of seismic waves, the distance of the point from the source, and the local geological structure. Furthermore, it is distinct from magnitude, which is a measure of earthquake size specified by a seismograph reading (see below Earthquake magnitude).

A number of different intensity scales have been set up during the past century and applied to both current and ancient destructive earthquakes. For many years the most widely used was the 10-point scale devised by Michele Stefano de Rossi and Fran�ois-Alphonse Forel in 1878. The scale now generally employed in North America is the Mercalli scale, as modified by Harry O. Wood and Frank Neumann in 1931, in which intensity is considered to be more uniformly graded. An abridged form of the modified Mercalli scale is provided below. Alternative scales have been developed in both Japan and Europe for local conditions. The European (MSK) scale of 12 grades is similar to the abridged version of the Mercalli.

Modified Mercalli Scale of Felt Intensity (1931; Abridged)

I. Not felt. Marginal and long-period effects of large earthquakes.

II. Felt by persons at rest, on upper floors, or otherwise favourably placed to sense tremors.

III. Felt indoors. Hanging objects swing. Vibrations like passing of light trucks. Duration can be estimated.

IV. Vibration like passing of heavy trucks (or sensation of a jolt like a heavy ball striking the walls). Standing motorcars rock. Windows, dishes, doors rattle. Glasses clink. Crockery clashes. In the upper range of IV, wooden walls and frames creak.

V. Felt outdoors; direction may be estimated. Sleepers wakened. Liquids disturbed, some spilled. Small objects displaced or upset. Doors swing, open, close. Pendulum clocks stop, start, change rate.

VI. Felt by all; many frightened and run outdoors. Persons walk unsteadily. Pictures fall off walls. Furniture moved or overturned. Weak plaster and masonry cracked. Small bells ring (church, school). Trees, bushes shaken.

VII. Difficult to stand. Noticed by drivers of motorcars. Hanging objects quiver. Furniture broken. Damage to weak masonry. Weak chimneys broken at roof line. Fall of plaster, loose bricks, stones, tiles, cornices. Waves on ponds; water turbid with mud. Small slides and caving along sand or gravel banks. Large bells ring. Concrete irrigation ditches damaged.

VIII. Steering of motorcars affected. Damage to masonry; partial collapse. Some damage to reinforced masonry; none to reinforced masonry designed to resist lateral forces. Fall of stucco and some masonry walls. Twisting, fall of chimneys, factory stacks, monuments, towers, elevated tanks. Frame houses moved on foundations if not bolted down; loose panel walls thrown out. Decayed piling broken off. Branches broken from trees. Changes in flow or temperature of springs and wells. Cracks in wet ground and on steep slopes.

IX. General panic. Weak masonry destroyed; ordinary masonry heavily damaged, sometimes with complete collapse; reinforced masonry seriously damaged. Serious damage to reservoirs. Underground pipes broken. Conspicuous cracks in ground. In alluvial areas, sand and mud ejected, earthquake fountains, sand craters.

X. Most masonry and frame structures destroyed with their foundations. Some well-built wooden structures and bridges destroyed. Serious damage to dams, dikes, embankments. Large landslides. Water thrown on banks of canals, rivers, lakes, etc. Sand and mud shifted horizontally on beaches and flat land. Railway rails bent slightly.

XI. Rails bent greatly. Underground pipelines completely out of service.

XII. Damage nearly total. Large rock masses displaced. Lines of sight and level distorted. Objects thrown into air.

With the use of an intensity scale, it is possible to summarize the macroseismic data for an earthquake by constructing isoseismal curves, which are the loci of points that demarcate areas of equal intensity. If there were complete symmetry about the vertical through the earthquake's focus, the isoseismals would be circles with the epicentre as centre. However, because of the many unsymmetrical factors influencing the intensity, the curves are often far from circular.

The most probable position of the epicentre based on macroseismic data will be at a point inside the area of highest intensity. In some cases, it is verified by instrumental data that the epicentre is satisfactorily determined in this way, but not infrequently the true epicentre lies outside the area of greatest intensity.


Tsunamis and seiches

Tsunamis

Very long water waves in oceans or seas, tsunamis (or seismic sea waves), sweep inshore following certain earthquakes. They sometimes reach great heights and may be extremely destructive. The immediate cause of a tsunami is a disturbance in an adjacent seabed sufficient to cause the sudden raising or lowering of a large body of water. This disturbance may be centred in the focal region of an earthquake or it may be a submarine landslide arising from an earthquake.

Following the initial disturbance to the sea surface, water waves spread out in all directions. Their speed of travel in deep water is given by (gh)1/2, where h is the sea depth and g is the acceleration of gravity. This speed may be considerable; e.g., 100 metres per second (224 miles per hour) when h is 1,000 metres (3,280 feet). The amplitude at the surface does not exceed a few metres in deep water, but the principal wavelength may be on the order of hundreds of kilometres; correspondingly, the principal wave period may be on the order of tens of minutes. Because of these features, the waves are not noticed by ships far out at sea.

When tsunamis approach shallow water, the wave amplitude increases. The waves may occasionally reach a height of 20 to 30 metres in U- and V-shaped harbours and inlets. They sometimes do a great deal of damage in low-lying ground around such inlets. Frequently the wave front in the inlet is nearly vertical, as, for example, in a tidal bore, and the speed of onrush may be on the order of 10 metres per second. In some cases there are several great waves separated by intervals of several minutes or more. The first of these waves is often preceded by an extraordinary recession of water from the shore, which may commence several minutes or even half an hour beforehand.

Organizations, notably in Japan, Siberia, Alaska, and Hawaii, have been set up to provide tsunami warnings. A key development is the Seismic Sea Wave Warning System (SSWWS), an internationally supported system designed to reduce loss of life in the Pacific Ocean. Centred in Honolulu, it issues alerts based on reports of earthquakes from circum-Pacific seismographic stations.

Seiches

These are rhythmic motions of water in nearly landlocked bays or lakes that are sometimes induced by earthquakes and by tsunamis (in the case of the former). Oscillations of this sort may last for hours or even for a day or two.

The great Lisbon earthquake of 1755 caused the waters of canals and lakes in areas as far away as Scotland and Sweden to go into observable oscillations. Seiche surges in Texas in the southwestern United States commenced between 30 and 40 minutes after the 1964 Alaska earthquake and were produced by seismic surface waves passing through the area.

Of course, P waves from an earthquake may pass through the sea following refraction through the seafloor. The speed of these waves is about 1.5 kilometres per second, the speed of sound in water. If such waves meet a ship with sufficient intensity, they give the impression that the ship has struck a submerged object. This phenomenon is called a seaquake.


Some great earthquakes

About 50,000 earthquakes large enough to be felt or noticed without the aid of instruments occur annually over the entire Earth. Of these, approximately 100 are of sufficient size to produce substantial damage if their centres are near areas of habitation. Very great earthquakes occur at an average rate of about one per year. Among the great earthquakes of the past are those of Lisbon in 1755; New Madrid, Mo., U.S., in December 1811 and January and February 1812; San Francisco in 1906; Tokyo-Yokohama in 1923; the coast of Chile in 1960; south-central Alaska in 1964; T'ang-shan, China, in 1976; and Mexico in 1985. Their devastating effects are briefly described below.

Lisbon

On Nov. 1, 1755, Lisbon was heavily damaged by a great earthquake that occurred at 9:40 AM. The source was situated some distance off the coast. The violent shaking demolished large public buildings and about 12,000 dwellings. As November 1 was All Saint's Day, a large part of the population was attending religious services; most of the churches were destroyed, resulting in many casualties. The total number of persons killed in Lisbon alone was estimated to be as high as 60,000, including those who perished by drowning and in the fire that burned for about six days following the shock. Damage was reported in Algiers, 1,100 kilometres to the east. The earthquake generated a tsunami that produced waves about six metres high at Lisbon and 20 metres high at C�diz, Spain. The waves traveled on to Martinique, a distance of 6,100 kilometres in 10 hours, and there rose to a height of four metres.

New Madrid

Three large earthquakes occurred near New Madrid in southern Missouri on Dec. 16, 1811, and Jan. 23 and Feb. 7, 1812. There were numerous aftershocks, of which 1,874 were large enough to be felt in Louisville, Ky., some 300 kilometres away. The principal shock produced waves of sufficient amplitude to shake down chimneys in Cincinnati, Ohio, about 600 kilometres away. The waves were felt as far as Canada in the north and the Gulf Coast in the south. The area of greatest shaking was about 100,000 square kilometres, considerably greater than the area involved in the San Francisco earthquake in 1906. It has been discovered that in continental earthquakes such as the Missouri shocks, the area of strong shaking can be abnormally large compared with that in shocks along the Pacific coast of the United States. In one region 240 kilometres long by 60 kilometres wide, the ground sank from one to three metres and was covered by inflowing river water. Sand liquefaction effects were widespread. In certain locations, forests were overthrown or ruined by the loss of soil shaken from the roots of the trees.

San Francisco

On April 18, 1906, at about 5:12 AM, the San Andreas Fault slipped over a segment about 430 kilometres long, extending from San Juan Bautista in San Benito County to the upper Mattole River in Humboldt County and from there perhaps out under the sea to an unknown distance. The shaking was felt from Los Angeles in the south to Coos Bay, Ore., in the north. Damage was severe in San Francisco and in other towns situated near the fault--e.g., San Jose, Salinas, and Santa Rosa (30 kilometres from the fault). Approximately 700 people were killed. In San Francisco the earthquake started a fire, which destroyed the central business district.

Tokyo-Yokohama

A great earthquake struck the Tokyo-Yokohama metropolitan area near noon on Sept. 1, 1923. The death toll from this shock was estimated at more than 140,000. Fifty-four percent of the brick buildings and 10 percent of the reinforced concrete structures collapsed. Many hundreds of thousands of houses were either shaken down or burned. The shock started a tsunami that reached a height of 12 metres at Atami on Sagami-nada (Sagami Gulf), where it destroyed 155 houses and killed 60 persons.

Chile

The source of this earthquake in 1960 extended over a distance of about 1,100 kilometres along the southern Chilean coast. Casualties included about 5,700 killed and 3,000 injured, and property damage amounted to many millions of dollars. Seismic sea waves excited by the earthquake caused death and destruction in Hawaii, Japan, and the Pacific coast of the United States.

Alaska

On March 27, 1964, a great earthquake with a Richter magnitude 8.3-8.5 (see below) occurred in south central Alaska. It released at least twice as much energy as the 1906 San Francisco earthquake and was felt on land over an area of almost 1,300,000 square kilometres. The death toll was only 131 because of the low density of the state's population, but property damage was very high. The earthquake tilted an area of at least 120,000 square kilometres. Landmasses were thrust up locally as high as 25 metres to the east of a line extending northeastward from Kodiak Island through the western part of Prince William Sound. To the west, land sank as much as 2.5 metres. Extensive damage in coastal areas resulted from submarine landslides and tsunamis. Tsunami damage occurred as far away as Crescent City, Calif. The occurrence of tens of thousands of aftershocks indicates that the region of faulting extended about 1,000 kilometres.

T'ang-shan

The coal-mining and industrial city of T'ang-shan, located about 110 kilometres east of Peking, was almost razed in the tragic earthquake of July 28, 1976. The death toll exceeded 240,000 persons, and probably another 500,000 were injured. Most persons were killed from the collapse of unreinforced masonry homes, where they were asleep.

Mexico

The main shock occurred at 7:18 AM on Sept. 19, 1985. The cause was a fault slip along the Benioff zone (a band of intermediate- and deep-earthquake foci along a planar dipping zone) under the Pacific coast of Mexico. Although 400 kilometres from the epicentre, Mexico City suffered major building damage and more than 10,000 of its inhabitants were reported killed. The highest intensity was in the central city, which is founded on a former lake bed. The ground motion there measured five times that in the outlying districts, which have different soil foundations.






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