underground shocks and vibrations of the earth’s surface brought about by natural causes, mainly tectonic processes. Insome regions of the earth earthquakes occur often and sometimes attain great force, breaking up the ground, destroyingbuildings, and causing loss of life. The number of earthquakes annually recorded on the earth reaches into the hundreds ofthousands. The overwhelming majority of them, however, are weak, and only a few of them reach catastrophic proportions.Among pre-20th century catastrophic earthquakes are the Lisbon earthquake of 1755, the Vernyi earthquake of 1887, whichdestroyed the city of Vernyi (now Alma-Ata), and the earthquakes in Greece in 1870-73. The largest earthquakes of the 20thcentury are indicated in Table 3. Earthquakes are classified by their intensity, that is, by their manifestations on the earth’ssurface, in accordance with the 12 points of the MSK-64 international seismic scale (see Table 1).
| Table 1. Seismic scale (abridged) | |
|---|---|
| NumberDesignation | Brief description |
| 1 Not felt.................... | Detected only by seismographs. |
| 2 Very weak............ | Felt by persons at rest. |
| 3 Weak................. | Felt only by a small part of the population. |
| 4 Moderate.............. | Recognized by light rattling and vibration of small objects, dishes, and windowpanes and bycreaking of doors and walls. |
| 5 Ratherstrong................. | General shaking of buildings, vibration of furniture; cracking of windowpanes and plaster; sleepersawakened. |
| 6 Strong.............. | Felt by all; pictures fall off walls; pieces of plaster broken off; light damage to buildings. |
| 7 Very strong........... | Cracks in walls of stone buildings; no damage to earthquake-proof and wooden structures. |
| 8 Destructive........... | Cracks on steep slopes and in wet ground; monuments shift position or fall; houses heavilydamaged. |
| 9 Ruinous............. | Stone houses heavily damaged or destroyed. |
| 10 Disastrous.............. | Large cracks in ground surface; landslides and cave-ins; stone structures destroyed; railroad railsbent. |
| 11Catastrophic................ | Wide cracks in ground surface; many landslides and cave-ins; stone dwellings completelydestroyed. |
| 12 Verycatastrophic......... | Large-scale changes in ground surface; numerous fissures, cave-ins, landslides; formation ofwaterfalls, dams on lakes; diversion of river courses; no structure left standing. |
The zone of origin of an underground shock—the focal point of an earthquake—is a region within the earth where energy isreleased that has accumulated over a long period of time. In a geological sense, the focus is a fault or group of faults, alongwhich the almost instantaneous displacement of masses takes place. The point at the center of the focus is conventionallydesignated the hypocenter. The projection of the hypocenter on the surface of the earth is called the epi-center. This issurrounded by the pleistoseismic zone, or zone of greatest destruction. The lines joining points of equal vibration intensity (inscale numbers) are known as isoseismic lines.
The relation between the number of underground shocks N and their intensity in the epicenterI0 is expressed approximately bythe formula log N = α +βI0 , where α and β are constants. Elastic seismic waves, which include longitudinal P waves andtransverse S waves, are propagated in all directions from the focus of the earthquake. Rayleigh and Love seismic surfacewaves radiate in all directions on the earth’s surface from the epicenter. Earthquake focuses originate at different depths (h).Most of them occur in the earth’s crust at depths ranging from 20 to 30 km. A large number of shocks originating in depths ofhundreds of kilometers (upper mantle) have been recorded in some regions.
An earthquake is a powerful manifestation of the interior forces of the earth. An enormous amount of kinetic energy E isreleased in the focus during each earthquake:E ~ 1015 joules (J) in Ashkhabad (1948), E ~ 1016 J in San Francisco (1906),and E ~ 1018 J in Alaska (1964). Elastic energy (in the form of earthquakes) on the order of 0.5 x 1019 J is released each yearover the entire earth; however, this amounts to less than 0.5 percent of the total energy of the earth’s endogenic (internal)processes.
Earthquake intensity, measurable according to numbers on a scale, characterizes the degree of vibration on the surface of theearth, which depends on the depth of occurrence of the earthquake focus. The magnitude (M) of an earthquake, which servesas a measure of total wave energy, is an arbitrary number proportional to the logarithm of the maximum amplitude of soilparticles; it is measured from observations at seismic stations and is expressed in relative units. The magnitude of the largestearthquakes does not exceed 9.
A relationship exists between the number of earthquakes (N) and their magnitude (M) which may be expressed approximatelyby the formula: log N =a - bM, where a and b are constants. Earthquake energy (E) is associated with magnitude by arelationship of the form: log E =a1 + b1M Different values are given for the coefficients a1 and b1 although approximate valuesof 4 for a1 and 1.6 for b1 are considered most appropriate. The valued = log E is sometimes called the energy class of anearthquake. In an earthquake for which M = 5, energy ~ 1012 J is released from the focus and K =12; where M = 8.0, E ~ 1017J and k = 17. The magnitude(M), intensity (I0), and depth of focus (h) are interrelated. Table 2 may be used for theapproximate determination of one of these quantities from the two others.
| Table 2. Approximate correlation of magnitude and intensity in relation to depth of focus | |||||
|---|---|---|---|---|---|
| Depth of locus | Magnitude | ||||
| 5 | 6 | 7 | 8 | ||
| 10 Km.................. | 7 | 8-9 | 10 | 11-12 | Intensity |
| 20 km................. | 6 | 7-8 | 9 | 10-11 | |
| 40 km....................... | 5 | 6-7 | 8 | 9-10 | |
In recent decades, much progress has been made in the development of detailed statistical methods of earthquake analysis.These have been used to compile charts of seismic activity and vibration maps (for comparison of earthquake frequency forvarious energy classes at a given point), as well as frequency graphs (for the relationship between the frequency andmagnitude of earthquakes). Earthquake distribution over the earth’s surface is extremely irregular. Earth-quakes occur inportions of the earth’s crust within which the most recent, differentiated tectonic movements have been developing. Two majorearthquake belts have been identified—the Mediterranean, which extends across southern Eurasia from the coast of Portugalin the west to the Malay Archipelago in the east, and the Pacific , which encompasses the perimeter of the Pacific Ocean.These belts include young folded mountain systems, that is, epigeosynclinal orogenies (Alpine, Apennine, Carpathian,Caucasian, Himalayan, Cordilleran, Andean), and the mobile zones of the underwater continental margins, which manyinvestigators interpret as modern geosynclinal regions or folded systems in the initial stages of development (the westernperiphery of the Pacific Ocean with the Aleutian, Kuril, Japanese, Malay, and New Zealand island arcs; the Caribbean andMediterranean seas). Beyond the limits of these belts, within the continents, earth-quake epicenters are confined to regions ofvery recent tec-tonic activity (epiplatform orogenies of the Tien-Shan type) and to rift zones, which are accompanied by theformation of fracture systems (the rifts of East Africa and the Red Sea, the Baikal system of rifts). Within oceans, themidocean ridges are characterized by considerable seismic activity. On platforms and on most of the ocean bottomearthquakes occur rarely and do not attain great force.
Detailed analysis of the origin of underground shocks indicates that earthquakes are a reaction of material of the earth’s crustor mantle to tectonic stresses that are continually building up within the interior of the earth. Compression stressespredominate, although tension stresses may also be observed in some places.
The analysis of seismic, geological, and geophysical data makes it possible to identify in advance those regions in whichearthquakes may be expected in the future and to evaluate their maximum intensity. This is the essence of seismic zonation.In the USSR a seismic zonation chart is an official document that must be taken into consideration by planning organizationsin earthquake regions. Strict observance of earthquake-proof construction specifications makes it possible to reduceconsiderably the destructive effects of earthquakes on buildings and other engineering structures. It is probable that in thefuture even the problem of earthquake prediction will be solved successfully. The basic method for solving this problem is torecord in detail the “foresigns” of earthquakes—weak preliminary tremors (foreshocks), deformations of the earth’s surface,variations in the parameters of geophysical fields, and other changes in the condition and characteristics of material in thezone of a future earth-quake focus.
Earthquakes have been described since ancient times. In the 19th century earthquake catalogs were compiled for the entireworld (J. Milne, R. Mallet) and for the Russian empire (I. V. Mushketov, A. P. Orlovj, and monographs dealing with the largestand best studied earthquakes (especially those in Italy) were published. In the early 20th century much attention was devotedto the geological aspect of earthquakes (for example, the works of K. I. Bogdanovich, V. N. Veber, and D. I. Mushketov inRussia; those of F. Montessus de Ballore and A. Sieberg abroad), to the development of seismometers, and to theestablishment of seismic stations (B. B. Golitsyn, P. M. Nikiforov, A. V. Vikhert, D. A. Kharin, D. P. Kirnos). Earthquakesbecame the subject of study of a special branch of knowledge—seismology.
Physical and mathematical methods have been developed in seismology which are being used to study not only earth-quakesbut the interior structure of the earth as well; seismology has also come to include prospecting for mineral deposits.Earthquake observations are carried out by special seismic surveys.
REFERENCES
Gutenberg, B.,andC. Richter.Seismichnost’ ZemlL Moscow, 1948. (Translated from English.)Savarenskii, E. F., and D. P. Kirnos. Elementy seismologii i seismometrii. Moscow, 1955.
Atlas zemletriasenii v SSSR. Moscow, 1962.
Seismicheskoe raionirovanie SSSR. Moscow, 1968.
| Table 3. Largest earthquakes of the 20th century | ||||
|---|---|---|---|---|
| Date(newstyle),GMT | Location of epicenter | Magnitude | Intensity | Description |
| Europe | ||||
| 1908, Dec.28 | Sicily (Italy).................. | 7.5 | – | Destroyed Messina and other population centers insouthern Italy; tsunami waves 14 m high; 100,000-160,000 persons perished. |
| 1927, Sept.1 1 | Southern Crimean shore, south ofYalta (USSR)................. | 6.5 | up to 8 | Many structures damaged (from Sevastopol toFeodosiia). |
| 1953, Aug.12 | Ionian Islands (Greece)............ | 7.5 | – | Settlements destroyed on Cephalonia; part ofisland sank below sea level. |
| 1963, July26 | Skopje (Yugoslavia).............. | 6 | 9-10 | Almost 80 percent of city’s buildings destroyed ordamaged; over 2,000 killed. |
| 1969, Feb. 8 | Southern and western coasts ofPortugal.......... | 8 | – | Damage to Lisbon, Casablanca; ground surfacecovered with fissures. |
| 1969, Oct.27 | Southwestern YugoslaviaAsia............ | 6.4 | 9 | Catastrophic; Banja Luka reduced to rubble. |
| 1902, Dec.16 | Fergana Valley, city of Andizhan(USSR)............ | – | 9 | More than 4,500 persons perished. |
| 1905,Apr. 4 | Himalayas............. | 8 | – | |
| 1905, July23 | Bolnai Mountains (MongolianPeople’s Republic).................... | 8.2 | – | Fissure 400 km long formed in region east of theKhan-Khukhei Mountains. |
| 1907, Oct.21 | Southern slopes of GissarMountains (USSR)........... | – | 9 | Karatag and about 150 settlements destroyed;1,500 persons perished. |
| 1911, Jan. 3 | Kebin Valley, southern slope ofTransili Alatau Range(USSR)............ | 8 | 9 | Vernyi (now Alma-Ata) destroyed; cave-ins,mountain rivers dammed up. |
| 1911, June15 | Ryukyu Islands (Japan).......... | 8.2 | – | Huge landslides and cave-ins; 100,000 personsperished. |
| 1923, Sept.1 | Honshu Island (Japan).............. | 8.2 | – | Catastrophic; Tokyo, Yokohama devastated;150,000 persons killed; 10 m high tsunami wavesin Sagami Bay. |
| 1927, Mar. 7 | Honshu Island (Japan)......... | 7.8 | – | Catastrophic; city of Mineyama left in ruins; over1,000 persons perished. |
| 1938, Feb. 1 | Banda Sea (Indonesia)............ | 8.2 | – | |
| 1939, Dec.26 | Anti-Taurus Mountains(Turkey)................ | 8.0 | – | Catastrophic; about 30,000-40,000 persons died;Black Sea water receded 50 m, then flooded coast20 m inland. |
| 1941, Apr.20 | Surkhob valley, settlement ofGarm (USSR).......... | 6.5 | 8-9 | Over 60 settlements destroyed. |
| 1946, Nov. 2 | Northern part of ChatkalMountains (USSR)........... | 7.5 | 9 | Damage to hundreds of buildings in Tashkent andother cities; deformation of earth’s crust. |
| 1948, Oct. 5 | Ashkhabad (USSR)........... | 7 | 9 | Catastrophic; much of city destroyed within 20sec. |
| 1949, July10 | Gissar-Alai Mountain System,Khait (USSR).......... | 7.5 | 9+ | Over 150 settlements damaged. |
| 1952, Nov. 4 | Kuril Islands southeast ofShipunskii Peninsula(USSR)............... | 8.2 | – | Catastrophic; 18 m high tsunamis caused heavydamage to coasts of Kamchatka and northern partof Kuril Islands. |
| 1957, June27 | Transbaikalia, Southern MuiaRange
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