Earthquakes
Earthquakes
2. seismology.
| 单词 | earthquakes | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| 释义 | EarthquakesEarthquakes2. seismology. EarthquakesEarthquakes(religion, spiritualism, and occult)From the very beginning of astrology, astrologers have been concerned with the correlations between celestial events and terrestrial traumas. The Mesopotamians, as well as other ancient peoples, viewed earthquakes, plagues, droughts, and the like as being tied to such unusual heavenly occurrences as eclipses and comets. For their part, contemporary astrologers have viewed everything from the heliocentric nodes of the planets to the interaction of Uranus with specific planetary configurations as influencing earthquakes. The problem with all current methods is that, after an upheaval has occurred, it is easy enough to look back at a chart for the given day and point out the various factors that appear to be correlated with the event. Prediction, however, is another matter. Although many have claimed to have found the key to earthquake prediction, no factor yet discovered dependably predicts such upheavals. The closest thing to a reliable “earthquake factor” is a syzygy (an alignment of three celestial bodies in a straight line) involving Earth, the Sun, and the Moon. Syzygies occur every new and full moon; exact syzygies occur during eclipses (a partial verification of ancient astrology). Researchers who have observed this correlation speculate that it is the larger gravitational force—generated by the combined gravity of the Sun and the Moon pulling together (or apart) along the same axis—that disturbs the crust of the Earth and sets the stage for an earthquake, which may occur immediately or within a week of the syzygy. In addition to the exactness of the alignment, people involved in earthquake prediction, such as Jim Berkland, author and publisher of Syzygy: An Earthquake Newsletter, also pay attention to the distance of the Moon from Earth (and, to a lesser extent, the distance of the Sun from Earth). The Moon’s orbit is elliptical rather than circular. The point farthest away from Earth is the Moon’s apogee; the closest point is the Moon’s perigee. The distance between Earth and the Moon varies 14 percent every 15 days. Clearly, the gravitational force exerted by the Moon on Earth is greatest during a perigee, making the potential for earthquakes greater when syzygies occur during perigees (e.g., the Santa Cruz, California, earthquake of October 1989 took place a few days after a perigean full moon). The combination of an eclipse with a lunar perigee is almost certain to lead to a terrestrial upheaval somewhere on our planet. A good reference source for determining both perigees and eclipses is Neil F. Michelsen’s Tables of Planetary Phenomena. Another, nonastrological factor that acts as a predictor is the odd behavior of animals prior to an earthquake. Berkland observes that the number of missing animals reported in newspapers, for instance, shoots up just before an earthquake, as if they were somehow responding to a change in Earth’s magnetic field. In the March 1992 issue of his newsletter, Berkland refers to a passage in Helmut Tributsch’s When the Snakes Awake, “based on a popular brochure printed in China in 1973,” that describes the behavior of animals before an earthquake: “Cattle, sheep, mules and horses do not enter corrals. Pigs do not eat, and dogs bark madly. Ducks do not enter water and stay on shore. Chickens fly up into trees and scream loudly. Rats move their homes and flee. Hibernating snakes leave their burrows early and may freeze. Big cats pick up little ones and run. Frightened pigeons continuously fly and do not return to nests. Rabbits raise their ears, jump aimlessly and bump things. Fish are frightened and jump above water surface. Bees move their hives, making noise.” Sources:Earthquakesunderground shocks and vibrations of the earth’s surface brought about by natural causes, mainly tectonic processes. In some regions of the earth earthquakes occur often and sometimes attain great force, breaking up the ground, destroying buildings, and causing loss of life. The number of earthquakes annually recorded on the earth reaches into the hundreds of thousands. 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, which destroyed the city of Vernyi (now Alma-Ata), and the earthquakes in Greece in 1870-73. The largest earthquakes of the 20th century are indicated in Table 3. Earthquakes are classified by their intensity, that is, by their manifestations on the earth’s surface, in accordance with the 12 points of the MSK-64 international seismic scale (see Table 1).
The zone of origin of an underground shock—the focal point of an earthquake—is a region within the earth where energy is released that has accumulated over a long period of time. In a geological sense, the focus is a fault or group of faults, along which the almost instantaneous displacement of masses takes place. The point at the center of the focus is conventionally designated the hypocenter. The projection of the hypocenter on the surface of the earth is called the epi-center. This is surrounded by the pleistoseismic zone, or zone of greatest destruction. The lines joining points of equal vibration intensity (in scale numbers) are known as isoseismic lines. The relation between the number of underground shocks N and their intensity in the epicenterI0 is expressed approximately by the formula log N = α +βI0 , where α and β are constants. Elastic seismic waves, which include longitudinal P waves and transverse S waves, are propagated in all directions from the focus of the earthquake. Rayleigh and Love seismic surface waves 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 of hundreds 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 is released 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 year over 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 the earth, which depends on the depth of occurrence of the earthquake focus. The magnitude (M) of an earthquake, which serves as a measure of total wave energy, is an arbitrary number proportional to the logarithm of the maximum amplitude of soil particles; it is measured from observations at seismic stations and is expressed in relative units. The magnitude of the largest earthquakes does not exceed 9. A relationship exists between the number of earthquakes (N) and their magnitude (M) which may be expressed approximately by the formula: log N =a - bM, where a and b are constants. Earthquake energy (E) is associated with magnitude by a relationship of the form: log E =a1 + b1M Different values are given for the coefficients a1 and b1 although approximate values of 4 for a1 and 1.6 for b1 are considered most appropriate. The valued = log E is sometimes called the energy class of an earthquake. In an earthquake for which M = 5, energy ~ 1012 J is released from the focus and K =12; where M = 8.0, E ~ 1017 J and k = 17. The magnitude(M), intensity (I0), and depth of focus (h) are interrelated. Table 2 may be used for the approximate determination of one of these quantities from the two others.
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 for various energy classes at a given point), as well as frequency graphs (for the relationship between the frequency and magnitude of earthquakes). Earthquake distribution over the earth’s surface is extremely irregular. Earth-quakes occur in portions of the earth’s crust within which the most recent, differentiated tectonic movements have been developing. Two major earthquake belts have been identified—the Mediterranean, which extends across southern Eurasia from the coast of Portugal in 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 many investigators interpret as modern geosynclinal regions or folded systems in the initial stages of development (the western periphery of the Pacific Ocean with the Aleutian, Kuril, Japanese, Malay, and New Zealand island arcs; the Caribbean and Mediterranean seas). Beyond the limits of these belts, within the continents, earth-quake epicenters are confined to regions of very recent tec-tonic activity (epiplatform orogenies of the Tien-Shan type) and to rift zones, which are accompanied by the formation of fracture systems (the rifts of East Africa and the Red Sea, the Baikal system of rifts). Within oceans, the midocean ridges are characterized by considerable seismic activity. On platforms and on most of the ocean bottom earthquakes 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 crust or mantle to tectonic stresses that are continually building up within the interior of the earth. Compression stresses predominate, 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 which earthquakes 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 organizations in earthquake regions. Strict observance of earthquake-proof construction specifications makes it possible to reduce considerably the destructive effects of earthquakes on buildings and other engineering structures. It is probable that in the future even the problem of earthquake prediction will be solved successfully. The basic method for solving this problem is to record 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 the zone of a future earth-quake focus. Earthquakes have been described since ancient times. In the 19th century earthquake catalogs were compiled for the entire world (J. Milne, R. Mallet) and for the Russian empire (I. V. Mushketov, A. P. Orlovj, and monographs dealing with the largest and best studied earthquakes (especially those in Italy) were published. In the early 20th century much attention was devoted to the geological aspect of earthquakes (for example, the works of K. I. Bogdanovich, V. N. Veber, and D. I. Mushketov in Russia; those of F. Montessus de Ballore and A. Sieberg abroad), to the development of seismometers, and to the establishment of seismic stations (B. B. Golitsyn, P. M. Nikiforov, A. V. Vikhert, D. A. Kharin, D. P. Kirnos). Earthquakes became 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-quakes but 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. REFERENCESGutenberg, 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.
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