Seismology
Seismology is the science that studies earthquakes and phenomena connected with them. Seismology is a branch of geophysics.
Seismology attempts to explain the origin of earthquakes, where, when, and why they occur, what accompanies them, and how to forecast them. Earthquakes were mentioned in written historical documents as early several thousands of years ago, but their serious study began only in the nineteenth century. As a rough guide, the earthquake is a vibration of the ground tangible in a definite place; the stronger these vibrations are, the more damage an earthquake can cause.
Two variables are usually used for describing the earthquake power: magnitude and intensity. Magnitude is an objective parameter, which is connected with the ground displacement at the point of its measurement; the bigger the displacement, the stronger is the earthquake. Earthquakes with magnitudes bigger than 5–6 are considered powerful ones. Intensity is a parameter that is not measured by a device. Different factors are taken into account to determine the intensity of an earthquake, and its value varies relative to locations accessed. The Modified Mercalli Intensity Scale determines earthquake intensity in the United States by gathering information, including witness accounts and building damage, and assigning a numeral scale from I (low intensity, no earth movement felt) to XII (visible earth movement seen, severe building damage).
The earth's crust, the upper layer of Earth's surface, consists of a solid medium with different values of parameters in different points, and is exposed to permanent action of different forces, which are also irregular. Action of these forces can lead to a situation in which some parts of the crust can occur under the condition of very high tension (for example, like a rod that is curved). If the tension remains too high, the crust is damaged in some points (the rod is broken). In this case, a very big amount of energy becomes free, and this energy transfers into elastic waves of different kinds, which can spread to great distances from the damage point. This illustrates a simplified model of an earthquake.
It is possible to distinguish the areas where earthquakes occur more often than in other places. Usually, these are mountain areas, and areas circumambient the Pacific (Japan, for example). Seismology studies these areas together with geology, in which during the late nineteenth century arose the special theory (plate tectonics) which explains a set of phenomena, in particular the occurrence of active seismic zones and non-active zones (platform regions). Earthquakes are practically nonexistent in nonactive zones (for example, the Russian platform can be referred to as one of these regions).
Both seismology and geology use the achievements of physics (such as elasticity theory and hydrodynamics) in the creation of theories such as plate tectonics. In studying earthquakes, the oscillation theory and the theory of wave propagation in elastic media are used. The main devices used for earthquake study are seismometers, which record media oscillations at the point of the device location. Nowadays, such devices are located in many points on the earth's surface, on the ocean bottom, in shafts—often they are joined in special nets. The analysis of seismometers (seismograms), which take into account wave propagation theory, performs another important role; it permits scientists to understand deep-Earth structure. Even with improved theory, calculation methods, and equipment used in seismology, reliable forecasts of earthquakes still cannot be achieved.
Another important aspect of seismology is that its applied methods permit scientists to search useful minerals, especially oil. Seismologic methods also give the most precise results in underground nuclear tests control.
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