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s, 1963–1998]] An earthquake is a sudden and sometimes catastrophic movement of a part of the Earth 's Crust . Earthquakes result from the dynamic release of elastic strain energy that radiates Seismic Wave s. Earthquakes typically result from the movement of Fault s, planar zones of deformation within the Earth's upper Crust . The word earthquake is also widely used to indicate the source region itself. The Earth's Lithosphere is a patch work of plates in slow but constant motion (see Plate Tectonics ). Earthquakes occur where the Stress resulting from the differential motion of these plates exceeds the strength of the crust. The highest stress (and possible weakest zones) are most often found at the boundaries of the Tectonic Plates and hence these locations are where the majority of earthquakes occur. Events located at plate boundaries are called Interplate Earthquake s; the less frequent events that occur in the interior of the lithospheric plates are called Intraplate Earthquake s (see, for example, New Madrid Seismic Zone ). Earthquakes related to plate tectonics are called tectonic earthquakes. Most earthquakes are tectonic, but they also occur in Volcanic regions and as the result of a number of anthropogenic sources, such as reservoir induced seismicity, mining and the removal or injection of fluids into the crust. Seismic waves including some strong enough to be felt by Humans can also be caused by explosions (chemical or nuclear), landslides, and collapse of old mine shafts, though these sources are not strictly earthquakes. These sources will also show a different seismogram than earthquakes. CHARACTERISTICS Large numbers of earthquakes occur on a daily basis on Earth, but the majority are detected only by Seismometers and cause no damage. Most earthquakes occur in narrow regions around plate boundaries at depths down to a few tens of kilometers in the earth's Crust where it is rigid enough to support elastic strain. Where the crust is thicker and colder, they will occur at greater depths, whereas the opposite applies in portions of the crust that are hot. Along Subduction Zones , places where plates descend into the Mantle , earthquakes have been recorded at depths of up to 600 km, although these deep earthquakes are caused by different mechanisms than the more common, shallow events. Some deep earthquakes may be due to the transition of Olivine to Spinel , which is more stable in the deep mantle. Most of the world's earthquakes (90%, and 81% of the largest) take place in the 40,000 km-long, horseshoe-shaped zone called the Circum-Pacific Seismic Belt , also known as the '''Pacific Ring of Fire''', which for the most part bounds the Pacific Plate . {Link without Title} {Link without Title} Large earthquakes can cause serious destruction and massive loss of life through a variety of agents of damage, including fault rupture, vibratory ground motion (shaking), inundation ( Tsunami , Seiche , or dam failure), various kinds of permanent ground failure ( Liquefaction , Landslide s), and Fire or a release of Hazardous Material s. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life; nonetheless, for most earthquakes shaking is the dominant and most widespread cause of damage. There are four types of Seismic Wave s that are all generated simultaneously and can be felt on the ground. Responsible for the shaking hazard, they are P-wave s (primary waves), S-wave s (secondary or shear waves) and two types of surfaces waves, ( Love Waves and Rayleigh Waves ). .]] .]] Most large earthquakes are accompanied by other, smaller ones that can occur either before or after the main shock; these are called ''' Foreshock s''' and ''' Aftershock s''', respectively. While almost all earthquakes have aftershocks, foreshocks occur in only about 10% of events. The power of an earthquake is always distributed over a significant area, but in large earthquakes, it can even spread over the entire planet. Ground motions caused by very distant earthquakes are called Teleseism s. The Rayleigh Waves from the Sumatra-Andaman Earthquake Of 2004 caused ground motion of over 1 cm even at Seismometers that were located far from it, although this displacement was abnormally large. Using such ground motion records from around the world, seismologists can identify a point from which the earthquake's Seismic Wave s apparently originated. That point is called its '''focus''' or ''' Hypocenter ''' and usually coincides with the point where the fault slip started. The location on the surface directly above the hypocenter is known as the ''' Epicenter '''. The total length of the section of a fault that slips, the '''rupture zone''', can be as long as 1,000 km for the biggest earthquakes. Earthquakes that occur below sea level and have large vertical displacements can give rise to Tsunami s, either as a direct result of the deformation of the sea bed due to the earthquake or as a result of submarine Landslide s directly or indirectly triggered by the quake. MEASURING AND MAPPING EARTHQUAKES The severity of an earthquake can be expressed as a magnitude and as an '''intensity'''. However, and though often confused, these two terms indicate quite different observations. ''Magnitude,'' usually expressed as an Arabic Numeral , characterizes the size of an earthquake and is a measure of the energy released. In contrast, ''intensity'' indicates the local effects and potential for damage produced by an earthquake on the Earth's surface as it affects humans, animals, structures, and natural objects such as Bodies Of Water . Intensities are usually expressed in Roman Numerals , each representing the severity of the shaking resulting from an earthquake. Any given earthquake can be described by only one ''magnitude'', but many ''intensities'' since the earthquake effects vary with circumstances such as distance from the Epicenter and local soil conditions. The difference between ''intensity'' and ''magnitude'' was best described by s. It applies in Seismology because Seismograph s, or the Receivers , record the waves of elastic disturbance, or radio waves, that are radiated from the earthquake source, or the Broadcasting Station . Magnitude can be compared to the power output in Kilowatt s of a Broadcasting Station . Local intensity on the Mercalli Scale is then comparable to the signal strength on a receiver at a given Locality ; in effect, the quality of the signal. Intensity, like signal strength, will generally fall off with distance from the source, although it also depends on the local conditions and the pathway from the source to the point." Two fundamentally different but equally important types of scales are commonly used by seismologists to describe earthquakes. The original force or energy of an earthquake is measured on a ''magnitude scale'', while the intensity of shaking occurring at any given point on the Earth's surface is measured on an ''intensity scale''. : Seismic intensity scales The first intensity classification was devised by Domenico Pignataro in 1780s. Advancements were later made by P.N.G. Egen in 1828 and Robert Mallet in 1850s. The first widely accepted intensity scale, the , while the ''' European Macroseismic Scale ''' is used in Europe , the ''' Shindo Scale ''' is used in Japan , and the ''' MSK-64 Scale ''' is used in Russia and throughout the CIS . Most of these scales have twelve degrees of intensity, which are roughly equivalent to one another in values but vary in the degree of sophistication employed in their formulation. Magnitude scales The first attempt to qualitatively define a single, absolute value to describe the size of earthquakes was the magnitude scale (the name being taking from similarly formulated scales used to represent the brightness of stars). The Richter scale. In the , they do not measure the overall power of the source and can be negatively affected by '''saturation''' at higher magnitude values—meaning that they fail to report higher magnitude values for larger events. Further, since these scales too are empirical, they provide no values that are meaningful from a physics perspective. This does not mean, though, that they are useless: They are because they can be rapidly calculated, catalogues of them dating back many years are available, and the public is familiar with them. The moment-magnitude scale. Because of the limitations of the magnitude scales, a new, more uniformly applicable extension of them, known as ''' Moment Magnitude ''', or MW, was developed. In particular, for very large earthquakes moment magnitude gives the most reliable estimate of earthquake size. This is because seismic moment is derived from the concept of Moment in physics and therefore provides clues to the physical size of an earthquake—the size of fault rupture and accompanying displacement and length of slippage—as of as well as the amount of energy released. So while seismic moment, too, is calculated from Seismogram s, it can also be obtained by working backwards from geologic estimates of the size of the fault rupture and displacement. The values of moments for different earthquakes range over several orders of magnitude, and because they are not influenced by variables such as local circumstances, the results obtained make it easy to objectively compare the sizes of different earthquakes. These characteristics, plus the seismic moment's immunity to saturation at higher magnitudes and compatibility with other magnitude scales, led Tom Hanks and Hiroo Kanamori to introduce in 1979 the Moment Magnitude (MW) scale for representing the absolute size of earthquakes. Earthquake size and frequency of occurrence : Seismic maps showing the instrument-recorded intensities of the Nisqually Earthquake of February 28 2001 .]] showing the intensity of shaking felt by humans during the Nisqually earthquake; locality divisions are by ZIP Code .]] To show the extent of various levels of seismic effects within a particular locality, seismologists compile special maps called isoseismal maps. An isoseismal map uses contours to outline areas of equal value in terms of ground shaking intensity, ground surface Liquefaction , shaking amplification, or other seismic effects. Typically, these maps are created by combining historical instrument-recorded data with responses to postal questionnaires that are sent to each post office near the earthquake and to a sparser sample of post offices with increasing distance from the earthquake. This way of preparing a seismic hazard map can take months to complete. In contrast to the old method, a newer method of information collection takes advantage of the Internet to generate initial hazard maps almost instantly. Data are received through a questionnaire on the Internet answered by people who actually experienced the earthquake, reducing the process of preparing and distributing a map for a particular earthquake from months to minutes. Seismic hazard maps have many applications. They are used by insurance companies to set insurance rates for properties located in earthquake-risky areas, by civil engineers to estimate the stability of hillsides, by organizations responsible for the safety of nuclear waste disposal facilities, and also by Building Code s developers as the basis of design requirements. Shaking-hazard maps are also used to develop and update seismic zone maps used in Building Code s. The seismic zone maps depict seismic hazards as zones of different risk levels. Such zones are typically designated as Seismic Zone 0, Seismic Zone 1, Seismic Zone 2 and so on. The seismic zone maps usually show the severity of expected earthquake shaking for a particular level of probability, such as the levels of shaking that have a 1-in-10 chance of being exceeded in a 50-year period. Buildings and other structures must be designed with adequate strength to withstand the probable forces induced by seismic ground motions within the Seismic Zone where the building or structure is being constructed. CAUSES Most earthquakes are powered by the release of the elastic strain that accumulates over time, typically, at the boundaries of the plates that make up the Earth's Lithosphere via a process called Elastic-rebound Theory . The Earth is made up of tectonic plates driven by the heat in the Earth's mantle and core. Where these plates meet stress accumulates. Eventually when enough stress accumulates, the plates move, causing an earthquake. Deep Focus Earthquake s, at depths of hundreds of kilometres, are possibly generated as subducted lithospheric material catastrophically undergoes a Phase Transition since at the pressures and temperatures present at such depth elastic strain cannot be supported. Some earthquakes are also caused by the movement of Magma in Volcano es, and such quakes can be an early warning of volcanic eruptions. A rare few earthquakes have been associated with the build-up of large masses of water behind Dam s, such as the Kariba Dam in Zambia , Africa , and with the injection or extraction of fluids into the Earth's crust (e.g. at certain Geothermal Power plants and at the Rocky Mountain Arsenal ). Such earthquakes occur because the strength of the Earth's crust can be modified by fluid pressure. Earthquakes have also been known to be caused by the removal of Natural Gas from subsurface deposits, for instance in the northern Netherlands . Finally, ground shaking can also result from the detonation of Explosive s. Thus scientists have been able to monitor, using the tools of Seismology , Nuclear Weapon s tests performed by governments that were not disclosing information about these tests along normal channels. Earthquakes such as these, that are caused by human activity, are referred to by the term Induced Seismicity . Another type of movement of the Earth is observed by Terrestrial Spectroscopy . These oscillations of the earth are either due to the deformation of the Earth by Tide caused by the Moon or the Sun , or other phenomena. A recently proposed theory suggests that some earthquakes may occur in a sort of Earthquake Storm , where one earthquake will trigger a series of earthquakes each triggered by the previous shifts on the fault lines, similar to aftershocks, but occurring years later. SIZE AND FREQUENCY OF OCCURRENCE Larger earthquakes occur less frequently than smaller earthquakes, the relationship being Exponential ; namely, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. For example, it has been calculated that the average recurrence for the United Kingdom can be described as follows:
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