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Earthquakes occur every day on Earth, but the vast majority of them are minor and cause no damage. Large earthquakes can cause serious destruction and massive loss of life via a variety of agents of damage including fault rupture, vibratory ground motion (i.e., shaking), inundation (e.g., tsunami, seiche, dam failure), various kinds of permanent ground failure (e.g. liquefaction, landslide), and fire or hazardous material s release. In a particular earthquake, any of these agents of damage can dominate, and historically each has caused major damage and great loss of life, but for most earthquakes shaking is the dominant and most widespread cause of damage.
Most large earthquakes are accompanied by other, smaller ones, known as foreshocks when they occur before the principal or mainshock and aftershocks when they occur following it. The source of an earthquake is distributed over a significant area -- in the case of the very largest earthquakes, in excess of a thousand kilometres -- but it is usually possible to identify a point from which the earthquake waves appear to emanate. That point is called its "focus" and usually proves to be the point at which fault rupture was initiated. The position of the focus is known as the " hypocenter" and the location on the surface directly above it is the " epicenter." Earthquakes, especially those that occur beneath sea- or ocean-covered areas, can give rise to tsunamis, either as a direct result of the deformation of the sea bed due to the earthquake, or as a result of submarine landslips or "slides" indirectly triggered by it.
A class of earthquakes known as silent earthquakes are thought to be caused by very slow slippage. They are of extremely low intensity but can last for days or weeks releasing as much energy as large earthquakes.
In the 1930sCenturies: 19th century 20th century 21st century Decades: 1880s 1890s 1900s 1910s 1920s 1930s 1940s 1950s 1960s 1970s 1980s Years: 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 Events and trends Technology Jet engine invented Link Trainer invented Sc, a California seismologist named Charles F. Richter devised a simple numerical scale (which he called the magnitudeIn science, magnitude refers to the numerical size of something: see orders of magnitude. In mathematics, the magnitude of an object is a non-negative real number, which in simple terms is its length. In astronomy, magnitude refers to the logarithmic meas) to describe the relative sizes of earthquakes, which has come to be called the Richter scale. Since Richter, seismologists have developed a number of magnitude scales. Most of the scales in use in the Western world (such as the moment magnitude scaleThe moment magnitude scale introduced by Hiroo Kanamori, is used by seismologists to compare the size of earthquakes. The moment magnitude is defined by the equation , where is the seismic moment measured in dyne- centimeters (dyn·cm 10-7N·m). In SI-Units) are mutually consistent to a sufficient extent that the term "Richter scale" is routinely used in reporting these numbers to the public. Other scales (and other ways of describing the size of earthquakes) are used in some non-Western countries, and by earthquake specialists. For example, the JapanJapan (, Nippon/Nihon literally "the origin of the sun") is a country in East Asia situated on a chain of islands east of the Asian continent on the western edge of the Pacific Ocean. The largest of these islands are, from north to south, Hokkaido , Honshese scale for measuring the force of earthquakes measures horizontal movement. The press sometimes mistakenly reports such values as "Richter magnitude", and this has given rise to public confusion.
A Shakemap recorded by the Pacific Northwest Seismograph NetworkThe Pacific Northwest Seismograph Network or PNSN operates seismograph stations and locates earthquakes in Washington and Oregon. The network is funded by the US Geological Survey ( USGS), the Department of Energy, and the State of Washington. External li that shows the instrument recorded intensityIn physics, intensity is a measure of the time-averaged energy flux. To find the intensity, take the energy density (that is, the energy per unit volume) and multiply it by the velocity at which the energy is moving. The resulting vector has the units of of the shaking of the Nisqually earthquake on Febuary 28 2001.
A Community Internet Intensity Map generated by the USGS that shows the intensity felt by humans by zipcode of the shaking of the Nisqually earthquake on Febuary 28 2001.
Earthquake effects are described in terms of intensity, a scale which attempts to quantify the severity of shaking at a given location. A number of intensity scales are in use, and there is a significant degree of consistency amongst them. The best known is the Mercalli (or Modified Mercalli, MM) scale, but the more consistent and analytical European Macroseismic Scale (EMS) is now increasingly widely used. In Japan the Japan Meterological Agency seismic intensity scale (JMA) is used.