1.
Ground zero
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In terms of nuclear explosions and other large bombs, the term ground zero describes the point on the Earths surface closest to a detonation. In the case of an explosion above the ground, ground zero refers to the point on the ground directly below the nuclear detonation and is called the hypocenter. Generally, the term is used in relation to earthquakes, epidemics. The term is distinguished from the zero point in that the latter can also be located in the air, underground. The origins of the ground zero began with the Trinity test in Jornada del Muerto desert near Socorro, New Mexico. William Laurence, a reporter with the Manhattan Project, reported that Zero was the code name given to the spot chosen for the test in 1945. The term was military slang, used at the Trinity site where the tower for the first nuclear weapon was at point zero. At Hiroshima, the hypocenter of the attack was Shima Hospital, during the Cold War, The Pentagon, the headquarters of the United States Department of Defense in Arlington County, Virginia, was an assured target in the event of nuclear war. The open space in the center of the Pentagon became known informally as ground zero, a snack bar that used to be located at the center of this open space was nicknamed Cafe Ground Zero. The destroyed World Trade Center site soon known as ground zero. Rescue workers also used the phrase The Pile, referring to the pile of rubble that was left after the buildings collapsed, in advance of the 10th anniversary of the attacks, New York City mayor Michael Bloomberg urged that the ground zero moniker be retired, saying. The time has come to call those 16 acres what they are, The World Trade Center and the National September 11th Memorial and Museum
2.
Earthquake
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An earthquake is the shaking of the surface of the Earth, resulting from the sudden release of energy in the Earths lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to people around. The seismicity or seismic activity of an area refers to the frequency, type, Earthquakes are measured using measurements from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe and these two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly imperceptible or weak and magnitude 7 and over potentially cause damage over larger areas. The largest earthquakes in historic times have been of magnitude slightly over 9, intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the damage to structures it causes. At the Earths surface, earthquakes manifest themselves by shaking and sometimes displacement of the ground, when the epicenter of a large earthquake is located offshore, the seabed may be displaced sufficiently to cause a tsunami. Earthquakes can also trigger landslides, and occasionally volcanic activity, in its most general sense, the word earthquake is used to describe any seismic event — whether natural or caused by humans — that generates seismic waves. Earthquakes are caused mostly by rupture of faults, but also by other events such as volcanic activity, landslides, mine blasts. An earthquakes point of rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter, tectonic earthquakes occur anywhere in the earth where there is sufficient stored elastic strain energy to drive fracture propagation along a fault plane. The sides of a fault move past each other smoothly and aseismically only if there are no irregularities or asperities along the surface that increase the frictional resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behavior, once the fault has locked, continued relative motion between the plates leads to increasing stress and therefore, stored strain energy in the volume around the fault surface. This continues until the stress has risen sufficiently to break through the asperity, suddenly allowing sliding over the portion of the fault. This energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface and this process of gradual build-up of strain and stress punctuated by occasional sudden earthquake failure is referred to as the elastic-rebound theory. It is estimated that only 10 percent or less of a total energy is radiated as seismic energy. Most of the energy is used to power the earthquake fracture growth or is converted into heat generated by friction
3.
Aftershock
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An aftershock is a smaller earthquake that occurs after a previous large earthquake, in the same area of the main shock. If an aftershock is larger than the shock, the aftershock is redesignated as the main shock. Aftershocks are formed as the crust around the fault plane adjusts to the effects of the main shock. Typically, aftershocks are found up to an equal to the rupture length away from the fault plane. The pattern of aftershocks helps confirm the size of area that slipped during the main shock, Aftershocks rates and magnitudes follow several well-established empirical laws. The frequency of aftershocks decreases roughly with the reciprocal of time after the main shock and this empirical relation was first described by Fusakichi Omori in 1894 and is known as Omoris law. It is expressed as n = k where k and c are constants, a modified version of Omoris law, now commonly used, was proposed by Utsu in 1961. N = k p p is a third constant which modifies the decay rate. According to these equations, the rate of aftershocks decreases quickly with time, the rate of aftershocks is proportional to the inverse of time since the mainshock and this relationship can be used to estimate the probability of future aftershock occurrence. These patterns describe only the behavior of aftershocks, the actual times, numbers and locations of the aftershocks are stochastic. N =10 a − b M Where, N is the number of greater or equal to M M is magnitude a and b are constants In summary, there are more small aftershocks. Aftershocks are dangerous because they are unpredictable, can be of a large magnitude. An aftershock sequence is deemed to have ended when the rate of seismicity drops back to a background level, i. e. no further decay in the number of events with time can be detected. Land movement around the New Madrid is reported to be no more than 0.2 mm a year, Aftershocks on the San Andreas are now believed to top out at 10 years while earthquakes in New Madrid are considered aftershocks nearly 200 years after the 1812 New Madrid earthquake. Some scientists have tried to use foreshocks to help predict upcoming earthquakes, on the East Pacific Rise however, transform faults show quite predictable foreshock behaviour before the main seismic event. Reviews of data of past events and their foreshocks showed that they have a low number of aftershocks, seismologists use tools such as the Epidemic-Type Aftershock Sequence model to study cascading aftershocks. Following a large earthquake and aftershocks, many people have reported feeling phantom earthquakes when in fact no earthquake was taking place and this condition, known as earthquake sickness is thought to be related to motion sickness, and usually goes away as seismic activity tails off. Earthquake Aftershocks Not What They Seemed at Live Science
4.
Intraplate earthquake
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An intraplate earthquake occurs in the interior of a tectonic plate, whereas an interplate earthquake is one that occurs at a plate boundary. Interplate earthquakes, which occur at plate boundaries, are more common, nonetheless, very large intraplate earthquakes can inflict heavy damage, particularly because such areas are not accustomed to earthquakes and buildings are usually not seismically retrofitted. The surface of the Earth is made up of seven primary and eight secondary tectonic plates, the large plates move very slowly, owing to convection currents within the mantle below the crust. Because they do not all move in the direction, plates often directly collide or move laterally along each other. Relatively few earthquakes occur in environments, most occur on faults near plate margins. By definition, intraplate earthquakes do not occur near plate boundaries, compared to earthquakes near plate boundaries, intraplate earthquakes are not well understood, and the hazards associated with them may be difficult to quantify. The Charleston quake was particularly surprising because, unlike Boston and New York, in 2001, a large intraplate earthquake devastated the region of Gujarat, India. The earthquake occurred far from any plate boundaries, which meant the region above the epicenter was unprepared for earthquakes, in particular, the Kutch district suffered tremendous damage, where the death toll was over 12,000 and the total death toll was higher than 20,000. The 1888 earthquake in Río de la Plata, Brazil, was also an intraplate quake, many cities live with the seismic risk of a rare, large intraplate earthquake. The cause of earthquakes is often uncertain. In many cases, the fault is deeply buried. Under these circumstances it is difficult to calculate the exact seismic hazard for a given city, some progress is being made in understanding the fault mechanics driving these earthquakes. Scientists continue to search for the causes of earthquakes. The best success has come with detailed micro-seismic monitoring, involving dense arrays of seismometers, in this manner, very small earthquakes associated with a causative fault can be located with great accuracy, and in most cases these line up in patterns consistent with faulting. Cryoseisms can sometimes be mistaken for intraplate earthquakes, New Madrid Seismic Zone Saint Lawrence rift system Stein, S. and S. Mazzotti. Continental Intraplate Earthquakes, Science and Policy Issues, Geological Society of America, Special Paper 425
5.
Slow earthquake
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A slow earthquake is a discontinuous, earthquake-like event that releases energy over a period of hours to months, rather than the seconds to minutes characteristic of a typical earthquake. That is, they are compared to a regular earthquake. Earthquakes generate seismic waves when the rupture in the system occurs, the causes that lead to slow earthquakes have only been theoretically investigated, by the formation of longitudinal shear cracks that were analysed using mathematical models. The different distributions of initial stress, sliding frictional stress, if the initial stress minus the sliding frictional stress is low, and the specific fracture energy or the strength of the crustal material is high then slow earthquakes will occur regularly. In other words, slow earthquakes are caused by a variety of stick-slip, asperities are tiny bumps and protrusions along the faces of fractures. Faulting takes place all over Earth, faults can include convergent, divergent, and transform faults, as of 2013 some of the locations that have been recently studied for slow earthquakes include, Cascadia, California, Japan, New Zealand, Mexico, and Alaska. The locations of earthquakes can provide new insights into the behavior of normal or fast earthquakes. By observing the location of tremors associated with slow-slip and slow earthquakes, seismologists can determine the extension of the system, ETS events can last for weeks as opposed to normal earthquakes occur in a matter of seconds. Several slow-earthquake events around the world appear to have triggered major, conversely, major earthquakes trigger post-seismic creep in the deeper crust and mantle. Just like regular earthquakes, slow earthquakes can cause devastating tsunamis, the earthquake that caused this registered a magnitude of 7.8 and struck offshore the Mentawai islands in western Indonesia, causing more than 400 human casualties. Seismologists characterized it as a slow earthquake due to disproportionately large tsunami waves, rupture duration near 125 seconds, shallow near-trench slip, every five years a year-long quake of this type occurs beneath the New Zealand capital, Wellington. It was first measured in 2003, and has reappeared in 2008 and 2013 and it lasts for around a year each time, releasing as much energy as a magnitude 7 quake. C. s slow earthquakes fueled by fluid
6.
Submarine earthquake
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A submarine, undersea, or underwater earthquake is an earthquake that occurs underwater at the bottom of a body of water, especially an ocean. They are the cause of tsunamis. The magnitude can be measured scientifically by the use of the moment magnitude scale, understanding plate tectonics helps to explain the cause of submarine earthquakes. The plates converge upon one another, and one subducts below the other, or, little movements called fault creep are minor and not measurable. The plates meet with other, and if rough spots cause the movement to stop at the edges. When the rough spots can no longer hold, the release of the built-up motion releases. This area of slippage both horizontally and vertically is called the epicenter, and has the highest magnitude, and causes the greatest damage. As with an earthquake the severity of the damage is not often caused by the earthquake at the rift zone. Submarine earthquakes can also damage submarine communications cables, leading to disruption of the Internet. This is particularly common in Asia, where many submarine links cross submarine earthquake zones such as the Pacific Ring of Fire, the different ways in which tectonic plates rub against each other under the ocean or sea floor to create submarine earthquakes. The type of friction created may be due to the characteristic of the fault or the plate boundary as follows. Some of the areas of large tsunami producing submarine earthquakes are the Pacific Ring of Fire. The older, and denser plate moves below the lighter plate, the further down it moves, the hotter it becomes, until finally melting altogether at the asthenosphere and inner mantle and the crust is actually destroyed. The location where the two oceanic plates actually meet become deeper and deeper creating trenches with each successive action, there is an interplay of various densities of lithosphere rock, asthenosphere magma, cooling ocean water and plate movement for example the Pacific Ring of Fire. Therefore, the site of the sub oceanic trench will be a site of submarine earthquakes, for example the Mariana Trench, Puerto Rico Trench, and the volcanic arc along the Great Sumatran fault. A transform-fault boundary, or simply a boundary is where two plates will slide past each other, and the irregular pattern of their edges may catch on each other. The lithosphere is neither added to from the asthenosphere nor is it destroyed as in convergent plate action, rising convection currents occur where two plates are moving away from each other. In the gap, thus produced hot magma rises up, meets the sea water, cools
7.
Supershear earthquake
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A supershear earthquake is an earthquake in which the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave velocity. This causes an effect analogous to a sonic boom, during seismic events along a fault surface the displacement initiates at the focus and then propagates outwards. Typically for large earthquakes the focus lies towards one end of the slip surface, theoretical studies have in the past suggested that the upper bound for propagation velocity is that of Rayleigh waves, approximately 0.92 of the shear wave velocity. Evidence of rupture propagation at velocities greater than S-wave velocities expected for the surrounding crust have been observed for large earthquakes associated with strike-slip faults. During strike-slip, the component of rupture propagation will be horizontal, in the direction of displacement. This contrasts with a dip-slip rupture where the direction of rupture propagation will be perpendicular to the displacement. 2013 Craig earthquakes, magnitude Mw 7.6 associated with strike-slip on the Queen Charlotte Fault - the first supershear event recognised on plate boundaries, slow earthquake Eric Dunhams webpage on Supershear Dynamics New Scientist article on Supershear earthquakes
8.
Fault (geology)
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In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earths crust result from the action of tectonic forces. Energy release associated with movement on active faults is the cause of most earthquakes. A fault plane is the plane that represents the surface of a fault. A fault trace or fault line is the intersection of a plane with the ground surface. A fault trace is also the line commonly plotted on maps to represent a fault. Since faults do not usually consist of a single, clean fracture, the two sides of a non-vertical fault are known as the hanging wall and footwall. By definition, the wall occurs above the fault plane. This terminology comes from mining, when working a tabular ore body, because of friction and the rigidity of rocks, they cannot glide or flow past each other easily, and occasionally all movement stops. A fault in ductile rocks can also release instantaneously when the rate is too great. The energy released by instantaneous strain-release causes earthquakes, a common phenomenon along transform boundaries, slip is defined as the relative movement of geological features present on either side of a fault plane, and is a displacement vector. A faults sense of slip is defined as the motion of the rock on each side of the fault with respect to the other side. In practice, it is only possible to find the slip direction of faults. Based on direction of slip, faults can be categorized as, strike-slip. Dip-slip, offset is predominantly vertical and/or perpendicular to the fault trace, oblique-slip, combining significant strike and dip slip. The fault surface is usually vertical and the footwall moves either left or right or laterally with very little vertical motion. Strike-slip faults with left-lateral motion are known as sinistral faults. Those with right-lateral motion are known as dextral faults
9.
Epicenter
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The term was coined by the Irish seismologist Robert Mallet. The word, however, is misused to mean center. In seismology, the epicenter is the point on the Earths surface directly above the point where the fault begins to rupture, however, in larger events, the length of the fault rupture is much longer, and damage can be spread across the rupture zone. During an earthquake seismic waves propagate spherically out from the hypocenter, outside of the seismic shadow zone both types of wave can be detected but, due to their different velocities and paths through the Earth, they arrive at different times. This distance is called the distance, commonly measured in °. Once epicentral distances have been calculated from at least three seismographic measuring stations, it is a matter to find out where the epicenter was located using trilateration. Epicentral distance is used in calculating seismic magnitudes developed by Richter. Epicenter is frequently misused when not employed in the context of seismology and it is often utilized as an alternative to centre. For example, Travel is restricted in the Chinese province thought to be the epicentre of the SARS outbreak, garners Modern American Usage gives several examples of such misuse. However, Garner notes that these misusages may be metaphorical uses of the term to describe focal points of unstable and potentially destructive environments
10.
Shadow zone
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A seismic shadow zone is an area of the Earths surface where seismographs can only barely detect an earthquake after its seismic waves have passed through the Earth. When an earthquake occurs, seismic waves radiate out spherically from the earthquakes focus, the primary seismic waves are refracted by the liquid outer core of the Earth and are not detected between 104° and 140° from the epicenter. The secondary seismic waves pass through the liquid outer core and are not detected more than 104° from the epicenter. P waves that have converted to s-waves on leaving the outer core may be detected beyond 140 degrees. The three properties are, incompressibility, density and rigidity, p-wave velocity is equal to / p whereas S-wave velocity is equal to and so S-wave velocity is entirely dependent on the rigidity of the material it travels through. Liquids, however, have zero rigidity, hence making the S-wave velocity overall zero. P-waves, however, are only dependent on rigidity and as such still maintain some velocity when travelling through a liquid. Analysis of the seismology of various recorded earthquakes and their shadow zones, led geologist Richard Oldham to deduce in 1906 the liquid nature of the Earths outer core
11.
Seismic wave
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Many other natural and anthropogenic sources create low-amplitude waves commonly referred to as ambient vibrations. Seismic waves are studied by geophysicists called seismologists, seismic wave fields are recorded by a seismometer, hydrophone, or accelerometer. The propagation velocity of the waves depends on density and elasticity of the medium, velocity tends to increase with depth and ranges from approximately 2 to 8 km/s in the Earths crust, up to 13 km/s in the deep mantle. Earthquakes create distinct types of waves with different velocities, when reaching seismic observatories, among the many types of seismic waves, one can make a broad distinction between body waves, which travel through the Earth, and surface waves, which travel at the Earths surface. Other modes of wave propagation exist than those described in article, though of comparatively minor importance for earth-borne waves. Body waves travel through the interior of the Earth, surface waves travel across the surface. Surface waves decay more slowly with distance than body waves, which travel in three dimensions, particle motion of surface waves is larger than that of body waves, so surface waves tend to cause more damage. Body waves travel through the interior of the Earth along paths controlled by the properties in terms of density. The density and modulus, in turn, vary according to temperature, composition and this effect resembles the refraction of light waves. Two types of particle motion result in two types of waves, Primary and Secondary waves. Primary waves are waves that are longitudinal in nature. P waves are waves that travel faster than other waves through the earth to arrive at seismograph stations first. These waves can travel through any type of material, including fluids, in air, they take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air,1450 m/s in water, Secondary waves are shear waves that are transverse in nature. Following an earthquake event, S-waves arrive at stations after the faster-moving P-waves. S-waves can travel only through solids, as fluids do not support shear stresses, S-waves are slower than P-waves, and speeds are typically around 60% of that of P-waves in any given material. Seismic surface waves travel along the Earths surface and they can be classified as a form of mechanical surface waves. They are called surface waves, as they diminish as they get further from the surface and they travel more slowly than seismic body waves
12.
P-wave
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The continuum is made up of gases, liquids, or solids, including the Earth. P-waves can be produced by earthquakes and recorded by seismographs, in isotropic and homogeneous solids, the mode of propagation of a P-wave is always longitudinal, thus, the particles in the solid vibrate along the axis of propagation of the wave energy. Of these, density shows the least variation, so the velocity is mostly controlled by K and μ. The elastic moduli P-wave modulus, M, is defined so that M = K +4 μ /3, the precise speed varies according to the region of the Earths interior, from less than 6 km/s in the Earths crust to 13 km/s through the core. Geologist Francis Birch discovered a relationship between the velocity of P waves and the density of the material the waves are traveling in, primary and secondary waves are body waves that travel within the Earth. The motion and behavior of both P-type and S-type in the Earth are monitored to probe the structure of the Earth. Discontinuities in velocity as a function of depth are indicative of changes in phase or composition, differences in arrival times of waves originating in a seismic event like an earthquake as a result of waves taking different paths allow mapping of the Earths inner structure. P-waves travel through the layers of the Earths interior. As a result, there is a P-wave shadow zone between 103° and 142° from the focus, where the initial P-waves are not registered on seismometers. In contrast, S-waves do not travel through liquids, rather, the effectiveness of advance warning depends on accurate detection of the P-waves and rejection of ground vibrations caused by local activity as otherwise false-positive warnings will result. Earthquake Early Warning systems can be automated to allow for immediate safety actions such as issuing alerts, earthquake Early Warning Lamb waves Love wave Surface wave Photo Glossary of Earthquakes
13.
S-wave
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The S-wave moves as a shear or transverse wave, so motion is perpendicular to the direction of wave propagation. The wave moves through elastic media, and the restoring force comes from shear effects. Unlike the P-wave, the S-wave cannot travel through the outer core of the Earth. They can still appear in the inner core, when a P-wave strikes the boundary of molten. And when the S-waves hit the boundary again they will in turn create P-waves and this property allows seismologists to determine the nature of the inner core. As transverse waves, S-waves exhibit properties, such as polarization and birefringence, S-waves polarized in the horizontal plane are classified as SH-waves. If polarized in the plane, they are classified as SV-waves. When an S- or P-wave strikes an interface at an other than 90 degrees. As described above, if the interface is between a solid and liquid, S becomes P or vice versa, however, even if the interface is between two solid media, mode conversion results. If a P-wave strikes an interface, four propagation modes may result, reflected and transmitted P, similarly, if an SV-wave strikes an interface, the same four modes occur in different proportions. The exact amplitudes of all waves are described by the Zoeppritz equations. The prediction of S-waves came out of theory in the 1800s, taking the divergence of seismic wave equation in homogeneous media, instead of the curl, yields an equation describing P-wave propagation. The steady-state SH waves are defined by the Helmholtz equation u =0 where k is the wave number, earthquake Early Warning Lamb waves Longitudinal wave Love wave P-wave Rayleigh wave Seismic wave Shear wave splitting Surface wave Shearer, Peter. Aki, Keiiti, Richards, Paul G. Quantitative Seismology
14.
Seismometer
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Seismometers are instruments that measure motion of the ground, including those of seismic waves generated by earthquakes, volcanic eruptions, and other seismic sources. Records of seismic waves allow seismologists to map the interior of the Earth, the seismometer was invented by the Chinese polymath Zhang Heng in AD132 during the Han dynasty. The first Western description of a comes from the French physicist and priest Jean de Hautefeuille in 1703. The modern seismometer was developed in the 19th century by John Milne, James Alfred Ewing, seismograph is another Greek term from seismós and γράφω, gráphō, to draw. The concerning technical discipline is called seismometry, a branch of seismology, a simple seismometer that is sensitive to up-down motions of the earth can be understood by visualizing a weight hanging on a spring. The spring and weight are suspended from a frame that moves along with the earthʼs surface, as the earth moves, the relative motion between the weight and the earth provides a measure of the vertical ground motion. Any movement of the moves the frame. The mass tends not to move because of its inertia, early seismometers used optical levers or mechanical linkages to amplify the small motions involved, recording on soot-covered paper or photographic paper. In some systems, the mass is held nearly motionless relative to the frame by a negative feedback loop. The motion of the relative to the frame is measured. The voltage needed to produce this force is the output of the seismometer, in other systems the weight is allowed to move, and its motion produces a voltage in a coil attached to the mass and moving through the magnetic field of a magnet attached to the frame. This design is used in the geophones used in seismic surveys for oil. Professional seismic observatories usually have instruments measuring three axes, north-south, east-west, and the vertical, if only one axis is measured, this is usually the vertical because it is less noisy and gives better records of some seismic waves. The foundation of a station is critical. A professional station is mounted on bedrock. The best mountings may be in deep boreholes, which avoid thermal effects, ground noise and tilting from weather, other instruments are often mounted in insulated enclosures on small buried piers of unreinforced concrete. Reinforcing rods and aggregates would distort the pier as the temperature changes, a site is always surveyed for ground noise with a temporary installation before pouring the pier and laying conduit. Originally, European seismographs were placed in an area after a destructive earthquake
15.
Earthquake prediction
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Prediction can be further distinguished from earthquake warning systems, which upon detection of an earthquake, provide a real-time warning of seconds to neighboring regions that might be affected. In the 1970s, scientists were optimistic that a method for predicting earthquakes would soon be found. Demonstrably successful predictions of earthquakes have not occurred and the few claims of success are controversial. For example, the most famous claim of a prediction is that alleged for the 1975 Haicheng earthquake. A later study said there was no valid short-term prediction. Extensive searches have reported many possible earthquake precursors, but, so far, predictions are deemed significant if they can be shown to be successful beyond random chance. Therefore, methods of statistical hypothesis testing are used to determine the probability that a such as is predicted would happen anyway. The predictions are then evaluated by testing whether they correlate with actual earthquakes better than the null hypothesis, in many instances, however, the statistical nature of earthquake occurrence is not simply homogeneous. Clustering occurs in space and time. In southern California about 6% of M≥3.0 earthquakes are followed by an earthquake of magnitude within 5 days and 10 km. In central Italy 9. 5% of M≥3.0 earthquakes are followed by an event within 30 km and 48 hours. It has been shown that a method based solely on clustering can successfully predict about 5% of earthquakes. E. g. it has reported that members of the Chinese Academy of Sciences were purged for having ignored scientific predictions of the disastrous Tangshan earthquake of summer 1976. But warning of an earthquake that does not occur also incurs a cost, not only the cost of the emergency measures themselves, false alarms, including alarms that are canceled, also undermine the credibility, and thereby the effectiveness, of future warnings. This was prompted by more than 30 unofficial earthquake warnings, in the past three years, none of which has been accurate. The acceptable trade-off between missed quakes and false alarms depends on the valuation of these outcomes. The rate of occurrence of both must be considered when evaluating any prediction method and he found that earthquakes kill less than ten people per year in Greece, and that most of those fatalities occurred in large buildings with identifiable structural issues. Therefore, Stiros stated that it would be much more cost-effective to focus efforts on identifying and upgrading unsafe buildings, Earthquake prediction may be intrinsically impossible
16.
Earthquake forecasting
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Both forecasting and prediction of earthquakes are distinguished from earthquake warning systems, which upon detection of an earthquake, provide a real-time warning to regions that might be affected. In the 1970s, scientists were optimistic that a method for predicting earthquakes would soon be found. Demonstrably successful predictions of earthquakes have not occurred and the few claims of success are controversial. Consequently, many scientific and government resources have been used for probabilistic seismic hazard estimates rather than prediction of individual earthquakes, such estimates are used to establish building codes, insurance rate structures, awareness and preparedness programs, and public policy related to seismic events. In addition to regional earthquake forecasts, such seismic hazard calculations can take such as local geological conditions into account. Anticipated ground motion can then be used to building design criteria. Methods for earthquake forecasting generally look for trends or patterns that lead to an earthquake, as these trends may be complex and involve many variables, advanced statistical techniques are often needed to understand them, therefore these are sometimes called statistical methods. These approaches tend to have long time periods, making them useful for earthquake forecasting. Even the stiffest of rock is not perfectly rigid, given a large force the earths crust will bend or deform. According to the elastic theory of Reid, eventually the deformation becomes great enough that something breaks. Slippage along the break allows the rock on each side to rebound to a deformed state. In the process energy is released in various forms, including seismic waves, the cycle of tectonic force being accumulated in elastic deformation and released in a sudden rebound is then repeated. The most studied earthquake faults appear to have distinct segments, the characteristic earthquake model postulates that earthquakes are generally constrained within these segments. As the lengths and other properties of the segments are fixed and these include the maximum magnitude, and the amount of accumulated strain needed to rupture the fault segment. Return periods are used for forecasting other rare events, such as cyclones and floods. Instrumentation was put in place in hopes of detecting precursors of the anticipated earthquake, however, forecasted earthquake did not occur until 2004. The failure of the Parkfield prediction experiment has raised doubt as to the validity of the characteristic earthquake model itself, at the contact where two tectonic plates slip past each other every section must eventually slip, as none get left behind. But they do not all slip at the time, different sections will be at different stages in the cycle of strain accumulation
17.
Shear wave splitting
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Shear wave splitting, also called seismic birefringence, is the phenomenon that occurs when a polarized shear wave enters an anisotropic medium. The incident shear wave splits into two polarized shear waves, Shear wave splitting is typically used as a tool for testing the anisotropy of an area of interest. These measurements reflect the degree of anisotropy and lead to an understanding of the area’s crack density. We can think of the anisotropy of an area as a black box. An incident shear wave may enter an anisotropic medium from a media by encountering a change in the preferred orientation or character of the medium. When a polarized shear wave enters a new, anisotropic medium, one of these shear waves will be faster than the other and oriented parallel to the cracks or crystals in the medium. The second wave will be slower than the first and sometimes orthogonal to both the first shear wave and the cracks or crystals in the media, the time delays observed between the slow and fast shear waves give information about the density of cracks in the medium. The orientation of the fast shear wave records the direction of the cracks in the medium, when plotted using polarization diagrams, the arrival of split shear waves can be identified by the abrupt changes in direction of the particle motion. Hess made the first measurements of P-wave azimuthal velocity variations in oceanic basins and this area was chosen for this study because oceanic basins are made of large, relatively uniform homogeneous rocks. This concept was tested using seismic refraction profiles from the Mendocino Fracture Zone, hess found that the slow shear waves propagated perpendicular to the plane of slip and the higher velocity component was parallel to it. He inferred that the structure of oceanic basins could be recorded quickly, ando focused on identifying shear-wave anisotropy in the upper mantle. This study focused on shear wave splitting recorded near the Chubu Volcanic Area in Japan, using newly implemented telemetric seismographic stations, they were able to record both P-wave and S-wave arrivals from earthquakes up to 260 km beneath the volcanic area. The depths of these earthquakes make this area ideal for studying the structure of the upper mantle and they noted the arrivals of two distinct shear waves with different polarizations approximately 0.7 seconds apart. It was concluded that the splitting was not caused by the earthquake source, data from other nearby stations were used to constrain the source of the seismic anisotropy. He found the anisotropy to be consistent with the area directly below the area and was hypothesized to occur due to oriented crystals in a deep rooted magma chamber. If the magma chamber contained elliptical inclusions oriented approximately N-S, then the maximum velocity direction would also be N-S, Crampin proposed the theory of earthquake prediction using shear wave splitting measurements. This theory is based on the fact that microcracks between the grains or crystals in rocks will open wider than normal at high stress levels, after the stress subsides, the microcracks will return to their original positions. This phenomenon of cracks opening and closing in response to changing stress conditions is called dilatancy, once the signatures for an area are recognized, they may then be applied to predict nearby earthquakes with the same signatures
18.
Earthquake engineering
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Earthquake engineering is an interdisciplinary branch of engineering that designs and analyzes structures, such as buildings and bridges, with earthquakes in mind. Its overall goal is to such structures more resistant to earthquakes. An earthquake engineer aims to construct structures that will not be damaged in minor shaking, the main objectives of earthquake engineering are, Foresee the potential consequences of strong earthquakes on urban areas and civil infrastructure. Design, construct and maintain structures to perform at earthquake exposure up to the expectations, a properly engineered structure does not necessarily have to be extremely strong or expensive. It has to be designed to withstand the seismic effects while sustaining an acceptable level of damage. Seismic loading means application of an earthquake-generated excitation on a structure and it happens at contact surfaces of a structure either with the ground, with adjacent structures, or with gravity waves from tsunami. The loading that is expected at a location on the Earths surface is estimated by engineering seismology. It is related to the hazard of the location. Earthquake or seismic performance defines an ability to sustain its main functions, such as its safety and serviceability, at. A structure is considered safe if it does not endanger the lives. A structure may be considered if it is able to fulfill its operational functions for which it was designed. On the other hand, it should remain operational for more frequent, engineers need to know the quantified level of the actual or anticipated seismic performance associated with the direct damage to an individual building subject to a specified ground shaking. Such an assessment may be performed either experimentally or analytically, Experimental evaluations are expensive tests that are typically done by placing a model of the structure on a shake-table that simulates the earth shaking and observing its behavior. Such kinds of experiments were first performed more than a century ago, only recently has it become possible to perform 1,1 scale testing on full structures. Due to the nature of such tests, they tend to be used mainly for understanding the seismic behavior of structures, validating models. Thus, once validated, computational models and numerical procedures tend to carry the major burden for the seismic performance assessment of structures. The technique as a concept is a relatively recent development. In general, seismic structural analysis is based on the methods of structural dynamics, for decades, the most prominent instrument of seismic analysis has been the earthquake response spectrum method which also contributed to the proposed building codes concept of today
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Seismite
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Seismites are sedimentary beds and structures deformed by seismic shaking. The German paleontologist Adolf Seilacher first used the term in 1969 to describe earthquake-deformed layers, today, the term is applied to both sedimentary layers and soft sediment deformation structures formed by shaking. This subtle change in usage accommodates structures that may not remain within a layer, caution is urged when applying the term to features observed in the field, as similar-looking features may be products of either seismic or non-seismic perturbation. Several informal classification systems exist to help distinguish seismites from other soft-sediment deformation features, though a formal. Geologists use seismites, in combination with other evidence, to understand the earthquake history of an area. If age and distribution of seismically-generated features can be determined, then recurrence interval and seismic hazard risk can be assessed
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Seismology
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Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth or through other planet-like bodies. A related field that uses geology to infer information regarding past earthquakes is paleoseismology, a recording of earth motion as a function of time is called a seismogram. A seismologist is a scientist who does research in seismology, scholarly interest in earthquakes can be traced back to antiquity. Early speculations on the causes of earthquakes were included in the writings of Thales of Miletus, Anaximenes of Miletus, Aristotle. In 132 CE, Zhang Heng of Chinas Han dynasty designed the first known seismoscope, in 1664, Athanasius Kircher argued that earthquakes were caused by the movement of fire within a system of channels inside the Earth. In 1703, Martin Lister and Nicolas Lemery proposed that earthquakes were caused by chemical explosions within the earth, the Lisbon earthquake of 1755, coinciding with the general flowering of science in Europe, set in motion intensified scientific attempts to understand the behaviour and causation of earthquakes. The earliest responses include work by John Bevis and John Michell, Michell determined that earthquakes originate within the Earth and were waves of movement caused by shifting masses of rock miles below the surface. From 1857, Robert Mallet laid the foundation of instrumental seismology and he is also responsible for coining the word seismology. In 1897, Emil Wiecherts theoretical calculations led him to conclude that the Earths interior consists of a mantle of silicates, surrounding a core of iron. In 1906 Richard Dixon Oldham identified the separate arrival of P-waves, S-waves and surface waves on seismograms, in 1910, after studying the 1906 San Francisco earthquake, Harry Fielding Reid put forward the elastic rebound theory which remains the foundation for modern tectonic studies. The development of this depended on the considerable progress of earlier independent streams of work on the behaviour of elastic materials. In 1926, Harold Jeffreys was the first to claim, based on his study of waves, that below the mantle. In 1937, Inge Lehmann determined that within the liquid outer core there is a solid inner core. By the 1960s, earth science had developed to the point where a comprehensive theory of the causation of seismic events had come together in the now well-established theory of plate tectonics, seismic waves are elastic waves that propagate in solid or fluid materials. There are two types of waves, Pressure waves or Primary waves and Shear or Secondary waves. S-waves are transverse waves that move perpendicular to the direction of propagation, therefore, they appear later than P-waves on a seismogram. Fluids cannot support perpendicular motion, so S-waves only travel in solids, the two main surface wave types are Rayleigh waves, which have some compressional motion, and Love waves, which do not. Rayleigh waves result from the interaction of vertically polarized P- and S-waves that satisfy the conditions on the surface
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Ancient Greek
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Ancient Greek includes the forms of Greek used in ancient Greece and the ancient world from around the 9th century BC to the 6th century AD. It is often divided into the Archaic period, Classical period. It is antedated in the second millennium BC by Mycenaean Greek, the language of the Hellenistic phase is known as Koine. Koine is regarded as a historical stage of its own, although in its earliest form it closely resembled Attic Greek. Prior to the Koine period, Greek of the classic and earlier periods included several regional dialects, Ancient Greek was the language of Homer and of fifth-century Athenian historians, playwrights, and philosophers. It has contributed many words to English vocabulary and has been a subject of study in educational institutions of the Western world since the Renaissance. This article primarily contains information about the Epic and Classical phases of the language, Ancient Greek was a pluricentric language, divided into many dialects. The main dialect groups are Attic and Ionic, Aeolic, Arcadocypriot, some dialects are found in standardized literary forms used in literature, while others are attested only in inscriptions. There are also several historical forms, homeric Greek is a literary form of Archaic Greek used in the epic poems, the Iliad and Odyssey, and in later poems by other authors. Homeric Greek had significant differences in grammar and pronunciation from Classical Attic, the origins, early form and development of the Hellenic language family are not well understood because of a lack of contemporaneous evidence. Several theories exist about what Hellenic dialect groups may have existed between the divergence of early Greek-like speech from the common Proto-Indo-European language and the Classical period and they have the same general outline, but differ in some of the detail. The invasion would not be Dorian unless the invaders had some relationship to the historical Dorians. The invasion is known to have displaced population to the later Attic-Ionic regions, the Greeks of this period believed there were three major divisions of all Greek people—Dorians, Aeolians, and Ionians, each with their own defining and distinctive dialects. Often non-west is called East Greek, Arcadocypriot apparently descended more closely from the Mycenaean Greek of the Bronze Age. Boeotian had come under a strong Northwest Greek influence, and can in some respects be considered a transitional dialect, thessalian likewise had come under Northwest Greek influence, though to a lesser degree. Most of the dialect sub-groups listed above had further subdivisions, generally equivalent to a city-state and its surrounding territory, Doric notably had several intermediate divisions as well, into Island Doric, Southern Peloponnesus Doric, and Northern Peloponnesus Doric. The Lesbian dialect was Aeolic Greek and this dialect slowly replaced most of the older dialects, although Doric dialect has survived in the Tsakonian language, which is spoken in the region of modern Sparta. Doric has also passed down its aorist terminations into most verbs of Demotic Greek, by about the 6th century AD, the Koine had slowly metamorphosized into Medieval Greek
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Nuclear explosion
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The blast effect is created by the coupling of immense amounts of energy, spanning the electromagnetic spectrum, with the surroundings. Locations such as submarine, surface, air burst, or exo-atmospheric determine how energy is produced as blast. In general, denser media around the bomb, like water, absorb more energy, when an air burst occurs, lethal blast and thermal effects proportionally scale much more rapidly than lethal radiation effects, as higher and higher yield nuclear weapons are used. Energy from an explosive is initially released in several forms of penetrating radiation. When there is a material such as air, rock, or water. This causes vaporization of surrounding material resulting in its rapid expansion, kinetic energy created by this expansion contributes to the formation of a shockwave. When a nuclear detonation occurs in air near sea level, much of the released energy interacts with the atmosphere, intense thermal radiation at the hypocenter forms a nuclear fireball and if the burst is low enough, it is often associated with a mushroom cloud. In a burst at high altitude, where the air density is low, more energy is released as ionizing gamma radiation and this would concern a nuclear reaction of two nitrogen atoms forming a carbon and an oxygen atom, with release of energy. This energy would heat up the remaining nitrogen enough to keep the reaction going until all nitrogen atoms were consumed. Hans Bethe was assigned the task of studying there was a possibility in the very early days. Richard Hamming, a mathematician, was asked to make a similar calculation just before Trinity, nevertheless, the notion has persisted as a rumor for many years, and was the source of black humor at the Trinity test. The high temperatures and radiation cause gas to move outward radially in a thin, the front acts like a piston that pushes against and compresses the surrounding medium to make a spherically expanding shock wave. At first, this wave is inside the surface of the developing fireball. For air bursts at or near sea-level, 50–60% of the energy goes into the blast wave, depending on the size. As a general rule, the blast fraction is higher for low yield weapons, furthermore, it decreases at high altitudes because there is less air mass to absorb radiation energy and convert it into blast. This effect is most important for altitudes above 30 km, corresponding to <1 per cent of air density. The effects of a rain storm during an Operation Castle nuclear explosion was found to dampen, or reduce. Much of the caused by a nuclear explosion is due to blast effects
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Deformation (mechanics)
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Deformation in continuum mechanics is the transformation of a body from a reference configuration to a current configuration. A configuration is a set containing the positions of all particles of the body, a deformation may be caused by external loads, body forces, or changes in temperature, moisture content, or chemical reactions, etc. Strain is a description of deformation in terms of displacement of particles in the body that excludes rigid-body motions. In a continuous body, a deformation field results from a field induced by applied forces or is due to changes in the temperature field inside the body. The relation between stresses and induced strains is expressed by constitutive equations, e. g. Hookes law for linear elastic materials, deformations which are recovered after the stress field has been removed are called elastic deformations. In this case, the continuum completely recovers its original configuration, on the other hand, irreversible deformations remain even after stresses have been removed. Another type of deformation is viscous deformation, which is the irreversible part of viscoelastic deformation. In the case of elastic deformations, the response function linking strain to the stress is the compliance tensor of the material. Strain is a measure of deformation representing the displacement between particles in the relative to a reference length. A general deformation of a body can be expressed in the form x = F where X is the position of material points in the body. Such a measure does not distinguish between rigid body motions and changes in shape of the body, a deformation has units of length. We could, for example, define strain to be ε ≐ ∂ ∂ X = F ′ − I, hence strains are dimensionless and are usually expressed as a decimal fraction, a percentage or in parts-per notation. Strains measure how much a given deformation differs locally from a rigid-body deformation, a strain is in general a tensor quantity. Physical insight into strains can be gained by observing that a given strain can be decomposed into normal and this could be applied by elongation, shortening, or volume changes, or angular distortion. However, it is sufficient to know the normal and shear components of strain on a set of three perpendicular directions. In this case, the undeformed and deformed configurations of the continuum are significantly different and this is commonly the case with elastomers, plastically-deforming materials and other fluids and biological soft tissue. Infinitesimal strain theory, also called small strain theory, small deformation theory, small displacement theory, in this case, the undeformed and deformed configurations of the body can be assumed identical. Large-displacement or large-rotation theory, which assumes small strains but large rotations, in each of these theories the strain is then defined differently
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Wave
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In physics, a wave is an oscillation accompanied by a transfer of energy that travels through a medium. Frequency refers to the addition of time, wave motion transfers energy from one point to another, which displace particles of the transmission medium–that is, with little or no associated mass transport. Waves consist, instead, of oscillations or vibrations, around almost fixed locations, there are two main types of waves. Mechanical waves propagate through a medium, and the substance of this medium is deformed, restoring forces then reverse the deformation. For example, sound waves propagate via air molecules colliding with their neighbors, when the molecules collide, they also bounce away from each other. This keeps the molecules from continuing to travel in the direction of the wave, the second main type, electromagnetic waves, do not require a medium. Instead, they consist of periodic oscillations of electrical and magnetic fields generated by charged particles. These types vary in wavelength, and include radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, waves are described by a wave equation which sets out how the disturbance proceeds over time. The mathematical form of this varies depending on the type of wave. Further, the behavior of particles in quantum mechanics are described by waves, in addition, gravitational waves also travel through space, which are a result of a vibration or movement in gravitational fields. While mechanical waves can be transverse and longitudinal, all electromagnetic waves are transverse in free space. A single, all-encompassing definition for the wave is not straightforward. A vibration can be defined as a back-and-forth motion around a reference value, however, a vibration is not necessarily a wave. An attempt to define the necessary and sufficient characteristics that qualify a phenomenon as a results in a blurred line. The term wave is often understood as referring to a transport of spatial disturbances that are generally not accompanied by a motion of the medium occupying this space as a whole. In a wave, the energy of a vibration is moving away from the source in the form of a disturbance within the surrounding medium and it may appear that the description of waves is closely related to their physical origin for each specific instance of a wave process. For example, acoustics is distinguished from optics in that sound waves are related to a rather than an electromagnetic wave transfer caused by vibration. Concepts such as mass, momentum, inertia, or elasticity and this difference in origin introduces certain wave characteristics particular to the properties of the medium involved
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Physics
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Physics is the natural science that involves the study of matter and its motion and behavior through space and time, along with related concepts such as energy and force. One of the most fundamental disciplines, the main goal of physics is to understand how the universe behaves. Physics is one of the oldest academic disciplines, perhaps the oldest through its inclusion of astronomy, Physics intersects with many interdisciplinary areas of research, such as biophysics and quantum chemistry, and the boundaries of physics are not rigidly defined. New ideas in physics often explain the mechanisms of other sciences while opening new avenues of research in areas such as mathematics. Physics also makes significant contributions through advances in new technologies that arise from theoretical breakthroughs, the United Nations named 2005 the World Year of Physics. Astronomy is the oldest of the natural sciences, the stars and planets were often a target of worship, believed to represent their gods. While the explanations for these phenomena were often unscientific and lacking in evidence, according to Asger Aaboe, the origins of Western astronomy can be found in Mesopotamia, and all Western efforts in the exact sciences are descended from late Babylonian astronomy. The most notable innovations were in the field of optics and vision, which came from the works of many scientists like Ibn Sahl, Al-Kindi, Ibn al-Haytham, Al-Farisi and Avicenna. The most notable work was The Book of Optics, written by Ibn Al-Haitham, in which he was not only the first to disprove the ancient Greek idea about vision, but also came up with a new theory. In the book, he was also the first to study the phenomenon of the pinhole camera, many later European scholars and fellow polymaths, from Robert Grosseteste and Leonardo da Vinci to René Descartes, Johannes Kepler and Isaac Newton, were in his debt. Indeed, the influence of Ibn al-Haythams Optics ranks alongside that of Newtons work of the same title, the translation of The Book of Optics had a huge impact on Europe. From it, later European scholars were able to build the devices as what Ibn al-Haytham did. From this, such important things as eyeglasses, magnifying glasses, telescopes, Physics became a separate science when early modern Europeans used experimental and quantitative methods to discover what are now considered to be the laws of physics. Newton also developed calculus, the study of change, which provided new mathematical methods for solving physical problems. The discovery of new laws in thermodynamics, chemistry, and electromagnetics resulted from greater research efforts during the Industrial Revolution as energy needs increased, however, inaccuracies in classical mechanics for very small objects and very high velocities led to the development of modern physics in the 20th century. Modern physics began in the early 20th century with the work of Max Planck in quantum theory, both of these theories came about due to inaccuracies in classical mechanics in certain situations. Quantum mechanics would come to be pioneered by Werner Heisenberg, Erwin Schrödinger, from this early work, and work in related fields, the Standard Model of particle physics was derived. Areas of mathematics in general are important to this field, such as the study of probabilities, in many ways, physics stems from ancient Greek philosophy
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Wavelength
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In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the waves shape repeats, and thus the inverse of the spatial frequency. Wavelength is commonly designated by the Greek letter lambda, the concept can also be applied to periodic waves of non-sinusoidal shape. The term wavelength is also applied to modulated waves. Wavelength depends on the medium that a wave travels through, examples of wave-like phenomena are sound waves, light, water waves and periodic electrical signals in a conductor. A sound wave is a variation in air pressure, while in light and other electromagnetic radiation the strength of the electric, water waves are variations in the height of a body of water. In a crystal lattice vibration, atomic positions vary, wavelength is a measure of the distance between repetitions of a shape feature such as peaks, valleys, or zero-crossings, not a measure of how far any given particle moves. For example, in waves over deep water a particle near the waters surface moves in a circle of the same diameter as the wave height. The range of wavelengths or frequencies for wave phenomena is called a spectrum, the name originated with the visible light spectrum but now can be applied to the entire electromagnetic spectrum as well as to a sound spectrum or vibration spectrum. In linear media, any pattern can be described in terms of the independent propagation of sinusoidal components. The wavelength λ of a sinusoidal waveform traveling at constant speed v is given by λ = v f, in a dispersive medium, the phase speed itself depends upon the frequency of the wave, making the relationship between wavelength and frequency nonlinear. In the case of electromagnetic radiation—such as light—in free space, the speed is the speed of light. Thus the wavelength of a 100 MHz electromagnetic wave is about, the wavelength of visible light ranges from deep red, roughly 700 nm, to violet, roughly 400 nm. For sound waves in air, the speed of sound is 343 m/s, the wavelengths of sound frequencies audible to the human ear are thus between approximately 17 m and 17 mm, respectively. Note that the wavelengths in audible sound are much longer than those in visible light, a standing wave is an undulatory motion that stays in one place. A sinusoidal standing wave includes stationary points of no motion, called nodes, the upper figure shows three standing waves in a box. The walls of the box are considered to require the wave to have nodes at the walls of the box determining which wavelengths are allowed, the stationary wave can be viewed as the sum of two traveling sinusoidal waves of oppositely directed velocities. Consequently, wavelength, period, and wave velocity are related just as for a traveling wave, for example, the speed of light can be determined from observation of standing waves in a metal box containing an ideal vacuum. In that case, the k, the magnitude of k, is still in the same relationship with wavelength as shown above
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Least squares
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The method of least squares is a standard approach in regression analysis to the approximate solution of overdetermined systems, i. e. sets of equations in which there are more equations than unknowns. Least squares means that the overall solution minimizes the sum of the squares of the made in the results of every single equation. The most important application is in data fitting, the best fit in the least-squares sense minimizes the sum of squared residuals. Least squares problems fall into two categories, linear or ordinary least squares and non-linear least squares, depending on whether or not the residuals are linear in all unknowns, the linear least-squares problem occurs in statistical regression analysis, it has a closed-form solution. The non-linear problem is solved by iterative refinement, at each iteration the system is approximated by a linear one. Polynomial least squares describes the variance in a prediction of the dependent variable as a function of the independent variable, when the observations come from an exponential family and mild conditions are satisfied, least-squares estimates and maximum-likelihood estimates are identical. The method of least squares can also be derived as a method of moments estimator, the following discussion is mostly presented in terms of linear functions but the use of least-squares is valid and practical for more general families of functions. Also, by iteratively applying local quadratic approximation to the likelihood, for the topic of approximating a function by a sum of others using an objective function based on squared distances, see least squares. The least-squares method is credited to Carl Friedrich Gauss. The accurate description of the behavior of bodies was the key to enabling ships to sail in open seas. The combination of different observations taken under the same conditions contrary to simply trying ones best to observe, the approach was known as the method of averages. The combination of different observations taken under different conditions, the method came to be known as the method of least absolute deviation. It was notably performed by Roger Joseph Boscovich in his work on the shape of the earth in 1757, the development of a criterion that can be evaluated to determine when the solution with the minimum error has been achieved. Laplace tried to specify a mathematical form of the probability density for the errors and he felt these to be the simplest assumptions he could make, and he had hoped to obtain the arithmetic mean as the best estimate. Instead, his estimator was the posterior median, the first clear and concise exposition of the method of least squares was published by Legendre in 1805. The technique is described as a procedure for fitting linear equations to data. The value of Legendres method of least squares was immediately recognized by leading astronomers, in 1809 Carl Friedrich Gauss published his method of calculating the orbits of celestial bodies. In that work he claimed to have been in possession of the method of least squares since 1795 and this naturally led to a priority dispute with Legendre
The dictionary definition of hypocenter at Wiktionary
Media related to Hypocenters at Wikimedia Commons
