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
The dictionary definition of hypocenter at Wiktionary
Media related to Hypocenters at Wikimedia Commons
