An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. Earthquakes are recorded with a seismometer, also known as a seismograph. The moment magnitude of an earthquake is conventionally reported, or the related and mostly obsolete Richter magnitude, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causing serious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale.
At the Earth's surface, earthquakes manifest themselves by a shaking and sometimes displacement of the ground. When a large earthquake epicenter is located offshore, the seabed sometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can also trigger landslides and occasionally volcanic activity.
In its most generic sense, the word earthquake is used to describe any seismic event—whether a natural phenomenon or an event caused by humans—that generates seismic waves. Earthquakes are caused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts, and nuclear experiments.
An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter means the point at ground level directly above this.
Naturally occurring earthquakes Most naturally occurring earthquakes are related to the tectonic nature of the Earth. Such earthquakes are called tectonic earthquakes. The Earth's lithosphere is a patchwork of plates in slow but constant motion caused by the release to space of the heat in the Earth's mantle and core. The heat causes the rock in the Earth to flow on geological timescales, so that the plates move slowly but surely. Plate boundaries lock as the plates move past each other, creating frictional stress. When the frictional stress exceeds a critical value, called local strength, a sudden failure occurs. The boundary of tectonic plates along which failure occurs is called the fault plane. When the failure at the fault plane results in a violent displacement of the Earth's crust, energy is released as a combination of radiated elastic strain seismic waves, frictional heating of the fault surface, and cracking of the rock, thus causing an earthquake. 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 an earthquake's total energy is radiated as seismic energy. Most of the earthquake's energy is used to power the earthquake fracture growth or is converted into heat generated by friction. Therefore, earthquakes lower the Earth's available elastic potential energy and raise its temperature, though these changes are negligible compared to the conductive and convective flow of heat out from the Earth's deep interior.
The majority of tectonic earthquakes originate at depths not exceeding tens of kilometers. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, Deep focus earthquakes may occur at much greater depths (up to seven hundred kilometers). These seismically active areas of subduction are known as Wadati-Benioff zones. These are earthquakes that occur at a depth at which the subducted lithosphere should no longer be brittle, due to the high temperature and pressure. A possible mechanism for the generation of deep focus earthquakes is faulting caused by olivine undergoing a phase transition into a spinel structure.
Earthquakes also often occur in volcanic regions and are caused there, both by tectonic faults and by the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions.
Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike a fault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similar to aftershocks but on adjacent segments of fault, these storms occur over the course of years, and with some of the later earthquakes as damaging as the early ones. Such a pattern was observed in the sequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20th century, the half dozen large earthquakes in New Madrid in 1811-1812, and has been inferred for older anomalous clusters of large earthquakes in the Middle East and in the Mojave Desert.
Size and frequency of occurrence Small earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Chile, Peru, Indonesia, Iran, the Azores in Portugal, New Zealand, Greece and Japan. Large earthquakes occur less frequently, the relationship being exponential; for example, roughly ten times as many earthquakes larger than magnitude 4 occur in a particular time period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, it has been calculated that the average recurrences are:
an earthquake of 3.7 - 4.6 every year
an earthquake of 4.7 - 5.5 every 10 years
an earthquake of 5.6 or larger every 100 years.
The number of seismic stations has increased from about 350 in 1931 to many thousands today. As a result, many more earthquakes are reported than in the past because of the vast improvement in instrumentation (not because the number of earthquakes has increased). The USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable. In fact, in recent years, the number of major earthquakes per year has actually decreased, although this is likely a statistical fluctuation. More detailed statistics on the size and frequency of earthquakes is available from the USGS.
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. Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.
With the rapid growth of mega-cities such as Mexico City, Tokyo or Tehran, in areas of high seismic risk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.
Effects & impacts of earthquakes There are many effects of earthquakes including, but not limited to the following:
Shaking and ground rupture Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings or other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation. The ground-shaking is measured by ground acceleration.
Specific local geological, geomorphological, and geostructural features can induce high levels of shaking on the ground surface even from low-intensity earthquakes. This effect is called site or local amplification. It is principally due to the transfer of the seismic motion from hard deep soils to soft superficial soils and to effects of seismic energy focalization owing to typical geometrical setting of the deposits.
Ground rupture is a visible breaking and displacement of the earth's surface along the trace of the fault, which may be of the order of few metres in the case of major earthquakes. Ground rupture is a major risk for large engineering structures such as dams, bridges and nuclear power stations and requires careful mapping of existing faults to identify any likely to break the ground surface within the life of the structure.
Landslides and avalanches Earthquakes can cause landslides and avalanches, which may cause damage in hilly and mountainous areas.
Fires Following an earthquake, fires can be generated by break of the electrical power or gas lines. In the event of water mains rupturing and a loss of pressure, it may also become difficult to stop the spread of a fire once it has started.
Soil liquefaction Soil liquefaction occurs when, because of the shaking, water-saturated granular material temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, as buildings or bridges, to tilt or sink into the liquefied deposits.
Tsunami Undersea earthquakes and earthquake-triggered landslides into the sea, can cause Tsunami. See, for example, the 2004 Indian Ocean earthquake.
Human impacts Earthquakes may result in disease, lack of basic necessities, loss of life, higher insurance premiums, general property damage, road and bridge damage, and collapse of buildings or destabilization of the base of buildings which may lead to collapse in future earthquakes.
Earthquakes in mythology and religion In Norse mythology, earthquakes were explained as the violent struggling of the god Loki. When Loki, god of mischief and strife, murdered Baldr, god of beauty and light, he was punished by being bound in a cave with a poisonous serpent placed above his head dripping venom. Loki's wife Sigyn stood by him with a bowl to catch the poison, but whenever she had to empty the bowl the poison would drip on Loki's face, forcing him to jerk his head away and thrash against his bonds, causing the earth to tremble.
In Greek mythology, Poseidon was the god of earthquakes.
In Christian mythology, certain saints were invoked as patrons against earthquakes, including Saint Gregory Thaumaturgus, Saint Agatha, Saint Francis Borgia, and Saint Emygdius.
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地震
地震是由于地壳中的能量突然释放而产生的震波。地震检波器，也被称为测振仪，可记录地震。据报道称地震的矩震级，或者与此相关的以前的里氏地震级，在三级或者以下的地震几乎是察觉不到的，而七级的地震就会造成大范围的严重损坏。震动的强度可以由改良的麦加利震级测量出来。
在地球表面，地震表现为震动，有时还伴随着地面的移动。一个大地震的震中位于海上时，有时海床经受的强烈移动会导致海啸。地震的震动还有可能引发山体滑坡，还有时候会导致火山活动。
大多数的观点认为，地震这个词常用于形容一切引发震波的地震活动，无论这是一个自然现象或者是人为的事件。地震的原因大部分是因为地质断层导致的地裂，也有可能是火山活动、山体滑坡、煤矿爆炸、以及核试验。
地震的最初地裂点称为震源（英文为focus或hypocenter）。震中意思是震源在地面上的直接对应点。
自然发生的地震
大部分自然发生的地震都与地球的构造有关。这样的地震被称为壳构地震。地球的岩石圈是由许多板块拼接起来的，它以缓慢的速度持续不断的移动着，这样的移动是由于地球的地幔与地核中热空间的释放。这热量导致了地球中的岩石依照地质时间表流动，因此，板块缓慢但是确定地移动着。随着板块相互经过，板块间的交接处锁定，产生了摩擦力。当这个摩擦力超过了临界值，称为“局部强度”，一个突然的断裂就发生了。沿着构造板块发生断裂的边缘被称为断层面。当断层面的断裂造成了剧烈的地壳移位时，由于辐射的弹性应变震波、断层表面摩擦加热、以及岩石断裂的同时作用，能量被释放出来，因此导致了地震。张力逐渐形成并且被偶尔的突然的地震断裂打断的过程就是弹力回跃理论。据估计，只有百分之十的地震总能量以地震能量的形式辐射。大部分的地震能量通常会加剧地震断层的增加，或者转化为摩擦力引起的热量。因此，地震减少了地球可用的弹性势能并且提高了它的温度，虽然这些改变比起源于地球深部的热对流来说是微不足道的。
大部分的壳构地震源于不到几十千米的深度。在隐没带，当更古老更冰冷的洋底壳下降到另一版块之下时，深层震源地震可能会发生在很深的地方（达到为七百千米）。这些地震活跃隐没区被称为达本尼奥夫带。有发生在隐没岩石圈因为高温和压力不应再易碎的深度的地震。发生深层震源地震的机制可能是由于橄榄石经历了到针状结构的相变而产生的断层作用。
地震可能常发生在火山地区，并且是由于构造断层与火山岩浆运动造成的。这样的地震可以作为火山喷发的预警。
有时候一系列的地震会发生在一种地震风暴中，地震袭击了一片断层中的某一个，其他所有的都会被先前的地震带来的震动或者压力的重新分布激发出来。就和余震相似，但是在断层临近的部分，这些风暴会在一段时间内发生，一些以后发生的地震会和先前发生的有一样的破坏性。这样的模式在二十世纪土尔其的北安那托利亚断层的十几次连续的地震中和1811至1812年新马德里的几次地震中被观察到，并且用于推断位于中东和莫哈韦沙漠的几次古老的反常的大地震。
地震的规模与频率
小地震几乎不断地在世界各地发生，如美国的加利福尼亚和阿拉斯加、智利、秘鲁、印尼、伊朗、葡萄牙的亚述尔群岛、新西兰、希腊，和日本。大地震发生的没有那么频繁，他们之间的关系是指数关系;举例而言，震级大于四级的地震在一特定时期内发生的频率约是大于五级的地震的十倍。举个例子，在英国（震级低），据计算地震的平均次数为：
3.7-4.6级的地震每年发生一次
4.7-5.5级的地震每十年发生一次
5.6级或以上的地震每一百年发生一次。
地震台站的数量从1931年的350个增加到了现在的几千个。结果，由于仪器方面的巨大进步（不是因为地震的数量增加了），比起以前来，更多的地震被报道了。美国地质勘探局估计，从1900年开始，每年平均有18次主要地震（7.0-7.9级）和一次大地震（8.0级或者以上）,并且这个平均数是相对稳定的。事实上，在最近几年，每年的主要地震数量实际上已经减少了，即使这像一个统计学上的波动。美国地质勘探局有更加具体的地震规模和频率统计表。
世界上大部分的地震（90%和81%的最大地震）发生在40000千米长的马蹄形区域，那被称为环太平洋地震带，也叫做太平洋火环，那里的大部分都与太平洋板块相连。大规模的地震也倾向于沿着其他板块边界发生，就像沿着喜马拉雅山脉。
随着百万人口城市的增加，就像：墨西哥城、东京或者德黑兰，在那些高地震危险的区域，一些地震学专家警告说一次地震就可以要了三百万人的性命。
地震的影响与冲击
地震的许多影响包括但不限于以下内容：
震动与地裂
震动与地裂是地震造成的主要影响，主要造成了或多或少对房屋以及其他刚性结构的严重破坏。局部影响的严重性取决于一个复杂的组合，包括地震的震级、距离震中的远近、以及局部地质和地貌条件，这个条件可能增强或者减小波的传播。地面的震动可通过地面加速度来测量。
即使是源于低强度的地震，特别的局部地质、地貌和地质结构特征也可能引起高级别的地面震动。这样的影响被称为定点或者局部扩大。这主要是由于地震运动从硬的深层的土壤到软的表层的土壤的传递，以及典型的沉淀物的几何定位造成的地震能量集中。
地面断裂是一个可见的、沿着断层发生地表的破坏和移位，这可能是主要地震中发生在地表几米之处。地面断裂是大型工程结构的主要威胁，如：水坝、桥梁、以及核电站，它要求对现存的断层进行仔细的计划来确定在建筑的寿命以内不会有任何可能破坏地表。
山体滑坡和雪崩
地震可能导致山体滑坡以及雪崩，它们可能在丘陵与山地造成损害。
火灾
伴随着地震，电力系统的破坏和煤气管道破裂都会导致火灾的发生。如果水管破裂，失去压力，那么如果火灾发生了，我们可能很难阻止火势的蔓延。
土壤液化
由于震动，水渗透颗粒的材质暂时失去了它的强度，并且由固体转变为液体，这时候就会发生土壤液化。土壤液化可能导致钢结构，如大楼和桥梁，倾斜或者沉入融化的土地。
海啸
海面下的地震和地震引起的山体滑坡到还中可能会导致海啸。举个例子，如2004年印度洋地震。
对人类的影响
地震可能导致疾病、缺乏基本必需品、失去生命、更高的保险费用、普遍的财产损失、道路桥梁损害、大楼坍塌或者可能导致在以后地震中坍塌的建筑物基础的不稳定。
神话与宗教中的地震
在挪威的神话中，地震被解释为洛基神的猛烈的斗争。当伤害与斗争之神，洛基谋杀了美与光之神巴德尔，他被惩罚，关在一个山洞里，一条毒蛇盘在他的头上滴下毒液。洛基的妻子锡因站在他身旁用碗接住毒液，但是每当她清空碗的时候，毒液就会滴到洛基的脸上，迫使他头部痉挛猛击他的手铐，导致了地球的震动。
在希腊神话中，波塞冬是地震之神。
在基督教神话中，某些圣人是对抗地震的，包括圣格雷戈里、圣阿加莎、圣弗朗西斯.波吉亚、以及圣爱米巨。