Natural Disaster

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A natural disaster is a sudden event that causes widespread destruction, lots of collateral damage or loss of life, brought about by forces other than the acts of human beings. A natural disaster might be caused by earthquakes, flooding, volcanic eruption, landslide, hurricanes etc. In order to be classified as a disaster it will have profound environmental effect and/or human loss and frequently incurs financial loss. Citations

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1 Ten deadliest natural disasters o 1.1 Ten deadliest natural disasters since 1900 2 Lists of natural disasters by cause o 2.1 Avalanches o 2.2 Blizzards o 2.3 Communicable diseases o 2.4 Cyclones (including hurricanes) o 2.5 Earthquakes o 2.6 Famines o 2.7 Floods and landslides o 2.8 Heat waves o 2.9 Lightning strikes o 2.10 Limnic eruptions o 2.11 Storms (non-cyclone) o 2.12 Tornadoes o 2.13 Tsunamis o 2.14 Volcanic eruptions o 2.15 Wildfires and bushfires 3 See also 4 References 5 External links

Drought
From Wikipedia, the free encyclopedia Jump to: navigation, search For other uses, see Drought (disambiguation).

Fields outside Benambra, Victoria, Australia suffering from drought conditions.

A drought is an extended period of months or years when a region notes a deficiency in its water supply whether surface or underground water. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region. Although droughts can persist for several years, even a short, intense drought can cause significant damage[1] and harm the local economy.[2] This global phenomenon has a widespread impact on agriculture. Lengthy periods of drought have long been a key trigger for mass migration and played a key role in a number of ongoing migrations and other humanitarian crises in the Horn of Africa and the Sahel.

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1 Consequences 2 Globally o 2.1 Regions 3 Causes 4 Types of drought 5 Drought protection and relief 6 See also 7 References 8 External links

[edit] Consequences

Dry earth in the Sonoran desert, Mexico.

Periods of drought can have significant environmental, agricultural, health, economic and social consequences. The effect varies according to vulnerability. For example, subsistence farmers are more likely to migrate during drought because they do not have alternative food sources. Areas with populations that depend on as a major food source are more vulnerable to drought-triggered famine. Drought can also reduce water quality, because lower water flows reduce dilution of pollutants and increase contamination of remaining water sources. Common consequences of drought include:
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Diminished crop growth or yield productions and carrying capacity for livestock Dust bowls, themselves a sign of erosion, which further erode the landscape Dust storms, when drought hits an area suffering from desertification and erosion Famine due to lack of water for irrigation Habitat damage, affecting both terrestrial and aquatic wildlife[3] Malnutrition, dehydration and related diseases Mass migration, resulting in internal displacement and international refugees Reduced electricity production due to reduced water flow through hydroelectric dams[4] Shortages of water for industrial users[5][6] Snake migration and increases in snakebites[7] Social unrest War over natural resources, including water and food Wildfires, such as Australian bushfires, are more common during times of drought.[8]

[edit] Globally
Drought is a normal, recurring feature of the climate in most parts of the world. It is among the earliest documented climatic events, present in the Epic of Gilgamesh and tied to the biblical story of Joseph's arrival in and the later Exodus from Ancient Egypt.[9] Hunter-gatherer migrations in 9,500 BC Chile have been linked to the phenomenon,[10] as has the exodus of early humans out of Africa and into the rest of the world around 135,000 years ago.[11] Modern peoples can effectively mitigate much of the impact of drought through irrigation and crop rotation. Failure to develop adequate drought mitigation strategies carries a grave human cost in the modern era, exacerbated by ever-increasing population densities.
[edit] Regions

Lake Chad in a 2001 satellite image, with the actual lake in blue. The lake has shrunk by 95% since the 1960s.[12][13]

Sheep on a drought affected paddock near Uranquinty, New South Wales.

Recurring droughts leading to desertification in the Horn of Africa have created grave ecological catastrophes, prompting massive food shortages, still recurring.[14] To the north-west of the Horn, the Darfur conflict in neighboring Sudan, also affecting Chad, was fueled by decades of drought; combination of drought, desertification and overpopulation are among the causes of the Darfur

conflict, because the Arab Baggara nomads searching for water have to take their livestock further south, to land mainly occupied by non-Arab farming peoples.[15] Approximately 2.4 billion people live in the drainage basin of the Himalayan rivers.[16] India, China, Pakistan, Bangladesh, Nepal and Myanmar could experience floods followed by droughts in coming decades. Drought in India affecting the Ganges is of particular concern, as it provides drinking water and agricultural irrigation for more than 500 million people.[17][18][19] The west coast of North America, which gets much of its water from glaciers in mountain ranges such as the Rocky Mountains and Sierra Nevada, also would be affected.[20][21] In 2005, parts of the Amazon basin experienced the worst drought in 100 years.[22][23] A 23 July 2006 article reported Woods Hole Research Center results showing that the forest in its present form could survive only three years of drought.[24][25] Scientists at the Brazilian National Institute of Amazonian Research argue in the article that this drought response, coupled with the effects of deforestation on regional climate, are pushing the rainforest towards a "tipping point" where it would irreversibly start to die. It concludes that the rainforest is on the brink of being turned into savanna or desert, with catastrophic consequences for the world's climate. According to the WWF, the combination of climate change and deforestation increases the drying effect of dead trees that fuels forest fires.[26] By far the largest part of Australia is desert or semi-arid lands commonly known as the outback. A 2005 study by Australian and American researchers investigated the desertification of the interior, and suggested that one explanation was related to human settlers who arrived about 50,000 years ago. Regular burning by these settlers could have prevented monsoons from reaching interior Australia.[27] In June 2008 it became known that an expert panel had warned of long term, maybe irreversible, severe ecological damage for the whole Murray-Darling basin if it does not receive sufficient water by October.[28] Australia could experience more severe droughts and they could become more frequent in the future, a government-commissioned report said on July 6, 2008.[29] Australian environmentalist Tim Flannery, predicted that unless it made drastic changes, Perth in Western Australia could become the world’s first ghost metropolis, an abandoned city with no more water to sustain its population.[30] East Africa currently faces its worst drought in decades,[31][32] with crops and livestock destroyed.[33] The U.N. World Food Programme recently said that nearly four million Kenyans urgently needed food.[34]

[edit] Causes

A Mongolian gazelle dead due to drought .

Ancient Meso-American civilizations of the Mayans and Aztecs likely amplified droughts in the Yucatán and southern Mexico by clearing rainforests to make room for pastures and farmland.

Generally, rainfall is related to the amount of water vapor in the atmosphere, combined with the upward forcing of the air mass containing that water vapor. If either of these are reduced, the result is a drought. This can be triggered by an above average prevalence of high pressure systems, winds carrying continental, rather than oceanic air masses (i.e. reduced water content), and ridges of high pressure areas form with behaviors which prevent or restrict the developing of thunderstorm activity or rainfall over one certain region. Oceanic and atmospheric weather cycles such as the El Niño-Southern Oscillation (ENSO) make drought a regular recurring feature of the Americas along the Midwest and Australia. Guns, Germs, and Steel author Jared Diamond sees the stark impact of the multi-year ENSO cycles on Australian weather patterns as a key reason that Australian aborigines remained a hunter-gatherer society rather than adopting agriculture.[35] Another climate oscillation known as the North Atlantic Oscillation has been tied to droughts in northeast Spain.[36]

Human activity can directly trigger exacerbating factors such as over farming, excessive irrigation,[37] deforestation, and erosion adversely impact the ability of the land to capture and hold water.[38] While these tend to be relatively isolated in their scope, activities resulting in global climate change are expected to trigger droughts with a substantial impact on agriculture[39] throughout the world, and especially in developing nations.[40][41][42] Overall, global warming will result in increased world rainfall.[43] Along with drought in some areas, flooding and erosion will increase in others. Paradoxically, some proposed solutions to global warming that focus on more active techniques, solar radiation management through the use of a space sunshade for one, may also carry with them increased chances of drought.[44]

[edit] Types of drought

Ship stranded by the retreat of the Aral Sea.

As a drought persists, the conditions surrounding it gradually worsen and its impact on the local population gradually increases. People tend to define droughts in three main ways:[45]
1. Meteorological drought is brought about when there is a prolonged period with less than average precipitation. Meteorological drought usually precedes the other kinds of drought. 2. Agricultural droughts are droughts that affect crop production or the ecology of the range. This condition can also arise independently from any change in precipitation levels when soil conditions and erosion triggered by poorly planned agricultural endeavors cause a shortfall in water available to the crops. However, in a traditional drought, it is caused by an extended period of below average precipitation. 3. Hydrological drought is brought about when the water reserves available in sources such as aquifers, lakes and reservoirs fall below the statistical average. Hydrological drought tends to show up more slowly because it involves stored water that is used but not replenished. Like an agricultural drought, this can be triggered by more than just a loss of rainfall. For instance, Kazakhstan was recently awarded a large amount of money by the World Bank to restore water that had been diverted to other nations from the Aral Sea under Soviet rule.[46] Similar circumstances also place their largest lake, Balkhash, at risk of completely drying out.[47]

[edit] Drought protection and relief

The effects of the drought brought on by El Niño. Waiting for water distribution (Ebeye, Marshall Islands.)

Strategies for drought protection, mitigation or relief include:
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Dams - many dams and their associated reservoirs supply additional water in times of drought. Cloud seeding - an artificial technique to induce rainfall.[48] Desalination of sea water for irrigation or consumption. Drought monitoring - Continuous observation of rainfall levels and comparisons with current usage levels can help prevent man-made drought. For instance, analysis of water usage in Yemen has revealed that their water table (underground water level) is put at grave risk by overuse to fertilize their Khat crop.[49] Careful monitoring of moisture levels can also help predict increased risk for wildfires, using such metrics as the Keetch-Byram Drought Index[8] or Palmer Drought Index. Land use - Carefully planned crop rotation can help to minimize erosion and allow farmers to plant less water-dependent crops in drier years. Outdoor water-use restriction - Regulating the use of sprinklers, hoses or buckets on outdoor plants, filling pools, and other water-intensive home maintenance tasks. Rainwater harvesting - Collection and storage of rainwater from roofs or other suitable catchments. Recycled water - Former wastewater (sewage) that has been treated and purified for reuse. Transvasement - Building canals or redirecting rivers as massive attempts at irrigation in drought-prone areas.

Earthquake
From Wikipedia, the free encyclopedia Jump to: navigation, search For other uses, see Earthquake (disambiguation).

Global earthquake epicenters, 1963–1998

Global plate tectonic movement

An earthquake (also known as a quake, tremor or temblor) is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time. Earthquakes are measured using observations from seismometers. The moment magnitude is the most common scale on which earthquakes larger than approximately 5 are reported for the entire globe. The more numerous earthquakes smaller than magnitude 5 reported by national seismological observatories are measured mostly on the local magnitude scale, also referred to as the Richter scale. These two scales are numerically similar over their range of validity. Magnitude 3 or lower earthquakes are mostly almost imperceptible and magnitude 7 and over potentially cause serious damage over large areas, depending on their depth. The largest earthquakes in historic times have been of magnitude slightly over 9, although there is no limit to the possible magnitude. The most recent large earthquake of magnitude 9.0 or larger was a 9.0 magnitude earthquake in Japan in 2011 (as of March 2011), and it was the largest Japanese earthquake since records began. Intensity of shaking is measured on the modified Mercalli scale. The shallower an earthquake, the more damage to structures it causes, all else being equal.[1]

At the Earth's 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 geological faults, but also by other events such as volcanic activity, landslides, mine blasts, and nuclear tests. An earthquake's point of initial rupture is called its focus or hypocenter. The epicenter is the point at ground level directly above the hypocenter.

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1 Naturally occurring earthquakes o 1.1 Earthquake fault types o 1.2 Earthquakes away from plate boundaries o 1.3 Shallow-focus and deep-focus earthquakes o 1.4 Earthquakes and volcanic activity o 1.5 Rupture dynamics o 1.6 Tidal forces o 1.7 Earthquake clusters  1.7.1 Aftershocks  1.7.2 Earthquake swarms  1.7.3 Earthquake storms 2 Size and frequency of occurrence 3 Induced seismicity 4 Measuring and locating earthquakes 5 Effects of earthquakes o 5.1 Shaking and ground rupture o 5.2 Landslides and avalanches o 5.3 Fires o 5.4 Soil liquefaction o 5.5 Tsunami o 5.6 Floods o 5.7 Human impacts 6 Major earthquakes 7 Prediction 8 Preparedness 9 Historical views 10 Earthquakes in culture o 10.1 Mythology and religion o 10.2 Popular culture 11 See also 12 References

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13 General references 14 External links

Naturally occurring earthquakes

Fault types

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 fault surface that increase the frictional resistance. Most fault surfaces do have such asperities and this leads to a form of stick-slip behaviour. 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 locked portion of the fault, releasing the stored energy. This 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.[2]
Earthquake fault types Main article: Fault (geology)

There are three main types of fault that may cause an earthquake: normal, reverse (thrust) and strike-slip. Normal and reverse faulting are examples of dip-slip, where the displacement along the fault is in the direction of dip and movement on them involves a vertical component. Normal faults occur mainly in areas where the crust is being extended such as a divergent boundary. Reverse faults occur in areas where the crust is being shortened such as at a convergent boundary. Strike-slip faults are steep structures where the two sides of the fault slip horizontally past each other; transform boundaries are a particular type of strike-slip fault. Many earthquakes are caused by movement on faults that have components of both dip-slip and strike-slip; this is known as oblique slip. Reverse faults, particularly those along convergent plate boundaries are associated with the most powerful earthquakes, including almost all of those of magnitude 8 or more. Strike-slip faults, particularly continental transforms can produce major earthquakes up to about magnitude 8. Earthquakes associated with normal faults are generally less than magnitude 7. This is so because the energy released in an earthquake, and thus its magnitude, is proportional to the area of the fault that ruptures[3] and the stress drop. Therefore, the longer the length and the wider the width of the faulted area, the larger the resulting magnitude. The topmost, brittle part of the Earth’s crust, and the cool slabs of the tectonic plates that are descending down into the hot mantle, are the only parts of our planet which can store elastic energy and release it in fault ruptures. Rocks hotter than about 300 degrees Celsius flow in response to stress; they do not rupture in earthquakes.[4][5] The maximum observed lengths of ruptures and mapped faults, which may break in one go are approximately 1000 km. Examples are the earthquakes in Chile, 1960; Alaska, 1957; Sumatra, 2004, all in subduction zones. The longest earthquake ruptures on strikeslip faults, like the San Andreas Fault (1857, 1906), the North Anatolian Fault in Turkey (1939) and the Denali Fault in Alaska (2002), are about half to one third as long as the lengths along subducting plate margins, and those along normal faults are even shorter.

Aerial photo of the San Andreas Fault in the Carrizo Plain, northwest of Los Angeles

The most important parameter controlling the maximum earthquake magnitude on a fault is however not the maximum available length, but the available width because the latter varies by a factor of 20. Along converging plate margins, the dip angle of the rupture plane is very shallow, typically about 10 degrees.[6] Thus the width of the plane within the top brittle crust of the Earth can become 50 to 100 km (Tohoku, 2011; Alaska, 1964), making the most powerful earthquakes possible. Strike-slip faults tend to be oriented near vertically, resulting in an approximate width of 10 km within the brittle crust,[7] thus earthquakes with magnitudes much larger than 8 are not possible. Maximum magnitudes along many normal faults are even more limited because many of them are located along spreading centers, as in Iceland, where the thickness of the brittle layer is only about 6 km.[8][9] In addition, there exists a hierarchy of stress level in the three fault types. Thrust faults are generated by the highest, strike slip by intermediate, and normal faults by the lowest stress levels.[10] This can easily be understood by considering the direction of the greatest principal stress, the direction of the force that ‘pushes’ the rock mass during the faulting. In the case of normal faults, the rock mass is pushed down in a vertical direction, thus the pushing force (greatest principal stress) equals the weight of the rock mass itself. In the case of thrusting, the rock mass ‘escapes’ in the direction of the least principal stress, namely upward, lifting the rock mass up, thus the overburden equals the least principal stress. Strike-slip faulting is intermediate between the other two types described above. This difference in stress regime in the three faulting environments can contribute to differences in stress drop during faulting, which contributes to differences in the radiated energy, regardless of fault dimensions.
Earthquakes away from plate boundaries Main article: Intraplate earthquake

Where plate boundaries occur within continental lithosphere, deformation is spread out over a much larger area than the plate boundary itself. In the case of the San Andreas fault continental transform, many earthquakes occur away from the plate boundary and are related to strains developed within the broader zone of deformation caused by major irregularities in the fault trace (e.g., the "Big bend" region). The Northridge earthquake was associated with movement on a blind thrust within such a zone. Another example is the strongly oblique convergent plate boundary between the Arabian and Eurasian plates where it runs through the northwestern part of the Zagros mountains. The deformation associated with this plate boundary is partitioned into nearly pure thrust sense movements perpendicular to the boundary over a wide zone to the southwest and nearly pure strike-slip motion along the Main Recent Fault close to the actual plate boundary itself. This is demonstrated by earthquake focal mechanisms.[11] All tectonic plates have internal stress fields caused by their interactions with neighbouring plates and sedimentary loading or unloading (e.g. deglaciation[12]). These stresses may be sufficient to cause failure along existing fault planes, giving rise to intraplate earthquakes.[13]
Shallow-focus and deep-focus earthquakes Main article: Depth of focus (tectonics)

The majority of tectonic earthquakes originate at the ring of fire in depths not exceeding tens of kilometers. Earthquakes occurring at a depth of less than 70 km are classified as 'shallow-focus' earthquakes, while those with a focal-depth between 70 and 300 km are commonly termed 'midfocus' or 'intermediate-depth' earthquakes. In subduction zones, where older and colder oceanic crust descends beneath another tectonic plate, deep-focus earthquakes may occur at much greater depths (ranging from 300 up to 700 kilometers).[14] These seismically active areas of subduction are known as Wadati-Benioff zones. Deep-focus earthquakes occur at a depth where 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.[15]
Earthquakes and volcanic activity

Earthquakes often occur in volcanic regions and are caused there, both by tectonic faults and the movement of magma in volcanoes. Such earthquakes can serve as an early warning of volcanic eruptions, as during the Mount St. Helens eruption of 1980.[16] Earthquake swarms can serve as markers for the location of the flowing magma throughout the volcanoes. These swarms can be recorded by seismometers and tiltmeters (a device that measures ground slope) and used as sensors to predict imminent or upcoming eruptions.[17]
Rupture dynamics

A tectonic earthquake begins by an initial rupture at a point on the fault surface, a process known as nucleation. The scale of the nucleation zone is uncertain, with some evidence, such as the rupture dimensions of the smallest earthquakes, suggesting that it is smaller than 100 m while other evidence, such as a slow component revealed by low-frequency spectra of some earthquakes, suggest that it is larger. The possibility that the nucleation involves some sort of preparation process is supported by the observation that about 40% of earthquakes are preceded by foreshocks. Once the rupture has initiated it begins to propagate along the fault surface. The mechanics of this process are poorly understood, partly because it is difficult to recreate the high sliding velocities in a laboratory. Also the effects of strong ground motion make it very difficult to record information close to a nucleation zone.[18] Rupture propagation is generally modeled using a fracture mechanics approach, likening the rupture to a propagating mixed mode shear crack. The rupture velocity is a function of the fracture energy in the volume around the crack tip, increasing with decreasing fracture energy. The velocity of rupture propagation is orders of magnitude faster than the displacement velocity across the fault. Earthquake ruptures typically propagate at velocities that are in the range 70– 90% of the S-wave velocity and this is independent of earthquake size. A small subset of earthquake ruptures appear to have propagated at speeds greater than the S-wave velocity. These supershear earthquakes have all been observed during large strike-slip events. The unusually wide zone of coseismic damage caused by the 2001 Kunlun earthquake has been attributed to the effects of the sonic boom developed in such earthquakes. Some earthquake ruptures travel at unusually low velocities and are referred to as slow earthquakes. A particularly dangerous form of slow earthquake is the tsunami earthquake, observed where the relatively low felt intensities,

caused by the slow propagation speed of some great earthquakes, fail to alert the population of the neighbouring coast, as in the 1896 Meiji-Sanriku earthquake.[18]
Tidal forces See also: Earthquake prediction#Tides

Research work has shown a robust correlation between small tidally induced forces and nonvolcanic tremor activity.[19][20][21][22]
Earthquake clusters

Most earthquakes form part of a sequence, related to each other in terms of location and time.[23] Most earthquake clusters consist of small tremors that cause little to no damage, but there is a theory that earthquakes can recur in a regular pattern.[24]
Aftershocks Main article: Aftershock

An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock is in the same region of the main shock but always of a smaller magnitude. If an aftershock is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Aftershocks are formed as the crust around the displaced fault plane adjusts to the effects of the main shock.[23]
Earthquake swarms Main article: Earthquake swarm

Earthquake swarms are sequences of earthquakes striking in a specific area within a short period of time. They are different from earthquakes followed by a series of aftershocks by the fact that no single earthquake in the sequence is obviously the main shock, therefore none have notable higher magnitudes than the other. An example of an earthquake swarm is the 2004 activity at Yellowstone National Park.[25]
Earthquake storms Main article: Earthquake storm

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 and has been inferred for older anomalous clusters of large earthquakes in the Middle East.[26][27]

Size and frequency of occurrence

It is estimated that around 500,000 earthquakes occur each year, detectable with current instrumentation. About 100,000 of these can be felt.[28][29] Minor earthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., as well as in Mexico, Guatemala, Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, New Zealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, including New York City, London, and Australia.[30] Larger 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, and an earthquake of 5.6 or larger every 100 years.[31] This is an example of the Gutenberg-Richter law.

The Messina earthquake and tsunami took as many as 200,000 lives on December 28, 1908 in Sicily and Calabria.[32]

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, but this is because of the vast improvement in instrumentation, rather than an increase in the number of earthquakes. The United States Geological Survey 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.[33] In recent years, the number of major earthquakes per year has decreased, though this is probably a statistical fluctuation rather than a systematic trend.[citation needed] More detailed statistics on the size and frequency of earthquakes is available from the United States Geological Survey (USGS).[34] A recent increase in the number of major earthquakes has been noted, which could be explained by a cyclical pattern of periods of intense tectonic activity, interspersed with longer periods of low-intensity. However, accurate recordings of earthquakes only began in the early 1900s, so it is too early to categorically state that this is the case.[35] 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, known as the Pacific Ring of Fire, which for the most part bounds the Pacific Plate.[36][37] Massive earthquakes tend to occur along other plate boundaries, too, such as along the Himalayan Mountains.[38]

With the rapid growth of mega-cities such as Mexico City, Tokyo and 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.[39]

Induced seismicity
Main article: Induced seismicity

While most earthquakes are caused by movement of the Earth's tectonic plates, human activity can also produce earthquakes. Four main activities contribute to this phenomenon: storing large amounts of water behind a dam (and possibly building an extremely heavy building), drilling and injecting liquid into wells, and by coal mining and oil drilling.[40] Perhaps the best known example is the 2008 Sichuan earthquake in China's Sichuan Province in May; this tremor resulted in 69,227 fatalities and is the 19th deadliest earthquake of all time. The Zipingpu Dam is believed to have fluctuated the pressure of the fault 1,650 feet (503 m) away; this pressure probably increased the power of the earthquake and accelerated the rate of movement for the fault.[41] The greatest earthquake in Australia's history is also claimed to be induced by humanity, through coal mining. The city of Newcastle was built over a large sector of coal mining areas. The earthquake has been reported to be spawned from a fault that reactivated due to the millions of tonnes of rock removed in the mining process.[42]

Measuring and locating earthquakes
Main article: Seismology

Earthquakes can be recorded by seismometers up to great distances, because seismic waves travel through the whole Earth's interior. The absolute magnitude of a quake is conventionally reported by numbers on the Moment magnitude scale (formerly Richter scale, magnitude 7 causing serious damage over large areas), whereas the felt magnitude is reported using the modified Mercalli intensity scale (intensity II–XII). Every tremor produces different types of seismic waves, which travel through rock with different velocities:
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Longitudinal P-waves (shock- or pressure waves) Transverse S-waves (both body waves) Surface waves — (Rayleigh and Love waves)

Propagation velocity of the seismic waves ranges from approx. 3 km/s up to 13 km/s, depending on the density and elasticity of the medium. In the Earth's interior the shock- or P waves travel much faster than the S waves (approx. relation 1.7 : 1). The differences in travel time from the epicentre to the observatory are a measure of the distance and can be used to image both sources of quakes and structures within the Earth. Also the depth of the hypocenter can be computed roughly.

In solid rock P-waves travel at about 6 to 7 km per second; the velocity increases within the deep mantle to ~13 km/s. The velocity of S-waves ranges from 2–3 km/s in light sediments and 4–5 km/s in the Earth's crust up to 7 km/s in the deep mantle. As a consequence, the first waves of a distant earthquake arrive at an observatory via the Earth's mantle. Rule of thumb: On the average, the kilometer distance to the earthquake is the number of seconds between the P and S wave times 8.[43] Slight deviations are caused by inhomogeneities of subsurface structure. By such analyses of seismograms the Earth's core was located in 1913 by Beno Gutenberg. Earthquakes are not only categorized by their magnitude but also by the place where they occur. The world is divided into 754 Flinn-Engdahl regions (F-E regions), which are based on political and geographical boundaries as well as seismic activity. More active zones are divided into smaller F-E regions whereas less active zones belong to larger F-E regions.

Effects of earthquakes

1755 copper engraving depicting Lisbon in ruins and in flames after the 1755 Lisbon earthquake, which killed an estimated 60,000 people. A tsunami overwhelms the ships in the harbor.

The effects of earthquakes include, but are not limited to, the following:
Shaking and ground rupture

Damaged buildings in Port-au-Prince, Haiti, January 2010.

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more or less severe damage to buildings and other rigid structures. The severity of the local effects depends on the complex combination of the earthquake magnitude, the distance from the epicenter, and the local geological and geomorphological conditions, which may amplify or reduce wave propagation.[44] 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 several 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 which are likely to break the ground surface within the life of the structure.[45]
Landslides and avalanches Main article: Landslide

Earthquakes, along with severe storms, volcanic activity, coastal wave attack, and wildfires, can produce slope instability leading to landslides, a major geological hazard. Landslide danger may persist while emergency personnel are attempting rescue.[46]
Fires

Fires of the 1906 San Francisco earthquake

Earthquakes can cause fires by damaging 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. For example, more deaths in the 1906 San Francisco earthquake were caused by fire than by the earthquake itself.[47]

Soil liquefaction Main article: Soil liquefaction

Soil liquefaction occurs when, because of the shaking, water-saturated granular material (such as sand) temporarily loses its strength and transforms from a solid to a liquid. Soil liquefaction may cause rigid structures, like buildings and bridges, to tilt or sink into the liquefied deposits. This can be a devastating effect of earthquakes. For example, in the 1964 Alaska earthquake, soil liquefaction caused many buildings to sink into the ground, eventually collapsing upon themselves.[48]
Tsunami

The tsunami of the 2004 Indian Ocean earthquake

A large ferry boat rests inland amidst destroyed houses after a 9.0 earthquake and subsequent tsunami struck Japan in March 2011. Main article: Tsunami

Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement of large volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers (62 mi), and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour (373–497 miles per hour), depending on water depth. Large waves produced by an earthquake or a submarine landslide can overrun nearby coastal areas in a matter of minutes. Tsunamis can also travel thousands of kilometers across open ocean and wreak destruction on far shores hours after the earthquake that generated them.[49] Ordinarily, subduction earthquakes under magnitude 7.5 on the Richter scale do not cause tsunamis, although some instances of this have been recorded. Most destructive tsunamis are caused by earthquakes of magnitude 7.5 or more.[49]
Floods Main article: Flood

A flood is an overflow of any amount of water that reaches land.[50] Floods occur usually when the volume of water within a body of water, such as a river or lake, exceeds the total capacity of the formation, and as a result some of the water flows or sits outside of the normal perimeter of the body. However, floods may be secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to dam rivers, which collapse and cause floods.[51] The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flood if the landslide dam formed by the earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impact projections suggest the flood could affect roughly 5 million people.[52]
Human impacts

An earthquake may cause injury and loss of life, road and bridge damage, general property damage (which may or may not be covered by earthquake insurance), and collapse or destabilization (potentially leading to future collapse) of buildings. The aftermath may bring disease, lack of basic necessities, and higher insurance premiums.

Major earthquakes

Earthquakes of magnitude 8.0 and greater since 1900. The apparent 3D volumes of the bubbles are linearly proportional to their respective fatalities.[53]

Main article: List of earthquakes

One of the most devastating earthquakes in recorded history occurred on 23 January 1556 in the Shaanxi province, China, killing more than 830,000 people (see 1556 Shaanxi earthquake).[54] Most of the population in the area at the time lived in yaodongs, artificial caves in loess cliffs, many of which collapsed during the catastrophe with great loss of life. The 1976 Tangshan earthquake, with death toll estimated to be between 240,000 to 655,000, is believed to be the largest earthquake of the 20th century by death toll.[55] The 1960 Chilean Earthquake is the largest earthquake that has been measured on a seismograph, reaching 9.5 magnitude on 22 May 1960.[28][29] Its epicenter was near Cañete, Chile. The energy released was approximately twice that of the next most powerful earthquake, the Good Friday Earthquake, which was centered in Prince William Sound, Alaska.[56][57] The ten largest recorded earthquakes have all been megathrust earthquakes; however, of these ten, only the 2004 Indian Ocean earthquake is simultaneously one of the deadliest earthquakes in history. Earthquakes that caused the greatest loss of life, while powerful, were deadly because of their proximity to either heavily populated areas or the ocean, where earthquakes often create tsunamis that can devastate communities thousands of kilometers away. Regions most at risk for great loss of life include those where earthquakes are relatively rare but powerful, and poor regions with lax, unenforced, or nonexistent seismic building codes.

Prediction
Main article: Earthquake prediction

Many different methods have been developed for predicting the time and place in which earthquakes will occur. Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month.[58] However, for wellunderstood faults the probability that a segment may rupture during the next few decades can be estimated.[59] Earthquake warning systems have been developed that can provide regional notification of an earthquake in progress, but before the ground surface has begun to move, potentially allowing people within the system's range to seek shelter before the earthquake's impact is felt.

Preparedness
The objective of earthquake engineering is to foresee the impact of earthquakes on buildings and other structures and to design such structures to minimize the risk of damage. Existing structures can be modified by seismic retrofitting to improve their resistance to earthquakes. Earthquake insurance can provide building owners with financial protection against losses resulting from earthquakes.

Emergency management strategies can be employed by a government or organization to mitigate risks and prepare for consequences.

Historical views

An image from a 1557 book

From the lifetime of the Greek philosopher Anaxagoras in the 5th century BCE to the 14th century CE, earthquakes were usually attributed to "air (vapors) in the cavities of the Earth."[60] Thales of Miletus, who lived from 625–547 (BCE) was the only documented person who believed that earthquakes were caused by tension between the earth and water.[60] Other theories existed, including the Greek philosopher Anaxamines' (585–526 BCE) beliefs that short incline episodes of dryness and wetness caused seismic activity. The Greek philosopher Democritus (460–371 BCE) blamed water in general for earthquakes.[60] Pliny the Elder called earthquakes "underground thunderstorms."[60]

Earthquakes in culture
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 dripped on Loki's face, forcing him to jerk his head away and thrash against his bonds, which caused the earth to tremble.[61] In Greek mythology, Poseidon was the cause and god of earthquakes. When he was in a bad mood, he struck the ground with a trident, causing earthquakes and other calamities. He also used earthquakes to punish and inflict fear upon people as revenge.[62]

In Japanese mythology, Namazu (鯰) is a giant catfish who causes earthquakes. Namazu lives in the mud beneath the earth, and is guarded by the god Kashima who restrains the fish with a stone. When Kashima lets his guard fall, Namazu thrashes about, causing violent earthquakes.
Popular culture

In modern popular culture, the portrayal of earthquakes is shaped by the memory of great cities laid waste, such as Kobe in 1995 or San Francisco in 1906.[63] Fictional earthquakes tend to strike suddenly and without warning.[63] For this reason, stories about earthquakes generally begin with the disaster and focus on its immediate aftermath, as in Short Walk to Daylight (1972), The Ragged Edge (1968) or Aftershock: Earthquake in New York (1998).[63] A notable example is Heinrich von Kleist's classic novella, The Earthquake in Chile, which describes the destruction of Santiago in 1647. Haruki Murakami's short fiction collection after the quake depicts the consequences of the Kobe earthquake of 1995. The most popular single earthquake in fiction is the hypothetical "Big One" expected of California's San Andreas Fault someday, as depicted in the novels Richter 10 (1996) and Goodbye California (1977) among other works.[63] Jacob M. Appel's widely anthologized short story, A Comparative Seismology, features a con artist who convinces an elderly woman that an apocalyptic earthquake is imminent.[64] In Pleasure Boating in Lituya Bay, one of the stories in Jim Shepard's Like You'd Understand, Anyway, the "Big One" leads to an even more devastating tsunami. In the film 2012 (2009), solar flares (geologically implausibly) affecting the Earth's core caused massive destabilization of the Earth's crust layers. This created destruction planet-wide with earthquakes and tsunamis, foreseen by the Mayan culture and myth surrounding the last year noted in the Mesoamerican calendar — 2012. Contemporary depictions of earthquakes in film are variable in the manner in which they reflect human psychological reactions to the actual trauma that can be caused to directly afflicted families and their loved ones.[65] Disaster mental health response research emphasizes the need to be aware of the different roles of loss of family and key community members, loss of home and familiar surroundings, loss of essential supplies and services to maintain survival.[66][67] Particularly for children, the clear availability of caregiving adults who are able to protect, nourish, and clothe them in the aftermath of the earthquake, and to help them make sense of what has befallen them has been shown even more important to their emotional and physical health than the simple giving of provisions.[68] As was observed after other disasters involving destruction and loss of life and their media depictions, such as those of the 2001 World Trade Center Attacks or Hurricane Katrina—and has been recently observed in the 2010 Haiti earthquake, it is also important not to pathologize the reactions to loss and displacement or disruption of governmental administration and services, but rather to validate these reactions, to support constructive problem-solving and reflection as to how one might improve the conditions of those affected.[69]

See also

    

Seismite Seismotectonics Submarine earthquake Triangle of Life Marine terrace

References
1. 2. 3. ^ "Earthquake FAQ". Crustal.ucsb.edu. http://www.crustal.ucsb.edu/outreach/faq.php. Retrieved 201107-24. ^ Spence, William; S. A. Sipkin, G. L. Choy (1989). "Measuring the Size of an Earthquake". United States Geological Survey. http://earthquake.usgs.gov/learning/topics/measure.php. Retrieved 2006-11-03. ^ Wyss, M. (1979). "Estimating expectable maximum magnitude of earthquakes from fault dimensions". Geology 7 (7): 336–340. Bibcode 1979Geo.....7..336W. doi:10.1130/00917613(1979)7<336:EMEMOE>2.0.CO;2. ^ Sibson R. H. (1982) "Fault Zone Models, Heat Flow, and the Depth Distribution of Earthquakes in the Continental Crust of the United States", Bulletin of the Seismological Society of America, Vol 72, No. 1, pp. 151–163 ^ Sibson, R. H. (2002) „Geology of the crustal earthquake source" International handbook of earthquake and engineering seismology, Volume 1, Part 1, page 455, eds. W H K Lee, H Kanamori, P C Jennings, and C. Kisslinger, Academic Press, ISBN / ASIN: 0124406521 ^ "Global Centroid Moment Tensor Catalog". Globalcmt.org. http://www.globalcmt.org/CMTsearch.html. Retrieved 2011-07-24. ^ "Instrumental California Earthquake Catalog". WGCEP. http://wgcep.org/data-inst_eq_cat. Retrieved 2011-07-24. ^ Hjaltadóttir S., 2010, "Use of relatively located microearthquakes to map fault patterns and estimate the thickness of the brittle crust in Southwest Iceland" ^ "Reports and publications | Seismicity | Icelandic Meteorological office". En.vedur.is. http://en.vedur.is/earthquakes-and-volcanism/reports-and-publications/. Retrieved 2011-07-24. ^ Schorlemmer, D.; Wiemer, S.; Wyss, M. (2005). "Variations in earthquake-size distribution across different stress regimes". Nature 437 (7058): 539–542. Bibcode 2005Natur.437..539S. doi:10.1038/nature04094. PMID 16177788. ^ Talebian, M; Jackson, J (2004). "A reappraisal of earthquake focal mechanisms and active shortening in the Zagros mountains of Iran". Geophysical Journal International 156 (3): 506–526. Bibcode 2004GeoJI.156..506T. doi:10.1111/j.1365-246X.2004.02092.x. ^ Nettles, M.; Ekström, G. (May 2010). "Glacial Earthquakes in Greenland and Antarctica". Annual Review of Earth and Planetary Sciences 38 (1): 467–491. Bibcode 2010AREPS..38..467N. doi:10.1146/annurevearth-040809-152414.Avinash Kumar edit ^ Noson, Qamar, and Thorsen (1988). Washington State Earthquake Hazards: Washington State Department of Natural Resources. Washington Division of Geology and Earth Resources Information Circular 85. ^ "M7.5 Northern Peru Earthquake of 26 September 2005" (PDF). National Earthquake Information Center. 17 October 2005. ftp://hazards.cr.usgs.gov/maps/sigeqs/20050926/20050926.pdf. Retrieved 2008-08-01. ^ Greene II, H. W.; Burnley, P. C. (October 26, 1989). "A new self-organizing mechanism for deep-focus earthquakes". Nature 341 (6244): 733–737. Bibcode 1989Natur.341..733G. doi:10.1038/341733a0. ^ Foxworthy and Hill (1982). Volcanic Eruptions of 1980 at Mount St. Helens, The First 100 Days: USGS Professional Paper 1249. ^ Watson, John; Watson, Kathie (January 7, 1998). "Volcanoes and Earthquakes". United States Geological Survey. http://pubs.usgs.gov/gip/earthq1/volcano.html. Retrieved May 9, 2009. ab ^ National Research Council (U.S.). Committee on the Science

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http://library.thinkquest.org/16132/frames.html

Natural disasters in India, many of them related to the climate of India, cause massive losses of Indian life and property. Droughts, flash floods, cyclones, avalanches, landslides brought on by torrential rains, and snowstorms pose the greatest threats. Other dangers include frequent summer dust storms, which usually track from north to south; they cause extensive property damage in North India[1] and deposit large amounts of dust from arid regions. Hail is also common in parts of India, causing severe damage to standing crops such as rice and wheat. Landslides are common in the Lower Himalayas. The young age of the region's hills result in labile rock formations, which are susceptible to slippages. Rising population and development pressures, particularly from logging and tourism, cause deforestation. The result is denuded hillsides which exacerbate the severity of landslides; since tree cover impedes the downhill flow of water.[2] Parts of the Western Ghats also suffer from low-intensity landslides. Avalanches occurrences are common in Kashmir, Himachal Pradesh, and Sikkim. Floods are the most common natural disaster in India. The heavy southwest monsoon rains cause the Brahmaputra and other rivers to distend their banks, often flooding surrounding areas. Though they provide rice paddy farmers with a largely dependable source of natural irrigation and fertilisation, the floods can kill thousands and displace millions. Excess, erratic, or untimely monsoon rainfall may also wash away or otherwise ruin crops.[3][4] Almost all of India is floodprone, and extreme precipitation events, such as flash floods and torrential rains, have become increasingly common in central India over the past several decades, coinciding with rising temperatures. Mean annual precipitation totals have remained steady due to the declining frequency of weather systems that generate moderate amounts of rain.[5]

Natural Calamities
In India, we have various kinds of natural disasters take place. The followings are the common natural disasters, which occur very often at different parts of the country. Droughts Drought is perhaps the manifestation of desertification, which may be because of unprecedented soil erosion, large scale deforestation and abrupt change in micro-climate thereby increasing the temperature and reducing rainfall etc., ultimately leads to fall of groundwater level and hence, loss of agricultural productivity of the land, due to lack of water resources. Since, Indian Agriculture is mostly rain-fed, the occurrences of Droughts are common at the different parts of the country. Floods When it rains heavily in the catchments of rivers and there is no dam, especially during monsoon, the rivers flood. Like drought, occurrence of flood is also quite common in the various parts of the country. Earthquakes The geological strata of the country belong to Gondwana land-mass; which is

comparatively new, younger and unstable geological formation. There are still many parts of the country under earthquakes-prone-regions. If history would be referred, there are many severe earthquakes had shacked the backbone of the country; the recent one is being the earthquake of Bhuj at the state of Gujarat. The great Himalayan Mountain range, which belong to comparatively the younger geological formations, which is still undergoing morphological changes, the construction of Tehri Dam, therefore, is a great-threat to the Garhwal region of the Himalayas. Cyclone Due to low pressure in the atmosphere and frequent formation of whirls; cyclones take place frequently at the eastern coast of India. In the Bay of Bengal of Indian Ocean, these Low- pressure Whirls are formed and gets transmitted to the coastal districts of Andhra Pradesh and Orissa. The recent super-cyclone at Orissa in October, 1999 took away the life of more than 25,000 people, destroyed the properties of more than thousands billion dollars and more than a million of people rendered jobless. Their livelihood security of the common mass was also got severely threatened. Hot waves In recent days, India has got highly affected by a new form of natural calamities i.e flowing of hot waves again in the east coast, killing the thousands of people in the Northen and Eartern parts of the country like, Uttar Pradesh, Bihar, Rajasthan, Gujarat, Orissa and Andhra Pradesh. The flow of hot waves is also known as ‘Sun-stroke,’ which in fact, is common in our country. In Orissa, alone about 151 people died of “ Sun-stock “ in 1999. The worst sufferers are physically weaker persons, Old men and women and the children. Cold Waves The incidents of death due to cold waves occurs in higher and lesser Himalayas especially in the States of Uttranchal, Sikkim, Himachal Pradesh and Northen Parts of West Bengal including Darjiling. In addition, there are also other natural calamities such as Tornado, Spiral tide Whirls etc, which occur very often in our country.

It is a well known fact that natural disasters strikes countries, both developed and developing, causing enormous destruction and creating human sufferings and producing negative impacts on national economies. Different types of natural disasters like floods, droughts, earthquakes, cyclones, landslides, and volcanoes, etc. strikes according to the vulnerability of the area. India is considered as one of the the world's most disaster prone country. It has witnessed devastating natural disasters in

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recent past like droughts, floods, cyclones, earthquakes, landslides, etc. Among all the natural disasters that country faces, river floods are the most frequent and often devastating. The shortfall in the rainfall cause droughts or drought like situation in various parts of the country. The country has faced some severe earthquakes causing widespread damage to the life and property. India has a coastline of about 8000 km, which is prone to very severe cyclonic formations in the Arabian Sea and Bay of Bengal.

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Though it is not possible to completely avoid the natural disasters, but the sufferings can be minimized by creating proper awareness of the likely disasters and its impact by developing a suitable warning system, disaster preparedness and management of disasters through application of information technology tools.
http://www.ready.gov/drought

An earthquake is a shaking of the ground caused by the sudden dislocation of material within the earth's outer layer, or crust. When forces pushing on a mass of rock overcome the friction holding the rock in place and blocks of rock slip against each other a earthquake may occur. Some earthquakes are so slight, and some occur in such remote areas, that they are barely felt. Others are so violent that they cause extensive damage.

Causes of Earthquakes

Earthquakes are caused by stresses below the earth's outer surface. These stresses usually build up until the rocks fracture along a "fault plane." This causes vibrations, also known as seismic waves. Seismic waves will then travel in all directions from the area of fracture. In large earthquakes seismic waves may be detected over the entire earth. Earthquakes can be caused by volcanoes in certain cases. Nuclear explosions under the ground can create waves that are very similar to natural seismic waves. The seismic energy created in a

atomic bomb is one hundred-thousandth that of the largest earthquake.

Kinds of Waves

Some seismic waves travel through the earth's interior while others travel only over the earth's surface. There are two types of body waves. The primary, or P wave, is the faster of the two. P waves always travel at higher speeds than S waves, so whenever an earthquake occurs, P waves are the first to arrive and to be recorded at geophysical research stations worldwide. These waves are transmitted through all parts of the earth. S waves, which are slower than P waves, are known as the shear waves. This type of wave causes lateral vibrations. S waves are not capable of travelling through liquids. S waves have not been detected in any part of the earth deeper than about 1,800 mi, which indicates that below this depth the earth is molten.

Intensity of EarthQuakes
The intensity of an earthquake becomes weaker outward from the epicenter. However, various types of ground respond differently to earthquake vibrations. Buildings on filled ground are damaged more than structures built on solid rock even though both may be at the same distance from the epicenter. The magnitude of a particular earthquake is a single number which does not vary from place to

place. Magnitude is the total energy released by an earthquake at its focus. Earthquakes of large magnitude are stronger and generally more destructive than those of small magnitude. The amount of destruction depends not only on the magnitude but on the kind of ground and types of buildings thereon, and on the location of the focus in relation to heavily populated areas. Large earthquakes are preceded by many aftershocks, which may persist for days or weeks. The first shock is the most damaging. However, sometimes an aftershock may be even more powerful than the original shock. The intensity of an earthquake is measured in terms of its geological effects and the overall damage it brings. There are two major scales in which earthquakes are measured. These two scales are the Mercalli Scale and the Richter Scale.

Mercalli Scale
The Mercalli scale was introduced at the turn of the 20th century by the Italian seismologist Giuseppe Mercalli. This scale measures the intensity of shaking with numbers from I to XII. Intensity I on this scale is defined as an event felt by very few people, whereas intensity XII is assigned to a catastrophic event that causes total destruction. Events of intensities II to III are roughly equivalent to quakes of magnitude 3 to 4 on the Richter scale, and XI to XII on the Mercalli scale correspond with magnitudes 8 to 9 on the Richter scale. I. Hardly felt II. Felt only by a few persons at rest, especially on upper floors of buildings. III. Can be felt by persons indoors, especially on upper floors of buildings. Many people do not recognize it as an earthquake. IV. Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doors disturbed. Standing motor cars rocked noticeably. V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned. VI. Felt by all VII. Considerable damage in poorly built or badly designed structures. VIII. Damage slight in specially designed structures. Damage great in poorly built structures. Heavy furniture overturned. IX. Damage considerable in specially designed structures. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations. X. Many objects destroyed, buildings collapse.

XI. Few structures remain standing. Bridges destroyed. Rails bent greatly. XII. Total Damage.

Richter Scale
The Richter scale was named after the American seismologist Charles Francis Richter. This scale measures the motion of the land surface 60 mi from the epicenter, or focus, of the earthquake. An estimated 800 quakes of magnitudes 5 to 6 occur worldwide each year. About 50,000 quakes of magnitudes 3 to 4 occur each year, and only about one of magnitude 8 to 9 each year. Between to 0-4.3 on the Richter scale, People at rest upstairs notice shaking. Shaking felt indoors; hanging objects swing. Between 4.3-4.8 Sleeping people are awakened. Dishes, doors and trees shake and rock. Between 4.8-6.2 Difficult to stand; people walk unsteadily. Windows break; plaster,bricks, and tiles fall. Between 6.2-7.3 General panic. Damage to foundations; buildings destoyed. Water thrown out of river. Between 7.3-8.9 Total destruction; roads break up, rocks fall. Large cracks appear in ground.

Location and Distribution of EarthQuakes
Earthquakes take place or have taken place in all parts of the world. Frequent activity occurs along certain belts. 80% of all seismic energy is generated from a belt that is found at the border of the Pacific Ocean. A great deal of volcanoes are also found there, and volcanoes set off many earthquakes. Japan, the Philippine Islands, New Guinea, and New Zealand are all part of the Pacific belt. A second seismic belt produces 15% of seismic activity. It goes through southern Asia to the region of the Mediterranean Sea. The final 5% of seismic energy comes from parts of the Arctic, Atlantic, and Indian Oceans. Antarctica and Australia experience the least amount of earthquake activity then any other areas of the world.

Effects of Earthquakes

Earthquakes produce various damaging effects to the areas they act upon. This includes damage to buildings and in worst cases the loss of human life. The effects of the rumbling produced by earthquakes usually leads to the destruction of structures such as buildings, bridges, and dams. They can also trigger landslides. An example of how an earthquake can lead to even more destruction is the 1959 earthquake near Hebgen, Montana. It caused a land slide that killed several people and blocked the Madison River. Due to the fact that the Madison River was blocked, a lake was created which later flooded the nearby town of Ennis.

Besides producing floods and destroying buildings, earthquakes that take place under the ocean can sometimes cause tsunamis, or tidal waves. Tsunamis are high and long walls of water which travel at a very rapid rate. They are notorious for destroying entire populations and cities near coastlines. In 1896 Sanriku, Japan, with a population of 20,000, suffered such a fate.

Attempts at predicting when and where earthquakes will occur have met with some success in recent years. Researchers have been able to use instruments such as seismographs, to find out the source of where seismic waves were produced. This allows the researchers to further study and eventually make accurate predictions about a coming earthquake and the damage it will bring. At present, China, Japan, Russia, and the United States are the countries most actively supporting such research. Other potential clues being investigated are tilting or bulging of the land surface and changes in the earth's magnetic field, in the water levels of wells, and even in animal behavior. A new method under study in the United States involves measuring the buildup of stress in the crust of the earth.

How to protect yourself during an earthquake

1.) If an earthquake is occuring the most important thing to do is to DROP and COVER. Drop and cover means to fall on to the floor and get under something for protection. During an earthquake, if you are indoors, it is very important to stay calm and take cover under a heavy object. 2.) If you are outdoors, stay as far away from buildings as possible. 3.) Stay away from glass or anything that could fall. 4.) If you are in a crowded area, do not even consider running for the nearest exit. Everyone will be doing that, and crowding will lead to even more injuries. Take cover under something heavy and stay away from things that could fall on you. It is also very important to remain as calm as possible. 5.) Be prepared for aftershocks after the initial earthquake has ended. Aftershocks are follow-up earthquakes. They are smaller than the first one, but still are very dangerous.

Types of Disaster

Earthquake Earthquake is an unexpected and rapid shaking of earth due to the breakage and shifting of underneath layers of Earth. Earthquake strikes all of a sudden at any time of day or night and quite violently. It gives no prior warning. If it happens in a populated area, the earthquake can cause great loss to human life and property. Tornado Tornado is one of the most violent storms on earth. It seems like a rotating and funnel shape cloud. It expands from the thunderstorm to the ground in the form of whirl winds reaching around 300 miles per hour. The damage path could move on to one mile wide and around 50 miles long. These storms can strike quickly without any warning. Flood Flood is also one of the most common hazards in the United States and other parts of the world. The effects of a flood can be local to a neighborhood or community. It can cast a larger impact, the whole river basin and multiple states could get affected. Every state is at its risk due to this hazard. Water Damage Water damage has a huge effect on your home, its neighborhood and your city. It is very much necessary that you should prepare for water damage. You must know what should be done during and after water damage. Hail Hail comes into existence when updrafts in the thunder clouds take the raindrops up towards the extremely cold regions in the atmosphere. They freeze and combine forming lumps of ice. As these lumps can be very heavy and are not supported by the updraft, they fall off with the speeds of about 100 km per hour or more. A Hail is created in the form of an enormous cloud, commonly known as thunderheads. Wildfire Wild forest areas catching fire is a very big problem for the people who live around these areas. The dry conditions caused several times in the year in different parts of United States can increase the possibility for wildfires. If you are well prepared in advance and know how to protect the buildings in your area, you can reduce much of the damage caused by wildfire. It is everyone�s duty to protect their home and neighborhood from wildfire. Hurricane

Hurricane also like the tornado is a wind storm, but it is a tropical cyclone. This is caused by a low pressure system that usually builds in the tropical. Huricanes comes with thunderstorms and a counterclockwise spread of winds near the surface of the earth. Winter Freeze Winter freeze storms are serious threats for people and their property. They include, snow, frozen rain, strong winds and extreme cold. Many precautions have to be taken in order to protect yourself, your family, home or property. Lightning Lightning is a much underestimated killer. Lightning is an abrupt electric expulsion which comes from cloud to cloud or from cloud to earth followed by an emission of light. Lightning is a common phenomenon after heavy rain and can also occur around 10 miles off from rainfall. Most lightning victims are people who are captivated outdoors in summer during the afternoon and evening. Volcano Volcano is a mountain that has an opening downwards to the reservoir of molten rock towards the surface of earth. Volcanoes are caused by the accrual of igneous products. As the pressure caused by gases in the molted rock becomes intense, the eruption takes place. The volcanic eruption can be of two kinds, quiet or volatile. The aftermaths of a volcano include flowing lava, flat landscapes, poisonous gases and fleeing ashes and rocks.Read on to know more on types of disasters.

What are Earthquakes?

Earthquakes are one of the many powerful natural disasters caused when there is a shift, collision, or sudden release of energy in the Earth’s crust. Sometimes called tremors and temblors, earthquakes usually occur on the boundaries of, or near, the lines where the Earth’s tectonic plates collide and slide past each other. This is called a fault line. Changes in the Earth’s surface usually result in earthquakes. Earthquakes consist of shaking and displacement of the ground. Seismographs and seismometers are instruments used to measure and record the seismic activity released by an earthquake. Depending on the intensity of the earthquakes, they can cause buildings to collapse, which sometimes results in fatalities. The Richter Magnitude Scale is used to classify and measure the magnitude of earthquakes. The Richter Scale was invented by Charles F. Richter in 1935. The Richter Magnitude Scale is also known as the Local Magnitude Scale. This scale measures the amount of seismic activity from the earthquake. The least intense earthquake would be rated 1.0 on the Richter Scale. The most intense earthquake would be a higher number, such as a 9.0. Most earthquakes range between 3.0 and 4.0 on the Richter Scale. An example of a devastating earthquake such as the one that struck Haiti in 2010, which registered 7.0 on the Richter scale. Earthquakes are very common in the western United States, west coast of South America, southern Europe, and east Asia. Many earthquakes occur in these locations because they are located on fault lines or the boundary lines of the Earth’s tectonic plates. Many of these sites are included in an area of the Earth known as the Ring of Fire. It is called the Ring of Fire because there are many earthquakes and volcanoes within this area. One of the world’s largest earthquakes took place in Valdivia, Chile, in 1960 and measured 9.5 in magnitude. The world’s second largest earthquake was in Prince William Sound in the United States. This earthquake occurred in 1964 and measured 9.2 in magnitude. In 2004, there was an earthquake in Sumatra, Indonesia. The earthquake in Indonesia measured 9.1 in magnitude and caused one of the world’s worst tsunamis which killed over 250,000 people.

Essay on the main causes of earthquake
RANJITA ESSAY IN ENGLISH LANGUAGE

Earthquake is a vibration or oscillation of the earth's surface. Just as waves are generated on the water surface of a pond when a stone strikes it, similarly, the earth's crust which has considerable elasticity is set into tremors by a sudden blow from internal or external sources. At times, the shocks are highly disastrous to human life and property. General destruction takes place in a few seconds; Earthquakes may be natural or artificial.

Causes of Earthquakes: The causes of earthquakes may be divided into three groups (i) surface causes, (ii) volcanic causes and (iii) tectonic causes. (i) Surface causes: Great explosions, landslides, slips on steep coasts, dashing of sea waves, avalanches, railway trains, heavy trucks, some large engineering projects cause minor tremors. Some of them are man made, others are natural. (ii) Volcanic causes: Vocanic eruptions produce earthquakes. Earthquakes may precede, accompany and frequently follow volcanic eruptions. They are caused by sudden violent displacments of lava within or beneath the conduit of the valcano. (iii) Tectonic causes: Structural disturbances resulting in the relative displacements of the parts of the lithosphere is the main cause of this type of earthquake. Most of the disastrous earthquakes belong to this category and occur in areas of great faults and fractures. Sudden yielding to strain produced on the rocks of accumulating stress causes displacements especially along old fault zones known as great transform faults. Reid proposed the idea of elastic rebound hypothesis. Stresses accumulate on the two sides of the fault plane and produce strain. The rock deforms bends and when the stress crosses the elastic limit, sudden displacement of the two sides of the fault plane takes place. This results in a strong blow to the rocks Elastic rebound and produces tremors. Earthquakes often occur on the ocean floor. This produces large sea waves known as tsunami that produces devastating effects on the sea coasts. Recently, the tsunami produced by the earthquake near the Sumatra coast affected far of places like Srilanka and South India and even African coast. Tectonic earthquakes are classified according to their depth of origin into: (i) Normal - When the depth of origin is less than 50 km. (ii) Intermediate- When the depth is in between 50 to 400 km and (iii) Deep seated - When the depth is more than 400 km.

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