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3 Meterological disasterso Blizzardso Cyclonic stormso Droughtso Hailstormso Heat waveso

Tornadoes 4 Fires

5 Health disasterso Epidemicso Famines

6 Space disasterso Impact eventso Solar flares

1 Geological disasters

Earthquakes

An earthquake is a sudden shake of the Earth's crust caused by the tectonic plates colliding.Thevibrations may vary in magnitude. The underground point of origin of the earthquake is called the "focus".The point directly above the focus on the surface is called the"epicenter". Earthquakes by themselvesrarely kill people or wildlife. It is usually the secondary events that they trigger, such as building collapse,fires, tsunamis (seismic sea waves) and volcanoes, that are actually the human disaster. Many of thesecould possibly be avoided by better construction, safety systems, early warning and evacuationplanning.Earthquakes are caused by the discharge of energy accumulated along geologic fault.

An earthquake (also known as a quake, tremorortemblor) is the result of a sudden release of energy

in the Earth's crust that creates seismic waves. The seismicity orseismic activity of an area refers tothe frequency, type and size of earthquakes experienced over a period of time. Earthquakes aremeasured with a seismometer; a device which also records is known as a seismograph. The momentmagnitude (or the related and mostly obsolete Richtermagnitude) of an earthquake is conventionallyreported, with magnitude 3 or lower earthquakes being mostly imperceptible and magnitude 7 causingserious damage over large areas. Intensity of shaking is measured on the modified Mercalli scale. Thedepth of the earthquake also matters: the more shallow the earthquake, the more damage to structures(all else being equal).

[1]At the Earth's surface, earthquakes manifest themselves by shaking and

sometimes displacing the ground. When a large earthquake epicenteris located offshore, the seabedsometimes suffers sufficient displacement to cause a tsunami. The shaking in earthquakes can alsotrigger landslides and occasionally volcanic activity.

In its most generic sense, the word earthquake is used to describe any seismic eventwhether anatural phenomenon or an event caused by humansthat generates seismic waves. Earthquakes arecaused mostly by rupture of geological faults, but also by volcanic activity, landslides, mine blasts,and nuclear tests. An earthquake's point of initial rupture is called its focus orhypocenter. Theterm epicenterrefers to the point at ground level directly above the hypocenter. Earthquake is one of themost destructivenatural hazard. They may occur at any time of the year, day or night, with sudden impact and littlewarning. They can destroy buildings and infrastructure in seconds, killing or injuring the inhabitants.Earthquakes not only destroy the entire habitation but may de-stabilize the government, economy andsocial structure of the country. But what is an earthquake? It is the sudden shaking of the earth crust.The impact of an earthquake is sudden and there is hardly any warning,making it impossible to predict.

http://en.wikipedia.org/wiki/Natural_disaster#Meterological_disastershttp://en.wikipedia.org/wiki/Natural_disaster#Blizzardshttp://en.wikipedia.org/wiki/Natural_disaster#Cyclonic_stormshttp://en.wikipedia.org/wiki/Natural_disaster#Droughtshttp://en.wikipedia.org/wiki/Natural_disaster#Hailstormshttp://en.wikipedia.org/wiki/Natural_disaster#Heat_waveshttp://en.wikipedia.org/wiki/Natural_disaster#Tornadoeshttp://en.wikipedia.org/wiki/Natural_disaster#Fireshttp://en.wikipedia.org/wiki/Natural_disaster#Health_disastershttp://en.wikipedia.org/wiki/Natural_disaster#Epidemicshttp://en.wikipedia.org/wiki/Natural_disaster#Famineshttp://en.wikipedia.org/wiki/Natural_disaster#Space_disastershttp://en.wikipedia.org/wiki/Natural_disaster#Impact_eventshttp://en.wikipedia.org/wiki/Natural_disaster#Solar_flareshttp://en.wikipedia.org/wiki/Natural_disaster#Geological_disastershttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Tsunamihttp://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Crust_(geology)http://en.wikipedia.org/wiki/Seismic_wavehttp://en.wikipedia.org/wiki/Seismometerhttp://en.wikipedia.org/wiki/Moment_magnitude_scalehttp://en.wikipedia.org/wiki/Moment_magnitude_scalehttp://en.wikipedia.org/wiki/Richter_magnitude_scalehttp://en.wiktionary.org/wiki/imperceptiblehttp://en.wikipedia.org/wiki/Mercalli_intensity_scalehttp://en.wikipedia.org/wiki/Earthquake#cite_note-0http://en.wikipedia.org/wiki/Earthquake#cite_note-0http://en.wikipedia.org/wiki/Earthquake#cite_note-0http://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Tsunamihttp://en.wikipedia.org/wiki/Phenomenonhttp://en.wikipedia.org/wiki/Fault_(geology)http://en.wikipedia.org/wiki/Underground_nuclear_testinghttp://en.wikipedia.org/wiki/Focus_(earthquake)http://en.wikipedia.org/wiki/Hypocenterhttp://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Hypocenterhttp://en.wikipedia.org/wiki/Focus_(earthquake)http://en.wikipedia.org/wiki/Underground_nuclear_testinghttp://en.wikipedia.org/wiki/Fault_(geology)http://en.wikipedia.org/wiki/Phenomenonhttp://en.wikipedia.org/wiki/Tsunamihttp://en.wikipedia.org/wiki/Epicenterhttp://en.wikipedia.org/wiki/Earthquake#cite_note-0http://en.wikipedia.org/wiki/Mercalli_intensity_scalehttp://en.wiktionary.org/wiki/imperceptiblehttp://en.wikipedia.org/wiki/Richter_magnitude_scalehttp://en.wikipedia.org/wiki/Moment_magnitude_scalehttp://en.wikipedia.org/wiki/Moment_magnitude_scalehttp://en.wikipedia.org/wiki/Seismometerhttp://en.wikipedia.org/wiki/Seismic_wavehttp://en.wikipedia.org/wiki/Crust_(geology)http://en.wikipedia.org/wiki/Earthhttp://en.wikipedia.org/wiki/Tsunamihttp://en.wikipedia.org/wiki/Earthquakehttp://en.wikipedia.org/wiki/Natural_disaster#Geological_disastershttp://en.wikipedia.org/wiki/Natural_disaster#Solar_flareshttp://en.wikipedia.org/wiki/Natural_disaster#Impact_eventshttp://en.wikipedia.org/wiki/Natural_disaster#Space_disastershttp://en.wikipedia.org/wiki/Natural_disaster#Famineshttp://en.wikipedia.org/wiki/Natural_disaster#Epidemicshttp://en.wikipedia.org/wiki/Natural_disaster#Health_disastershttp://en.wikipedia.org/wiki/Natural_disaster#Fireshttp://en.wikipedia.org/wiki/Natural_disaster#Tornadoeshttp://en.wikipedia.org/wiki/Natural_disaster#Heat_waveshttp://en.wikipedia.org/wiki/Natural_disaster#Hailstormshttp://en.wikipedia.org/wiki/Natural_disaster#Droughtshttp://en.wikipedia.org/wiki/Natural_disaster#Cyclonic_stormshttp://en.wikipedia.org/wiki/Natural_disaster#Blizzardshttp://en.wikipedia.org/wiki/Natural_disaster#Meterological_disasters

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Cause of Earthquake :The earths crust is a rocky layer of varying thickness ranging from a depth of about 10 kilometers underthe sea to 65 kilometersunder the continents. The crust is not onepiece but consists of portions called plateswhich vary in size from a few hundred to thousands of kilometers (Fig 2.1.1). Thetheory of plate tectonics holds that theplates ride up on the more mobile mantle,and are driven by some yet unconfirmed mechanisms, perhaps

thermal convection currents. When these plates contact eachother, stress arises in the crust.These stresses can be classified accordingto the type of movement along the plates boundaries:a) pulling away from each other,b) pushing against one another andc) sliding sideways relative to each other.All these movements are associated with earthquakes.The areas of stress at plate boundaries whichrelease accumulated energy by slipping or rupturing are known as 'faults'.The theory of 'elasticity' saysthat the crust is continuously stressed by the movement of the tectonic plates; it eventually reaches apoint of maximum supportable strain. A rupture then occurs along the fault and the rock rebounds underits own elastic stresses until the strain is relieved. The fault rupture generates vibration called seismic

(from the Greek 'seismos' meaning shock orearthquake) waves, which radiates from thefocus in all directions

Naturally occurring earthquakes

Tectonic earthquakes will occur anywhere within the earth where there is sufficient stored elastic strainenergy to drive fracture propagation along a fault plane. In the case oftransform orconvergent type plateboundaries, which form the largest fault surfaces on earth, they will move past each other smoothlyand aseismically only if there are no irregularities orasperities along the boundary that increase thefrictional resistance. Most boundaries do have such asperities and this leads to a form ofstick-slipbehaviour. Once the boundary has locked, continued relative motion between the plates leads toincreasing stress and therefore, stored strain energy in the volume around the fault surface. Thiscontinues until the stress has risen sufficiently to break through the asperity, suddenly allowing slidingover the locked portion of the fault, releasing the stored energy. This energy is released as a combinationof 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 only10 percent or less of an earthquake's total energy is radiated as seismic energy. Most of the earthquake'senergy 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 theEarth's deep interior.

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Earthquake fault types

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 thedirection ofdip and movement on them involves a vertical component. Normal faults occur mainly inareas where the crust is being extended such as a divergent boundary. Reverse faults occur in areaswhere the crust is being shortened such as at a convergent boundary. Strike-slip faults are steepstructures where the two sides of the fault slip horizontally past each other ; transform boundaries are aparticular 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.

Aftershocks

An aftershock is an earthquake that occurs after a previous earthquake, the mainshock. An aftershock isin the same region of the main shock but always of a smaller magnitude. If an aftershock is larger thanthe main shock, the aftershock is redesignated as the main shock and the original main shock isredesignated as a foreshock. Aftershocks are formed as the crust around the displacedfault plane adjuststo the effects of the main shock.

[11]

Earthquake sw arms

Sometimes a series of earthquakes occur in a sort of earthquake storm, where the earthquakes strike afault in clusters, each triggered by the shaking or stress redistribution of the previous earthquakes. Similarto 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 thesequence of about a dozen earthquakes that struck the North Anatolian Fault in Turkey in the 20thcentury and has been inferred for older anomalous clusters of large earthquakes in the Middle East.

There are around 500,000 earthquakes each year. About 100,000 of these can actually be felt. Minorearthquakes occur nearly constantly around the world in places like California and Alaska in the U.S., aswell as in Guatemala. Chile, Peru, Indonesia, Iran, Pakistan, the Azores in Portugal, Turkey, NewZealand, Greece, Italy, and Japan, but earthquakes can occur almost anywhere, includingNew YorkCity, London, and Australia. 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

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period than earthquakes larger than magnitude 5. In the (low seismicity) United Kingdom, for example, ithas been calculated that the average recurrences are: an earthquake of 3.7 - 4.6 every year, anearthquake of 4.7 - 5.5 every 10 years, and an earthquake of 5.6 or larger every 100 years .

[19]This is an

example of the Gutenberg-Richter law.

The number of seismic stations has increased from about 350 in 1931 to many thousands today. As aresult, 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 USGS estimates that,since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one greatearthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.

[21]In

recent years, the number of major earthquakes per year has decreased, although this is thought likely tobe a statistical fluctuation rather than a systematic trend. More detailed statistics on the size andfrequency 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, known as the Pacific Ring of Fire, whichfor the most part bounds the Pacific Plate.

[23][24]Massive earthquakes tend to occur along other plate

boundaries, too, such as along the Himalayan Mountains.

With the rapid growth ofmega-cities such as Mexico City, Tokyo and Tehran, in areas of high seismicrisk, some seismologists are warning that a single quake may claim the lives of up to 3 million people.

Induced seismicity

While most earthquakes are caused by movement of the Earth's tectonic plates, human activity can alsoproduce earthquakes. Four main activities contribute to this phenomenon: constructinglargedams and buildings, drilling and injecting liquid into wells, and by coal mining and oil drilling. Perhapsthe best known example is the 2008 Sichuan earthquake in China's Sichuan Province in May; this tremorresulted in 69,227 fatalities and is the 19th deadliest earthquake of all time. The Zipingpu Dam is believedto have fluctuated the pressure of the fault 1,650 feet (503 m) away; this pressure probably increased thepower of the earthquake and accelerated the rate of movement for the fault.

[27]The greatest earthquake in

Australia's history was also induced by humanity, through coal mining.The city of Newcastle was builtover a large sector of coal mining areas. The earthquake was spawned from a fault which reactivated dueto the millions of tonnes of rock removed in the mining process.

Measuring and locating earthquakes

Earthquakes can be recorded by seismometers up to great distances, because seismic waves travelthrough the whole Earth's interior. The absolute magnitude of a quake is conventionally reported bynumbers on the Moment magnitude scale (formerly Richter scale, magnitude 7 causing serious damageover large areas), whereas the felt magnitude is reported using the modified Mercalli intensity scale

Every tremor produces different types of seismic waves which travel through rock with different velocities:the longitudinal P-waves (shock- or pressure waves), the transverse S-waves (both body waves) andseveral surface waves (Rayleigh and Love waves). The propagation velocity of the seismic waves rangesfrom approx. 3 km/s up to 13 km/s, depending on the density and elasticity of the medium. In the Earth'sinterior the shock- or P waves travel much faster than the S waves (approx. relation 1.7 : 1). Thedifferences in travel time from the epicentre to the observatory are a measure of the distance and can beused to image both sources of quakes and structures within the Earth. Also the depth ofthe hypocentercan be computed roughly.

In solid rock P-waves travel at about 6 to 7 km per second; the velocity increases within the deep mantleto ~13 km/s. The velocity of S-waves ranges from 23 km/s in light sediments and 45 km/s in the Earth'scrust up to 7 km/s in the deep mantle. As a consequence, the first waves of a distant earth quake arrive atan observatory via the Earth's mantle.

Rule of thumb: On the average, the kilometer distance to the earthquake is the number of secondsbetween the P and S wave times 8.Slight deviations are caused by inhomogeneities of subsurfacestructure. By such analyses of seismograms the Earth's core was located in 1913 by Beno Gutenberg.

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Earthquakes are not only categorized by their magnitude but also by the place where they occur. Theworld is divided into 754 Flinn-Engdahl regions (F-E regions), which are based on political andgeographical boundaries as well as seismic activity. More active zones are divided into smaller F-Eregions whereas less active zones belong to larger F-E regions.

Effects/impacts of earthquakes

The effects of earthquakes include, but are not limited to, the following:

1 Shaking and ground rupture

2 Landslides and avalanches

3 Fires

4 Soil liquefaction

5 Tsunami

6 Floods

7 Tidal forces

8 Human impacts

Shaking and ground rupture

Shaking and ground rupture are the main effects created by earthquakes, principally resulting in more orless severe damage to buildings and other rigid structures. The severity of the local effects depends onthe complex combination of the earthquake magnitude, the distance from the epicenter, and the localgeological and geomorphological conditions, which may amplify or reduce wave propagation.

[30]The

ground-shaking is measured by ground acceleration.

Specific local geological, geomorphological, and geostructural features can induce high levels of shakingon 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 toeffects 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 majorrisk for large engineering structures such as dams, bridges and nuclear power stations and requirescareful mapping of existing faults to identify any likely to break the ground surface within the life of thestructure.

Landslides and avalanches

Earthquakes, along with severe storms, volcanic activity, coastal wave attack, and wildfires, can produceslope instability leading to landslides, a major geological hazard. Landslide danger may persist whileemergency personnel are attempting rescue.

Fires

Earthquakes can cause fires by damaging electrical poweror gas lines. In the event of water mainsrupturing 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 theearthquake itself.

Soil liquefaction

Soil liquefaction occurs when, because of the shaking, water-saturated granularmaterial (such as sand)temporarily loses its strength and transforms from asolid to a liquid. Soil liquefaction may cause rigidstructures, like buildings and bridges, to tilt or sink into the liquefied deposits. This can be a devastatingeffect of earthquakes. For example, in the 1964 Alaska earthquake, soil liquefaction caused manybuildings to sink into the ground, eventually collapsing upon themselves.

[34]

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Tsunami

Tsunamis are long-wavelength, long-period sea waves produced by the sudden or abrupt movement oflarge volumes of water. In the open ocean the distance between wave crests can surpass 100 kilometers(62 miles), and the wave periods can vary from five minutes to one hour. Such tsunamis travel 600-800 kilometers per hour (373497 miles per hour), depending on water depth. Large waves produced byan 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 farshores hours after the earthquake that generated them.

[35]

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 byearthquakes of magnitude 7.5 or more.

[35]

Floods

A flood is an overflow of any amount of water that reaches land.[36]

Floods occur usually when the volumeof water within a body of water, such as a river or lake, exceeds the total capacity of the formation, and asa result some of the water flows or sits outside of the normal perimeter of the body. However, floods maybe secondary effects of earthquakes, if dams are damaged. Earthquakes may cause landslips to damrivers, which then collapse and cause floods.

[37]

The terrain below the Sarez Lake in Tajikistan is in danger of catastrophic flood if the landslidedam formed by the earthquake, known as the Usoi Dam, were to fail during a future earthquake. Impactprojections suggest the flood could affect roughly 5 million people.

[38]

Tidal forces

Research work has shown a robust correlation between small tidally induced forces and non-volcanictremor activity.

Human impacts

Earthquakes may lead to disease, lack of basic necessities, loss of life, higher insurance premiums,general property damage, road and bridge damage, and collapse or destabilization (potentially leading tofuture collapse) of buildings. Earthquakes can also precede volcanic eruptions, which cause furtherproblems; for example, substantial crop damage, as in the "Year Without a Summer" (1816).

Major earthquakes

The largest earthquake that has been measured on a seismograph reached 9.5 magnitude, occurring on22 May 1960 Its epicenter was near Caete, Chile. The energy released was approximately twice that ofthe next most powerful earthquake, the Good Friday Earthquake, which was centered in Prince WilliamSound, Alaska. The ten largest recorded earthquakes have all been megathrust earthquakes; however, ofthese ten, only the 2004 Indian Ocean earthquake is simultaneously one of the deadliest earthquakes inhistory.

The earthquakes with the greatest amount of loss of life, while powerful, were deadly because of their

proximity to either heavily populated areas or the ocean, where earthquakes can potentially createtsunamis which can devastate communities thousands of miles away. Regions that are most at risk forgreat loss of life include those where earthquakes are relatively rare but powerful, and poor regions withlax, unenforced, or nonexistent seismic building codes.

Preparation

In order to determine the likelihood of future seismic activity, geologists and other scientists examine therock of an area to determine if the rock appears to be "strained". Studying the faults of an area to studythe buildup time it takes for the fault to build up stress sufficient for an earthquake also serves as an

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effective prediction technique.[46]

Measurements of the amount of accumulated strain energy on the faulteach year, time passed since the last major temblor, and the energy and power of the last earthquake aremade.

[46]Together the facts allow scientists to determine how much pressure it takes for the fault to

generate an earthquake. Though this method is useful, it has only been implemented on California's SanAndreas Fault.

[46]

Today, there are ways to protect and prepare possible sites of earthquakes from severe damage, through

the following processes: earthquake engineering, earthquake preparedness, household seismicsafety, seismic retrofit (including special fasteners, materials, and techniques), seismic hazard, mitigationof seismic motion, and earthquake prediction. Seismic retrofitting is the modification ofexisting structures to make them more resistant to seismic activity, ground motion, orsoil failure due toearthquakes. With better understanding of seismic demand on structures and with our recent experienceswith large earthquakes near urban centers, the need of seismic retrofitting is well acknowledged. Prior tothe introduction ofmodern seismic codes in the late 1960s for developed countries (US, Japan etc.) andlate 1970s for many other parts of the world (Turkey, China etc.), many structures were designed withoutadequate detailing and reinforcement for seismic protection. In view of the imminent problem, variousresearch work has been carried out. Furthermore, state-of-the-art technical guidelines for seismicassessment, retrofit and rehabilitation have been published around the world - such as the ASCE-SEI41 and the New Zealand Society for Earthquake Engineering (NZSEE)'s guidelines.

Volcanic eruptions

Volcanoes can cause widespread destruction and consequent disaster through several ways.The effects include the volcanic eruption itself that may cause harm following the explosion of the volcanoor the fall of rock. Second, lava may be produced during the eruption of a volcano. As it leaves thevolcano the lava destroys any buildings and plants it encounters. Third, volcanic ash generally meaningthe cooled ash - may form a cloud, and settle thickly in nearby locations. When mixed with water thisforms a concrete-like material. In sufficient quantity ash may cause roofs to collapse under its weight buteven small quantities will harm humans if inhaled. Since the ash has the consistency of ground glass it

causes abrasion damage to moving parts such as engines. The main killer of humans in the immediatesurrounding of an volcanic eruption is the pyroclastic flows, which consist of a cloud of hot volcanic ashwhich builds up in the air above the volcano and rushes down the slopes when the eruption no longersupports the lifting of the gases. It is believed that Pompeii was destroyed by a pyroclastic flow. A laharisa volcanic mudflow or landslide. A specific type of volcano is the supervolcano. According to the Tobacatastrophe theory 70 to 75 thousand years ago a super volcanic event at Lake Toba reduced the humanpopulation to 10,000 or even 1,000 breeding pairs creating a bottleneck in human evolution. It also killedthree quarters of all plant life in the northern hemisphere. The main danger from a supervolcano is theimmense cloud of ash which has a disastrous global effect on climate and temperature for many years.

During a volcanic eruption, lava, tephra (ash, lapilli, volcanic bombs and blocks), and various gases areexpelled from a volcanic vent orfissure. Several types of volcanic eruptions have been distinguishedby volcanologists. These are often named after famous volcanoes where that type of behavior has beenobserved. Some volcanoes may exhibit only one characteristic type of eruption during a period of activity,while others may display an entire sequence of types all in one eruptive series.

There are three different metatypes of eruptions. The most well-observed are magmatic eruptions, whichinvolve the decompression of gas within magma that propels it forward. Phreatomagmatic eruptions areanother type of volcanic eruption, driven by the compression of gas within magma, the direct opposite ofthe process powering magmatic activity. The last eruptive metatype is the Phreatic eruption, which isdriven by the superheating ofsteam via contact withmagma; these eruptive types often exhibit nomagmatic release, instead causing the granulation of existing rock.

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Within these wide-defining eruptive types are several subtypes. The weakestare Hawaiian and submarine, then Strombolian, followed by Vulcanian andSurtseyan. The strongereruptive types are Pelean eruptions, followed by Plinian eruptions; the strongest eruptions are called"Ultra Plinian." Subglacial andPhreatic eruptions are defined by their eruptive mechanism, and vary instrength. An important measure of erruptive strength is Volcanic Explosivity Index(VEI),a magnitudic scale ranging from 0 to 8 that often correlates to eruptive types.

Eruption mechanisms

Volcanic eruptions arise through three main mechanisms:

Gas release under decompression causing magmatic eruptions.

Thermal contraction from chilling on contact with water causing phreatomagmatic eruptions.

Ejection of entrained particles during steam eruptions causing phreatic eruptions.[1]

There are two types of eruptions in terms of activity, explosive eruptions and effusive eruptions. Explosiveeruptions are characterized by gas-driven explosions that propels magma and tephra.

[1]Effusive

eruptions, meanwhile, are characterized by the outpouring oflava without significant explosive eruption.[2]

Volcanic eruptions vary widely in strength. On the one extreme there are effusive Hawaiian eruptions,which are characterized by lava fountains and fluid lava flows, which are typically not very dangerous. Onthe other extreme, Plinian eruptions are large, violent, and highly dangerous explosive events. Volcanoesare not bound to one eruptive style, and frequently display many different types, both passive andexplosive, even the span of a single eruptive cycle.

[3]Volcanoes do not always erupt vertically from a

single crater near their peak, either. Some volcanoes exhibit lateral and fissure eruptions. Notably,many Hawaiian eruptions start from rift zones,

[4]and some of the strongest Surtseyan eruptions develop

along fracture zones.[5]

2 Hydrological disasters

1 Limnic eruptionsA limnic eruption occurs when a gas, usually CO2suddenly erupts from deep lake water, posing the threatof suffocating wildlife, livestock and humans. Such an eruption may also cause tsunamis in the lake asthe rising gas displaces water. Scientists believe landslides, volcanic activity, or explosions can triggersuch an eruption. To date, only two limnic eruptions have been observed and recorded

2 Floods

A flood is an overflow of an expanse of water that submerges land.[1]

The EU Floods directive defines aflood as a temporary covering by water of land not normally covered by water.

[2]In the sense of "flowing

water", the word may also be applied to the inflow of the tide. Flooding may result from the volume of

water within a body of water, such as a riverorlake, which overflows or breaks levees, with the result thatsome of the water escapes its usual boundaries.

[3]

While the size of a lake or other body of water will vary with seasonal changes in precipitation and snowmelt, it is not a significant flood unless such escapes of water endanger land areas used by man like avillage, city or other inhabited area.

Floods can also occur in rivers, when flow exceeds the capacity of the river channel, particularly at bendsor meanders. Floods often cause damage to homes and businesses if they are placed in natural floodplains of rivers. While flood damage can be virtually eliminated by moving away from rivers and other

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bodies of water, since time out of mind, people have lived and worked by the water to seek sustenanceand capitalize on the gains of cheap and easy travel and commerce by being near water. That humanscontinue to inhabit areas threatened by flood damage is evidence that the perceived value of living nearthe water exceeds the cost of repeated periodic flooding.

The word "flood" comes from the Old English flod, a word common to Germanic languages (compareGerman Flut, Dutch vloedfrom the same root as is seen in flow, float; also compare with

Latin fluctus, flumen). Deluge myths are mythical stories of a great flood sent by a deity or deities todestroy civilization as an act ofdivine retribution, and are featured in the mythology of many cultures.

Principal types and causes

1 River ine

Slow kinds: Runoff from sustained rainfall or rapid snow melt exceeding the capacity of a river'schannel. Causes include heavy rains from monsoons, hurricanes and tropical depressions, foreignwinds and warm rain affecting snow pack. Unexpected drainage obstructions such as landslides, ice,ordebriscan cause slow flooding upstream of the obstruction.

Fast kinds: include flash floods resulting from convective precipitation (intense thunderstorms) orsudden release from an upstream impoundment created behind a dam, landslide, orglacier.

2 Estuar ine

Commonly caused by a combination of sea tidal surges caused by storm-force winds. A storm surge,from either a tropical cyclone or an extratropical cyclone, falls within this category.

3 Coastal

Caused by severe sea storms, or as a result of another hazard (e.g. tsunami or hurricane). A stormsurge, from either a tropical cyclone or an extratropical cyclone, falls within this category.

4 Catastrophic

Caused by a significant and unexpected event e.g. dam breakage, or as a result of another hazard(e.g. earthquake or volcanic eruption).

5Muddy

A muddy flood is generated by run off on crop land.

A muddy flood is produced by an accumulation of runoff generated on cropland. Sediments are thendetached by runoff and carried as suspended matter or bed load. Muddy runoff is more likely detectedwhen it reaches inhabited areas.

Muddy floods are therefore a hill slope process, and confusion with mudflows produced by massmovements should be avoided.

6 Other

Floods can occur if water accumulates across an impermeable surface (e.g. from rainfall) and cannot

rapidly dissipate (i.e. gentle orientation or low evaporation). A series of storms moving over the same area.

Dam-building beavers can flood low-lying urban and rural areas, often causing significant damage.

Effects

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1 Primary effects

Physical damage - Can damage any type of structure, including bridges, cars,buildings, sewerage systems, roadways, and canals.

Casualties - People and livestock die due to drowning. It can also lead to epidemics and waterbornediseases.

2Secondary effects

Water supplies - Contamination of water. Clean drinking waterbecomes scarce.

Diseases - Unhygienic conditions. Spread ofwater-borne diseases.

Crops and food supplies - Shortage of food crops can be caused due to loss of entireharvest.

[4]However, lowlands near rivers depend upon river silt deposited by floods in order to add

nutrients to the local soil.

Trees - Non-tolerant species can die from suffocation.[5]

3 Tertiary/long -term effects

Economic - Economic hardship, due to: temporary decline in tourism, rebuilding costs, food shortageleading to price increase, etc.

ControlIn many countries across the world, rivers prone to floods are often carefully managed. Defensessuch as levees,

[6]bunds, reservoirs, and weirs are used to prevent rivers from bursting their banks.

When these defenses fail, emergency measures such as sandbags or portable inflatable tubes areused. Coastal flooding has been addressed in Europe and the Americas with coastal defences, suchas sea walls, beach nourishment, and barrier islands.

Benefits

There are many disruptive effects of flooding on human settlements and economic activities. However,floods (in particular the more frequent/smaller floods) can bring many benefits, such as recharging groundwater, making soil more fertile and providing nutrients in which it is deficient. Flood waters provide muchneeded water resources in particular in arid and semi-arid regions where precipitation events can be veryunevenly distributed throughout the year. Freshwater floods in particular play an important role inmaintaining ecosystems in river corridors and are a key factor in maintaining floodplainbiodiversity.

[14]Flooding adds a lot of nutrients to lakes and rivers which leads to improved fisheries for a

few years, also because of the suitability of a floodplain for spawning (little predation and a lot ofnutrients).

[15]Fish like the weather fish make use of floods to reach new habitats. Together with fish also

birds profit from the boost in production caused by flooding.[16]

Periodic flooding was essential to the well-being of ancient communities along the Tigris-Euphrates Rivers, the Nile River, the Indus River, the Ganges and theYellow River, among others. The

viability for hydrological based renewable sources of energy is higher in flood prone regions.

Computer modelling

While flood modelling is a fairly recent practice, attempts to understand and manage the mechanisms atwork in floodplains have been made for at least six millennia.

[17]The recent development in computational

flood modelling has enabled engineers to step away from the tried and tested "hold or break" approachand its tendency to promote overly engineered structures. Various computational flood models have beendeveloped in recent years either 1D models (flood levels measured in the channel) and 2D models (flooddepth measured for the extent of the floodplain). HEC-RAS,

[18]the Hydraulic Engineering Centre model, is

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currently among the most popular if only because it is available for free. Other models such asTUFLOW

[19]combine 1D and 2D components to derive flood depth in the floodplain. So far the focus has

been on mapping tidal and fluvial flood events but the 2007 flood events in the UK have shifted theemphasis onto the impact of surface water flooding

Tsunamis

A tsunami wave ortidal wave is a series of water waves (called a tsunami wave train) caused by thedisplacement of a large volume of a body of water, usually an ocean, but can occur in large lakes.Tsunamis are a frequent occurrence in Japan; approximately 195 events have been recorded. Due to theimmense volumes of water and energy involved, tsunamis can devastate coastal regions.

Earthquakes, volcanic eruptions and otherunderwater explosions (including detonations ofunderwaternuclear devices), landslides and othermass movements, meteorite ocean impacts or similarimpact events, and other disturbances above or below water all have the potential to generate a tsunami.

The Greek historian Thucydides was the first to relate tsunami to submarine earthquakes, butunderstanding of tsunami's nature remained slim until the 20th century and is the subject of ongoingresearch. Many early geological, geographical, and oceanographic texts refer to tsunamis as "seismic

sea waves."

Some meteorological conditions, such as deep depressions that cause tropical cyclones, can generatea storm surge, called a meteotsunami, which can raise tides several metres above normal levels. Thedisplacement comes from low atmospheric pressure within the centre of the depression. As these stormsurges reach shore, they may resemble (though are not) tsunamis, inundating vast areas of land. Such astorm surge inundated Burma in May 2008.

Etymology

The term tsunamicomes from the Japanese, meaning "harbor" and "wave". (For the plural, one caneither follow ordinary English practice and add an s, or use an invariable plural as in the Japanese.)

Tsunami are sometimes referred to as tidal waves. In recent years, this term has fallen out of favor,

especially in the scientific community, because tsunami actually have nothing to do with tides. The once-popular term derives from their most common appearance, which is that of an extraordinarily high tidalbore. Tsunami and tides both produce waves of water that move inland, but in the case of tsunami theinland movement of water is much greater and lasts for a longer period, giving the impression of anincredibly high tide. Although the meanings of "tidal" include "resembling" or "having the form or characterof"the tides, and the term tsunamiis no more accurate because tsunami are not limited to harbours, useof the term tidal wave is discouraged by geologists andoceanographers.

There are only a few other languages that have a native word for this disastrous wave. In the Tamillanguage, the word is aazhi peralai. In the Acehnese language, it is or (Depending on the dialect. Notethat in the fellow Austronesian language ofTagalog, a major language in thePhilippines,alon means"wave".) On Simeulue island, off the western coast of Sumatra in Indonesia, in the Defayan language theword is smong, while in theSigulai language it is emong.

Generation mechanismsThe principal generation mechanism (or cause) of a tsunami is the displacement of a substantial volumeof water or perturbation of the sea.

[11]This displacement of water is usually attributed to either

earthquakes, landslides, volcanic eruptions, or more rarely by meteorites and nuclear tests The wavesformed in this way are then sustained by gravity. It is important to note that tides do not play any part inthe generation of tsunamis, hence referring to tsunamis as 'tidal waves' is inaccurate.

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Seismicity generated tsunamis

Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlyingwater. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth'scrustal deformation; when these earthquakes occur beneath the sea, the water above the deformed areais displaced from its equilibrium position.

[14]More specifically, a tsunami can be generated when thrust

faults associated with convergent or destructive plate boundaries move abruptly, resulting in water

displacement, due to the vertical component of movement involved. Movement on normal faults will alsocause displacement of the seabed, but the size of the largest of such events is normally too small to giverise to a significant tsunami.

Tsunamis have a small amplitude (wave height) offshore, and a very long wavelength (often hundreds ofkilometers long), which is why they generally pass unnoticed at sea, forming only a slight swell usuallyabout 300 millimetres (12 in) above the normal sea surface. They grow in height when they reachshallower water, in a wave shoaling process described below. A tsunami can occur in any tidal state andeven at low tide can still inundate coastal areas.

On April 1, 1946, a magnitude-7.8 (Richter Scale) earthquake occurred near the Aleutian Islands, Alaska.It generated a tsunami which inundated Hilo on the island of Hawai'i with a 14 metres (46 ft) high surge.The area where the earthquake occurred is where the Pacific Ocean floor is subducting (or being pusheddownwards) underAlaska.

Examples of tsunami at locations away from convergent boundaries include Storegga about 8,000 yearsago, Grand Banks 1929, Papua New Guinea 1998 (Tappin, 2001). The Grand Banks and Papua NewGuinea tsunamis came from earthquakes which destabilized sediments, causing them to flow into theocean and generate a tsunami. They dissipated before traveling transoceanic distances.

The cause of the Storegga sediment failure is unknown. Possibilities include an overloading of thesediments, an earthquake or a release of gas hydrates (methane etc.)

The 1960 Valdivia earthquake (Mw9.5) (19:11 hrs UTC), 1964 Alaska earthquake (Mw 9.2), and 2004Indian Ocean earthquake (Mw 9.2) (00:58:53 UTC) are recent examples ofpowerful megathrustearthquakes that generated tsunamis (known as teletsunamis) that can cross entireoceans. Smaller (Mw 4.2) earthquakes in Japan can trigger tsunamis (called local and regionaltsunamis) that can only devastate nearby coasts, but can do so in only a few minutes.

In the 1950s, it was discovered that larger tsunamis than had previously been believed possible could becaused by giant landslides. These phenomena rapidly displace large water volumes, as energy fromfalling debris or expansion transfers to the water at a rate faster than the water can absorb. Theirexistence was confirmed in 1958, when a giant landslide in Lituya Bay, Alaska, caused the highest waveever recorded, which had a height of 524 metres (over 1700 feet). The wave didn't travel far, as it struckland almost immediately. Two people fishing in the bay were killed, but another boat amazingly managedto ride the wave. Scientists named these waves megatsunami.

Scientists discovered that extremely large landslides from volcanic island collapses cangenerate megatsunami, that can travel trans-oceanic distances.

Characteristics

While everyday wind waves have a wavelength (from crest to crest) of about 100 metres (330 ft) and aheight of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a wavelength of about 200 kilometres(120 mi). Such a wave travels at well over 800 kilometres per hour (500 mph), but due to the enormouswavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has anamplitude of only about 1 metre (3.3 ft).

[15]This makes tsunamis difficult to detect over deep water. Ships

rarely notice their passage.

As the tsunami approaches the coast and the waters become shallow, wave shoaling compresses thewave and its velocity slows below 80 kilometres per hour (50 mph). Its wavelength diminishes to less than20 kilometres (12 mi) and its amplitude grows enormously, producing a distinctly visible wave. Since the

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wave still has such a long wavelength, the tsunami may take minutes to reach full height. Except for thevery largest tsunamis, the approaching wave does not break (like a surf break), but rather appears like afast moving tidal bore.

[16]Open bays and coastlines adjacent to very deep water may shape the tsunami

further into a step-like wave with a steep-breaking front.

When the tsunami's wave peak reaches the shore, the resulting temporary rise in sea level is termed 'runup'. Run up is measured in metres above a reference sea level.

[16]A large tsunami may feature multiple

waves arriving over a period of hours, with significant time between the wave crests. The first wave toreach the shore may not have the highest run up.

[17]

About 80% of tsunamis occur in the Pacific Ocean, but are possible wherever there are large bodies ofwater, including lakes. They are caused by earthquakes, landslides, volcanic explosions, and bolides.

Drawback

If the first part of a tsunami to reach land is a troughcalled a drawbackrather than a wave crest, thewater along the shoreline recedes dramatically, exposing normally submerged areas.

A drawback occurs because the water propagates outwards with the trough of the wave at its front.Drawback begins before the wave arrives at an interval equal to half of the wave's period. Drawback canexceed hundreds of metres, and people unaware of the danger sometimes remain near the shore to

satisfy their curiosity or to collect fish from the exposed seabed. During the Indian Ocean tsunami, thesea withdrew and many people went onto the exposed sea bed to investigate.[citation needed]

Photos showpeople walking on the normally submerged areas with the advancing wave in the background.

[citation

needed]Few survived.

[citation needed]

Scales of intensity and magnitude

As with earthquakes, several attempts have been made to set up scales of tsunami intensity ormagnitude to allow comparison between different events.

[18]

Intensity scales

The first scales used routinely to measure the intensity of tsunami were the Sieberg-Ambraseys scale,used in the Mediterranean Sea and the Imamura-Iida intensity scale, used in the Pacific Ocean. The latterscale was modified by Soloviev, who calculated the Tsunami intensity Iaccording to the formula

where Hav is the average wave height along the nearest coast. This scale, known as the Soloviev-

Imamura tsunami intensity scale, is used in the global tsunami catalogues compiled bytheNGDC/NOAA and the Novosibirsk Tsunami Laboratory as the main parameter for the size of thetsunami.

Magnitude scales

The first scale that genuinely calculated a magnitude for a tsunami, rather than an intensity at aparticular location was the ML scale proposed by Murty & Loomis based on the potentialenergy.

[18]Difficulties in calculating the potential energy of the tsunami mean that this scale is rarely

used. Abe introduced the tsunami magnitude scale Mt, calculated from,

where h is the maximum tsunami-wave amplitude (in m) measured by a tide gauge at adistance Rfrom the epicenter, a, b & D are constants used to make the M t scale match as closelyas possible with the moment magnitude scale.

Warnings and predictions

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Drawbacks can serve as a brief warning. People who observe drawback (many survivors report anaccompanying sucking sound), can survive only if they immediately run for high ground or seek the upperfloors of nearby buildings. In 2004, ten-year old Tilly Smith ofSurrey, England, was on Maikhaobeach inPhuket, Thailand with her parents and sister, and having learned about tsunamis recently inschool, told her family that a tsunami might be imminent. Her parents warned others minutes before thewave arrived, saving dozens of lives. She credited her geography teacher, Andrew Kearney.

In the 2004 Indian Ocean tsunami drawback was not reported on the African coast or any other easterncoasts it reached. This was because the wave moved downwards on the eastern side of the fault line andupwards on the western side. The western pulse hit c