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Earthquake

An earthquake (also known as a quake, tremor or temblor) is the result of a

sudden release of energy in the Earth's crustthat creates seismic waves.

 The seismicity, seismism or seismic activity of an area refers to the frequency,

type and sie of earthquakes e!perienced over a period of time.

Earthquakes are measured using o"servations from seismometers. The moment

magnitude is the most common scale on which earthquakes larger than

appro!imately # are reported for the entire glo"e. The more numerous earthquakes

smaller than magnitude # reported "y national seismological o"servatories are

measured mostly on the local magnitude scale, also referred to as the $ichter

magnitude scale. These two scales are numerically similar over their range of

validity. %agnitude & or lower earthquakes are mostly almost impercepti"le or weak

and magnitude and over potentially cause serious damage over larger areas,

depending on their depth. The largest earthquakes in historic times have "een of

magnitude slightly over , although there is no limit to the possi"le magnitude. Themost recent large earthquake of magnitude . or larger was a. magnitude

earthquake in *apan in + (as of %arch +-), and it was the largest *apanese

earthquake since records "egan. ntensity of shaking is measured on the

modi/ed %ercalli scale. The shallower an earthquake, the more damage to

structures it causes, all else "eing equal.01

At the Earth's surface, earthquakes manifest themselves "y shaking and sometimes

displacement of the ground. 2hen theepicenter of a large earthquake is located

o3shore, the sea"ed may "e displaced su4ciently to cause a tsunami. Earthquakes

can also trigger landslides, and occasionally volcanic activity.

n its most general sense, the word earthquake is used to descri"e any seismic

event 5 whether natural or caused "y humans 5 that generates seismic waves.

Earthquakes are caused mostly "y rupture of geological faults, "ut also "y other

events such as volcanic activity, landslides, mine "lasts, and nuclear tests. An

earthquake's point of initial rupture is called itsfocus or hypocenter. The epicenter is

the point at ground level directly a"ove the hypocenter.

Contents

  0hide1

• 6aturally occurring earthquakes

o . Earthquake fault types

o .+ Earthquakes away from plate "oundaries

o .& 7hallow8focus and deep8focus earthquakes

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o .- Earthquakes and volcanic activity

o .# $upture dynamics

o .9 Tidal forces

o . Earthquake clusters

.. Aftershocks

..+ Earthquake swarms

..& Earthquake storms

• + 7ie and frequency of occurrence

• & nduced seismicity

• - %easuring and locating earthquakes

• # E3ects of earthquakes

o #. 7haking and ground rupture

o #.+ :andslides and avalanches

o #.& ;ires

o #.- 7oil liquefaction

o #.# Tsunami

o #.9 ;loods

o #. <uman impacts

• 9 %a=or earthquakes

• >rediction

• ? >reparedness

•

<istorical views

• Earthquakes in culture

o . %ythology and religion

o .+ n popular culture

• 7ee also

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• + $eferences

• & 7ources

• - E!ternal links

6aturally occurring earthquakes

;ault types

 Tectonic earthquakes occur anywhere in the earth where there is su4cient 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. %ost fault surfaces do have such asperities and this leads to a form

of stick8slip "ehaviour. @nce the fault has locked, continued relative motion

"etween 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

su4ciently to "reak through the asperity, suddenly allowing sliding over the locked

portion of the fault, releasing the stored energy.0+1 This energy is released as a

com"ination 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 "uild8up of strain and stress punctuated "y occasional sudden earthquake

failure is referred to as the elastic8re"ound theory. t is estimated that only

percent or less of an earthquake's total energy is radiated as seismic energy. %ost

of the earthquake's energy is used to power the earthquake fracture growth or is

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converted into heat generated "y friction. Therefore, earthquakes lower the Earth's

availa"le elastic potential energy and raise its temperature, though these changes

are negligi"le compared to the conductive and convective ow of heat out from

the Earth's deep interior.0&1

Earthquake fault types

Main article: Fault (geology)

 There are three main types of fault, all of which may cause an interplate

earthquakeB normal, reverse (thrust) and strike8slip. 6ormal and reverse faulting are

e!amples of dip8slip, where the displacement along the fault is in the direction

of dip and movement on them involves a vertical component. 6ormal faults occur

mainly in areas where the crust is "eing e!tended such as a divergent "oundary.

$everse faults occur in areas where the crust is "eing shortened such as at a

convergent "oundary. 7trike8slip faults are steep structures where the two sides of

the fault slip horiontally past each otherC transform "oundaries are a particular

type of strike8slip fault. %any earthquakes are caused "y movement on faults that

have components of "oth dip8slip and strike8slipC this is known as o"lique slip.

$everse faults, particularly those along convergent plate "oundaries are associated

with the most powerful earthquakes, megathrust earthquakes, including almost all

of those of magnitude ? or more. 7trike8slip faults, particularly

continental transforms, can produce ma=or earthquakes up to a"out magnitude ?.

Earthquakes associated with normal faults are generally less than magnitude . ;or

every unit increase in magnitude, there is a roughly thirtyfold increase in the energy

released. ;or instance, an earthquake of magnitude 9. releases appro!imately &

times more energy than a #. magnitude earthquake and a . magnitudeearthquake releases times (& D &) more energy than a #. magnitude of

earthquake. An ?.9 magnitude earthquake releases the same amount of energy as

, atomic "om"s that were used in 2orld 2ar .0-1

 This is so "ecause the energy released in an earthquake, and thus its magnitude, is

proportional to the area of the fault that ruptures 0#1 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, "rittle part of the Earth's crust, and the cool

sla"s 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. $ocks hotter than a"out & degrees elsius ow in response to stressCthey do not rupture in earthquakes.09101 The ma!imum o"served lengths of ruptures

and mapped faults (which may "reak in a single rupture) are appro!imately

km. E!amples are the earthquakes in hile, 9C Alaska, #C 7umatra,

+-, all in su"duction ones. The longest earthquake ruptures on strike8slip faults,

like the 7an Andreas ;ault (?#, 9), the 6orth Anatolian ;ault in Turkey (&)

and the Fenali ;ault in Alaska (++), are a"out half to one third as long as the

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lengths along su"ducting plate margins, and those along normal faults are even

shorter.

Aerial photo of the 7an Andreas ;ault in the arrio >lain, northwest of :os Angeles

 The most important parameter controlling the ma!imum earthquake magnitude on

a fault is however not the ma!imum availa"le length, "ut the availa"le width

"ecause the latter varies "y a factor of +. Along converging plate margins, the dip

angle of the rupture plane is very shallow, typically a"out degrees. 0?1 Thus thewidth of the plane within the top "rittle crust of the Earth can "ecome # to km

( *apan, +C Alaska, 9-), making the most powerful earthquakes possi"le.

7trike8slip faults tend to "e oriented near vertically, resulting in an appro!imate

width of km within the "rittle crust,01 thus earthquakes with magnitudes much

larger than ? are not possi"le. %a!imum magnitudes along many normal faults are

even more limited "ecause many of them are located along spreading centers, as in

celand, where the thickness of the "rittle layer is only a"out 9 km.0101

n addition, there e!ists a hierarchy of stress level in the three fault types. Thrust

faults are generated "y the highest, strike slip "y intermediate, and normal faults "ythe lowest stress levels.0+1 This can easily "e understood "y considering the

direction of the greatest principal stress, the direction of the force that 'pushes' the

rock mass during the faulting. n 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. n 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 over"urden equals the least principal stress. 7trike8slip

faulting is intermediate "etween the other two types descri"ed a"ove. This

di3erence in stress regime in the three faulting environments can contri"ute to

di3erences in stress drop during faulting, which contri"utes to di3erences in theradiated energy, regardless of fault dimensions.

Earthquakes away from plate boundaries

Main article: Intraplate earthquake

2here plate "oundaries occur within the continental lithosphere, deformation is

spread out over a much larger area than the plate "oundary itself. n the case of

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the 7an Andreas fault continental transform, many earthquakes occur away from

the plate "oundary and are related to strains developed within the "roader one of

deformation caused "y ma=or irregularities in the fault trace (e.g., the GHig "endG

region). The 6orthridge earthquake was associated with movement on a "lind thrust

within such a one. Another e!ample is the strongly o"lique convergent plate

"oundary "etween the Ara"ian and Eurasian plates where it runs through thenorthwestern part of the Iagros mountains. The deformation associated with this

plate "oundary is partitioned into nearly pure thrust sense movements

perpendicular to the "oundary over a wide one to the southwest and nearly pure

strike8slip motion along the %ain $ecent ;ault close to the actual plate "oundary

itself. This is demonstrated "y earthquake focal mechanisms.0&1

All tectonic plates have internal stress /elds caused "y their interactions with

neigh"ouring plates and sedimentary loading or unloading (e.g. deglaciation).0-1 These stresses may "e su4cient to cause failure along e!isting fault planes,

giving rise to intraplate earthquakes.0#1

Shallow-focus and deep-focus earthquakes

Main article: Depth of focus (tectonics)

ollapsed Jran <otel "uilding in the 7an 7alvador metropolis, after the

shallow ?9 7an 7alvador earthquakeduring mid civil war El 7alvador.

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Huildings fallen on their foundations after the shallow ?9 7an 7alvador

earthquake, El 7alvador.

leveled structures after the shallow?9 7an 7alvador earthquake, El 7alvador.

 The ma=ority of tectonic earthquakes originate at the ring of /re in depths not

e!ceeding tens of kilometers. Earthquakes occurring at a depth of less than km

are classi/ed as 'shallow8focus' earthquakes, while those with a focal8depth

"etween and & km are commonly termed 'mid8focus' or 'intermediate8depth'

earthquakes. n su"duction ones, where older and colder oceanic crust descends"eneath another tectonic plate, deep8focus earthquakes may occur at much greater

depths (ranging from & up to kilometers).091 These seismically active areas of

su"duction are known as 2adati8Henio3 ones. Feep8focus earthquakes occur at a

depth where the su"ducted lithosphere should no longer "e "rittle, due to the high

temperature and pressure. A possi"le mechanism for the generation of deep8focus

earthquakes is faulting caused "y olivine undergoing a phase transition into

a spinel structure.01

Earthquakes and volcanic activity

Earthquakes often occur in volcanic regions and are caused there, "oth"y tectonic faults and the movement of magma in volcanoes. 7uch earthquakes can

serve as an early warning of volcanic eruptions, as during the %ount 7t.

<elens eruption of ?.0?1 Earthquake swarms can serve as markers for the

location of the owing magma throughout the volcanoes. These swarms can "e

recorded "y seismometers and tiltmeters (a device that measures ground slope)

and used as sensors to predict imminent or upcoming eruptions.01

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Rupture dynamics

A tectonic earthquake "egins "y an initial rupture at a point on the fault surface, a

process known as nucleation. The scale of the nucleation one is uncertain, with

some evidence, such as the rupture dimensions of the smallest earthquakes,

suggesting that it is smaller than m while other evidence, such as a slowcomponent revealed "y low8frequency spectra of some earthquakes, suggest that it

is larger. The possi"ility that the nucleation involves some sort of preparation

process is supported "y the o"servation that a"out -K of earthquakes are

preceded "y foreshocks. @nce the rupture has initiated it "egins to propagate along

the fault surface. The mechanics of this process are poorly understood, partly

"ecause it is di4cult to recreate the high sliding velocities in a la"oratory. Also the

e3ects of strong ground motion make it very di4cult to record information close to

a nucleation one.0+1

$upture propagation is generally modeled using a fracture mechanics approach,

likening the rupture to a propagating mi!ed mode shear crack. The rupture velocityis 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 LK of the 78

wave velocity and this is independent of earthquake sie. A small su"set of

earthquake ruptures appear to have propagated at speeds greater than the 78wave

velocity. These supershear earthquakes have all "een o"served during large strike8

slip events. The unusually wide one of coseismic damage caused "y the +

Munlun earthquake has "een attri"uted to the e3ects of the sonic "oom developed

in such earthquakes. 7ome 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, o"served where the relatively low felt

intensities, caused "y the slow propagation speed of some great earthquakes, fail to

alert the population of the neigh"ouring coast, as in the ?9 %ei=i87anriku

earthquake.0+1

Tidal forces

$esearch work has shown a ro"ust correlation "etween small tidally induced forces

and non8volcanic tremor activity.0+10++10+&10+-1

Earthquake clusters

%ost earthquakes form part of a sequence, related to each other in terms of location

and time.0+#1 %ost earthquake clusters consist of small tremors that cause little to no

damage, "ut there is a theory that earthquakes can recur in a regular pattern. 0+91

Aftershocks

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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 "ut always of a

smaller magnitude. f an aftershock is larger than the main shock, the aftershock is

redesignated as the main shock and the original main shock is redesignated asa foreshock. Aftershocks are formed as the crust around the displaced fault

plane ad=usts to the e3ects of the main shock.0+#1

Earthquake swarms

Main article: Earthquake swarm

Earthquake swarms are sequences of earthquakes striking in a speci/c area within a

short period of time. They are di3erent from earthquakes followed "y a series

of aftershocks"y the fact that no single earthquake in the sequence is o"viously the

main shock, therefore none have nota"le higher magnitudes than the other. An

e!ample of an earthquake swarm is the +- activity at Nellowstone 6ational >ark.0+1 n August ++, a swarm of earthquakes shook 7outhern alifornia's mperial

Oalley, showing the most recorded activity in the area since the s. 0+?1

Earthquake storms

Main article: Earthquake storm

7ometimes a series of earthquakes occur in a sort of earthquake storm, where the

earthquakes strike a fault in clusters, each triggered "y the shaking or stress

redistri"ution of the previous earthquakes. 7imilar to aftershocks "ut on ad=acent

segments of fault, these storms occur over the course of years, and with some ofthe later earthquakes as damaging as the early ones. 7uch a pattern was o"served

in the sequence of a"out a doen earthquakes that struck the 6orth Anatolian

;ault in Turkey in the +th century and has "een inferred for older anomalous

clusters of large earthquakes in the %iddle East.0+10&1

7ie and frequency of occurrence

t is estimated that around #, earthquakes occur each year, detecta"le with

current instrumentation. A"out , of these can "e felt. 0&10&+1 %inor earthquakes

occur nearly constantly around the world in places like alifornia and Alaska in the

P.7., as well as in El7alvador, %e!ico, Juatemala, hile, >eru, ndonesia, ran, >akistan,

the Aoresin >ortugal, Turkey, 6ew Iealand, Jreece, taly, ndia and *apan, "ut

earthquakes can occur almost anywhere, including Fownstate 6ew Nork, England,

and Australia.0&&1 :arger earthquakes occur less frequently, the relationship

"eing e!ponentialC for e!ample, roughly ten times as many earthquakes larger than

magnitude - occur in a particular time period than earthquakes larger than

magnitude #. n the (low seismicity) Pnited Mingdom, for e!ample, it has "een

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calculated that the average recurrences areB an earthquake of &.L-.9 every year,

an earthquake of -.L#.# every years, and an earthquake of #.9 or larger every

years.0&-1 This is an e!ample of the Juten"ergL$ichter law.

 The %essina earthquake and tsunami took as many as +, lives on Fecem"er

+?, ? in 7icily andala"ria.0&#1

 The !" El Salvador earthquake

 The num"er of seismic stations has increased from a"out &# in & to many

thousands today. As a result, many more earthquakes are reported than in the past,"ut this is "ecause of the vast improvement in instrumentation, rather than an

increase in the num"er of earthquakes. The Pnited 7tates Jeological

7urvey estimates that, since , there have "een an average of ? ma=or

earthquakes (magnitude .L.) and one great earthquake (magnitude ?. or

greater) per year, and that this average has "een relatively sta"le.0&91n recent

years, the num"er of ma=or earthquakes per year has decreased, though this is

pro"a"ly a statistical uctuation rather than a systematic trend.0&1 %ore detailed

statistics on the sie and frequency of earthquakes is availa"le from the Pnited

7tates Jeological 7urvey (P7J7).0&?1 A recent increase in the num"er of ma=or

earthquakes has "een noted, which could "e e!plained "y a cyclical pattern of

periods of intense tectonic activity, interspersed with longer periods of low8intensity.

<owever, accurate recordings of earthquakes only "egan in the early s, so it is

too early to categorically state that this is the case.0&1

%ost of the world's earthquakes (K, and ?K of the largest) take place in the

-, km long, horseshoe8shaped one called the circum8>aci/c seismic "elt,

known as the >aci/c $ing of ;ire, which for the most part "ounds the >aci/c >late.0-1

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0-1 %assive earthquakes tend to occur along other plate "oundaries, too, such as

along the <imalayan %ountains.0-+1

2ith the rapid growth of mega8cities such as %e!ico ity, Tokyo and Tehran, in areas

of high seismic risk, some seismologists are warning that a single quake may claim

the lives of up to & million people.

0-&1

nduced seismicity

Main article: Induced seismicity 

2hile most earthquakes are caused "y movement of the Earth's tectonic plates,

human activity can also produce earthquakes. ;our main activities contri"ute to this

phenomenonB storing large amounts of water "ehind a dam (and possi"ly "uilding

an e!tremely heavy "uilding), drilling and in=ecting liquid into wells, and "y coal

mining and oil drilling.0--1 >erhaps the "est known e!ample is the +? 7ichuan

earthquake in hina's 7ichuan >rovince in %ayC this tremor resulted in 9,++

fatalities and is the th deadliest earthquake of all time. The Iipingpu Fam is

"elieved to have uctuated the pressure of the fault ,9# feet (#& m) awayC this

pressure pro"a"ly increased the power of the earthquake and accelerated the rate

of movement for the fault.0-#1 The greatest earthquake in Australia's history is also

claimed to "e induced "y humanity, through coal mining. The city of 6ewcastle was

"uilt over a large sector of coal mining areas. The earthquake has "een reported to

"e spawned from a fault that reactivated due to the millions of tonnes of rock

removed in the mining process.0-91

%easuring and locating earthquakes

Main article: eismology 

Earthquakes can "e recorded "y seismometers up to great distances,

"ecause seismic waves travel through the whole Earth's interior. The a"solute

magnitude of a quake is conventionally reported "y num"ers on the moment

magnitude scale (formerly $ichter scale, magnitude causing serious damage over

large areas), whereas the felt magnitude is reported using the modi/ed %ercalli

intensity scale (intensity LQ).

Every tremor produces di3erent types of seismic waves, which travel through rock

with di3erent velocitiesB

• :ongitudinal >8waves (shock8 or pressure waves)

•  Transverse 78waves ("oth "ody waves)

• 7urface waves 5 ($ayleigh and :ove waves)

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>ropagation velocity of the seismic waves ranges from appro!. & kmRs up to

& kmRs, depending on the density and elasticity of the medium. n the Earth's

interior the shock8 or > waves travel much faster than the 7 waves (appro!. relation

. B ). The di3erences in travel time from the epicentre to the o"servatory are a

measure of the distance and can "e used to image "oth sources of quakes and

structures within the Earth. Also the depth of the hypocenter can "e computedroughly.

n solid rock >8waves travel at a"out 9 to km per secondC the velocity increases

within the deep mantle to S& kmRs. The velocity of 78waves ranges from +L& kmRs

in light sediments and -L# kmRs in the Earth's crust up to kmRs in the deep mantle.

As a consequence, the /rst waves of a distant earthquake arrive at an o"servatory

via the Earth's mantle.

@n average, the kilometer distance to the earthquake is the num"er of seconds

"etween the > and 7 wave times #.0-1 7light deviations are caused "y

inhomogeneities of su"surface structure. Hy such analyses of seismograms theEarth's core was located in & "y Heno Juten"erg.

Earthquakes are not only categoried "y their magnitude "ut also "y the place

where they occur. The world is divided into #- ;linnLEngdahl regions (;8E regions),

which are "ased on political and geographical "oundaries as well as seismic activity.

%ore active ones are divided into smaller ;8E regions whereas less active ones

"elong to larger ;8E regions.

7tandard reporting of earthquakes includes its magnitude, date and time of

occurrence, geographic coordinates of its epicenter, depth of the epicenter,

geographical region, distances to population centers, location uncertainty, anum"er of parameters that are included in P7J7 earthquake reports (num"er of

stations reporting, num"er of o"servations, etc.), and a unique event F.0-?1

E3ects of earthquakes

## copper engraving depicting:is"on in ruins and in ames after the## :is"on

earthquake, which killed an estimated 9, people. A tsunamioverwhelms the

ships in the har"or.

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 The e3ects of earthquakes include, "ut are not limited to, the followingB

Shaking and ground rupture

Famaged "uildings in >ort8au8>rince, <aiti, *anuary +.

7haking and ground rupture are the main e3ects created "y earthquakes, principally

resulting in more or less severe damage to "uildings and other rigid structures. The

severity of the local e3ects depends on the comple! com"ination of theearthquake magnitude, the distance from the epicenter, and the local geological

and geomorphological conditions, which may amplify or reduce wave propagation.0-1 The ground8shaking is measured "y ground acceleration.

7peci/c local geological, geomorphological, and geostructural features can induce

high levels of shaking on the ground surface even from low8intensity earthquakes.

 This e3ect is called site or local ampli/cation. t is principally due to the transfer of

the seismic motion from hard deep soils to soft super/cial soils and to e3ects of

seismic energy focaliation owing to typical geometrical setting of the deposits.

Jround rupture is a visi"le "reaking and displacement of the Earth's surface alongthe trace of the fault, which may "e of the order of several metres in the case of

ma=or earthquakes. Jround rupture is a ma=or risk for large engineering structures

such as dams, "ridges and nuclear power stations and requires careful mapping of

e!isting faults to identify any which are likely to "reak the ground surface within the

life of the structure.0#1

$andslides and avalanches

Main article: !andslide

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:andslides "ecame a sym"ol of the devastation the + El 7alvador

earthquakes left, killing hundreds in its wake.

Earthquakes, along with severe storms, volcanic activity, coastal wave attack, and

wild/res, can produce slope insta"ility leading to landslides, a ma=or geological

haard. :andslide danger may persist while emergency personnel are attempting

rescue.0#1

%ires

;ires of the 9 7an ;rancisco earthquake

Earthquakes can cause /res "y damaging electrical power or gas lines. n the eventof water mains rupturing and a loss of pressure, it may also "ecome di4cult to stop

the spread of a /re once it has started. ;or e!ample, more deaths in the 9 7an

;rancisco earthquake were caused "y /re than "y the earthquake itself. 0#+1