how earthquakes work

8/21/2019 How Earthquakes Work

1/21

How Earthquakes Workby Tom Harris

Introduction to How Earthquakes Work

An earthquake is one of the most terrifying phenomena that nature can dish up. Wegenerally think of the ground we stand on as "rock-solid" and completely stable. Anearthquake can shatter that perception instantly and often with e!treme iolence.

#p until relatiely recently scientists only had unsubstantiated guesses as to what actuallycaused earthquakes. Een today there is still a certain amount of mystery surroundingthem but scientists hae a much clearer understanding.

Earthquake Image Gallery

$hoto courtesy #%&%

A section of Interstate 880 in Oakland, alifornia, damaged !ythe magnitude "#$ earthquake that shook the

%an &rancisco area in $'8'# %ee moreearthquake (ictures#

There has been enormous progress in the past century' %cientists hae identified theforces that cause earthquakes and deeloped technology that can tell us an earthquake(smagnitude and origin. The ne!t hurdle is to find a way of predicting earthquakes so theydon(t catch people by surprise.

)n this article we(ll find out what causes earthquakes and we(ll also find out why they canhae such a deastating effect on us.

*our +rowser ,oes ot %upport irames%haking GroundAn earthquake is a )i!rationthat traels through the earth(s crust. Technically a largetruck that rumbles down the street is causing a mini-earthquake if you feel your houseshaking as it goes by but we tend to think of earthquakes as eents that affect a fairlylarge area such as an entire city. All kinds of things can cause earthquakes'

olcanic eruptions
http://science.howstuffworks.com/about-author.htm#tom%20harrishttp://science.howstuffworks.com/enlarge-image.htm?terms=earthquake&page=0http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://science.howstuffworks.com/enlarge-image.htm?terms=earthquake&page=0http://science.howstuffworks.com/enlarge-image.htm?terms=earthquake&page=0http://science.howstuffworks.com/earthquake.htm/volcano.htmhttp://science.howstuffworks.com/enlarge-image.htm?terms=earthquake&page=0http://science.howstuffworks.com/enlarge-image.htm?terms=earthquake&page=0http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://science.howstuffworks.com/enlarge-image.htm?terms=earthquake&page=0http://science.howstuffworks.com/earthquake.htm/volcano.htmhttp://science.howstuffworks.com/about-author.htm#tom%20harris


2/21

meteor impacts

underground e!plosions /an underground nuclear test for e!ample0

collapsing structures /such as a collapsing mine0

+ut the ma1ority of naturally-occurring earthquakes are caused by moements of theearth(s (lates as we(ll see in the ne!t section.

We only hear about earthquakes in the news eery once in a while but they are actuallyan eeryday occurrence on our planet. According to the #nited %tates &eological %ureymore than three million earthquakes occur eery year. That(s about 2333 a day or oneeery 44 seconds5

$hoto courtesyE6A

*esidential damage caused !y the $''+ earthquake inorthridge, alifornia#

The ast ma1ority of these 7 million quakes are e!tremely weak. The law of probabilityalso causes a good number of stronger quakes to happen in uninhabited places where noone feels them. )t is the big quakes that occur in highly populated areas that get ourattention.

Earthquakes hae caused a great deal of property damage oer the years and they haeclaimed many lies. )n the last hundred years alone there hae been more than 4.8million earthquake-related fatalities. #sually it(s not the shaking ground itself that claimslies -- it(s the associated destruction of manmade structures and the instigation of othernatural disasters such as tsunamis aalanches and landslides.
http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.fema.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.fema.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.fema.gov


3/21

$hoto courtesy &,9

*esidential damage in -rince William %ound, Alaska, due to liquefaction caused !y a$'.+ '#/magnitude earthquake#

)n the ne!t section we(ll e!amine the powerful forces that cause this intense tremblingand find out why earthquakes occur much more often in certain regions.

-late 1ectonics

The biggest scientific breakthrough in the history of seismology-- the study ofearthquakes -- came in the middle of the :3th century with the deelopment of thetheory of (late tectonics. %cientists proposed the idea of plate tectonics to e!plain anumber of peculiar phenomenon on earth such as the apparent moement of continents

$hoto courtesy #%&%One of the !est known faults is

the %an Andreas fault inalifornia# 1he fault, which marksthe (late !oundary !etween the

-acific oceanic (late and theorth American continental (late,

e2tends o)er .30 miles 4$,030km5 of land#
http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.ngdc.noaa.gov/ngdc.htmlhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.ngdc.noaa.gov/ngdc.htmlhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.gov


4/21

oer time the clustering of olcanic actiity in certain areas and the presence of hugeridges at the bottom of the ocean.

The basic theory is that the surface layer of the earth -- the lithos(here-- is comprised ofmany (latesthat slide oer the lubricating athenos(herelayer. At the boundariesbetween these huge plates of soil and rock three different things can happen'

-lates can mo)e a(art- )f two plates are moing apart from each other hotmolten rock flows up from the layers of mantle below the lithosphere. Thismagmacomes out on the surface /mostly at the bottom of the ocean0 where it iscalled laa. As the laa cools it hardens to form new lithosphere material fillingin the gap. This is called a di)ergent (late !oundary.

-lates can (ush together- )f the two plates are moing toward each other oneplate typically pushes under the other one. This su!ductingplate sinks into thelower mantle layers where it melts. At some boundaries where two plates meetneither plate is in a position to subduct under the other so they both pushagainst each other to form mountains. The lines where plates push toward eachother are called con)ergent (late !oundaries.

-lates slide against each other- At other boundaries plates simply slide by

each other -- one moes north and one moes south for e!ample. While theplates don(t drift directly into each other at these transform !oundaries theyare pushed tightly together. A great deal of tension builds at the boundary.

%ee the (late!oundary diagram#

Where these plates meet you(ll find faults-- breaks in the earth(s crust where the blocksof rock on each side are moing in different directions. Earthquakes are much morecommon along fault lines than they are anywhere else on the planet.

)n the ne!t section we(ll look at some different types of faults and see how theirmoement creates earthquakes.

&aults

%cientists identify four types of faults characteri;ed by the position of the fault (lanethe break in the rock and the moement of the two rock blocks'

)n a normal fault/see animation below0 the fault plane is nearly ertical. Thehanging wall the block of rock positioned aboe the plane pushes down acrossthe footwall which is the block of rock below the plane. The footwall in turnpushes up against the hanging wall. These faults occur where the crust is beingpulled apart due to the pull of a diergent plate boundary.
http://science.howstuffworks.com/earthquake.htm/earthquake-graphic.htmhttp://science.howstuffworks.com/earthquake.htm/earthquake-graphic.htm


5/21

6our !rowser does not su((ort 7a)a%cri(t or it is disa!led#

ormal fault The fault plane in a re)erse faultis also nearly ertical but the hanging wall

pushes up and the footwall pushes down. This sort of fault forms where a plate isbeing compressed.

A thrust faultmoes the same way as a reerse fault but the fault line isnearly hori;ontal. )n these faults which are also caused by compression the rockof the hanging wall is actually pushed up on top of the footwall. This is the sortof fault that occurs in a conerging plate boundary.

6our !rowser does notsu((ort 7a)a%cri(t or it is disa!led#

*e)erse fault )n a strikesli( fault the blocks of rock moe in opposite hori;ontal directions.

These faults form when the crust pieces are sliding against each other as in atransform plate boundary


6/21

6our !rowser does not

su((ort 7a)a%cri(t or it is disa!led#%trikesli( fault

)n all of these types of faults the different blocks of rock push ery tightly togethercreating a good deal of friction as they moe. )f this friction leel is high enough the twoblocks become locked-- the friction keeps them from sliding against each other. Whenthis happens the forces in the plates continue to push the rock increasing the pressureapplied at the fault.

)f the pressure increases to a high enough leel then it will oercome the force of thefriction and the blocks will suddenly snap forward. To put it another way as the tectonicforces push on the "locked" blocks potential energy builds. When the plates are finallymoed this built-up energy becomes kinetic. %ome fault shifts create isible changes atthe earth(s surface but other shifts occur in rock well under the surface and so don(tcreate a surface rupture.

$hoto courtesy #%&%

ro( rows offset !y a lateral strike sli( fault shifting in the$'". earthquake that shook El -rogresso, Guatemala#
http://www.howstuffworks.com/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://www.howstuffworks.com/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.gov


7/21

The initial break that creates a fault along with these sudden intense shifts along alreadyformed faults are the main sources of earthquakes. 6ost earthquakes occur around plateboundaries because this is where the strain from the plate moements is felt mostintensely creating fault ones groups of interconnected faults. )n a fault ;one therelease of kinetic energy at one fault may increase the stress -- the potential energy -- in anearby fault leading to other earthquakes. This is one of the reasons that seeral

earthquakes may occur in an area in a short period of time.


8/21

$hoto courtesy #%&%

*ailroad tracks shifted !y the $'". Guatemala earthquake
http://www.howstuffworks.com/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://www.howstuffworks.com/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.gov


9/21

Eery now and then earthquakes do occur in the middle of plates. )n fact one of the mostpowerful series of earthquakes eer recorded in the #nited %tates occurred in the middleof the orth American continental plate. These earthquakes which shook seeral states in4244 and 424: originated in 6issouri. )n the 4 to 2 kps0 depending on the material they(re moingthrough. This speed is greater than the speed of other waes so $ waes arrie

first at any surface location. They can trael through solid liquid and gas and sowill pass completely through the body of the earth. As they trael through rockthe waes moe tiny rock particles back and forth -- pushing them apart and thenback together -- in line with the direction the wae is traeling. These waestypically arrie at the surface as an abrupt thud.

%econdary wa)es also called % wa)esor shear wa)es lag a little behind the $waes. As these waes moe they displace rock particles outward pushing themperpendicular to the path of the waes. This results in the first period of rollingassociated with earthquakes. #nlike $ waes % waes don(t moe straight
http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.gov


10/21

through the earth. They only trael through solid material and so are stopped atthe liquid layer in the earth(s core.

6our !rowser does notsu((ort 7a)a%cri(t or it is disa!led#

lick the (lay !utton to start the earthquake#When - and % wa)es reach the earth:s surface,they form ; wa)es# 1he most intense ; wa)es

radiate out from the e(icenter#

+oth sorts of body waes do trael around the earth howeer and can be detected on theopposite side of the planet from the point where the earthquake began. At any gienmoment there are a number of ery faint seismic waes moing all around the planet.

%urface waes are something like the waes in a body of water -- they moe the surface ofthe earth up and down. This generally causes the worst damage because the wae motionrocks the foundations of manmade structures. ? waes are the slowest moing of allwaes so the most intense shaking usually comes at the end of an earthquake.

)n the ne!t section we(ll see how scientists can calculate the origin of an earthquake bydetecting these different waes.

%eismology


11/21

We saw in the last section that there are three differenttypes of seismic waes and that these waes trael atdifferent speeds. While the e!act speed of $ and % waesaries depending on the composition of the materialthey(re traeling through the ratio between the speeds ofthe two waes will remain relatiely constant in any

earthquake. $ waes generally trael 4.= times fasterthan % waes.

#sing this ratio scientists can calculate the distance between any point on the earth(ssurface and the earthquake(s focus the breaking point where the ibrations originated.They do this with a seismograph a machine that registers the different waes. To find the

distance between the seismograph and the focus scientists also need to know the time theibrations arried. With this information they simply note how much time passed betweenthe arrial of both waes and then check a special chart that tells them the distance thewaes must hae traeled based on that delay.

)f you gather this information from three or more points you can figure out the location ofthe focus through the process of trilateration. +asically you draw an imaginary spherearound each seismograph location with the point of measurement as the center and themeasured distance /let(s call it @0 from that point to the focus as the radius. The surface ofthe circle describes all the points that are @ miles away from the seismograph. The focusthen must be somewhere along this sphere. )f you come up with two spheres based oneidence from two different seismographs you(ll get a two-dimensional circle where theymeet. %ince the focus must be along the surface of both spheres all of the possible focus

points are located on the circle formed by the intersection of these two spheres. A thirdsphere will intersect only twice with this circle giing you two possible focus points. Andbecause the center of each sphere is on the earth(s surface one of these possible pointswill be in the air leaing only one logical focus location.

or a more thorough discussion of trilateral calculation check out How &$% eceiersWork.

*ichter %cale

Wheneer a ma1or earthquake is in the news you(ll probably hear about its ichter %calerating. *ou might also hear about its


12/21

$hoto courtesy &,9

=estruction caused !y a 4*ichter5 magnitude .#. earthquake inaracas, >eneuela# 1he $'." earthquake took /+0 li)es and

caused more than ?30 million worth of (ro(erty damage#

The ichter %cale is used to rate the magnitudeof an earthquake -- the amount of energyit released. This is calculated using information gathered by a seismograph. The ichter%cale is logarithmic meaning that whole-number 1umps indicate a tenfold increase. )n thiscase the increase is in wae amplitude. That is the wae amplitude in a leel >earthquake is 43 times greater than in a leel 8 earthquake and the amplitude increases433 times between a leel = earthquake and a leel < earthquake. The amount of energyreleased increases 74.= times between whole number alues.

The largest earthquake on record registered an


13/21

-rince William %ound earthquake# 1he earthquake, which killed$@$ (eo(le and caused ?3@8 million of (ro(erty damage,

registered an '#/ on the *ichter %cale#

ichter ratings only gie you a rough idea of the actual impact of an earthquake. As we(eseen an earthquake(s destructie power aries depending on the composition of the

ground in an area and the design and placement of manmade structures. The e!tent ofdamage is rated on the


14/21

Earthquake -redictions

We understand earthquakes a lot better than we did een 83 years ago but we still can(tdo much about them. They are caused by fundamental powerful geological processes thatare far beyond our control. These processes are also fairly unpredictable so it(s notpossible at this time to tell people e!actly when an earthquake is going to occur. The firstdetected seismic waes will tell us that more powerful ibrations are on their way butthis only gies us a few minutes warning at most.

$hoto courtesy #%&%

=amage in downtown Anchorage, Alaska, caused !y the $'.+-rince William %ound earthquake#

%cientists can say where ma1or earthquakes are likely to occur based on the moement ofthe plates in the earth and the location of fault ;ones. They can also make general guessesof when they might occur in a certain area by looking at the history of earthquakes in the

region and detecting where pressure is building along fault lines. These predictions aree!tremely ague howeer -- typically on the order of decades. %cientists hae had moresuccess predicting aftershocks additional quakes following an initial earthquake. Thesepredictions are based on e!tensie research of aftershock patterns. %eismologists canmake a good guess of how an earthquake originating along one fault will cause additionalearthquakes in connected faults.

Another area of study is the relationship between magnetic and electrical charges in rockmaterial and earthquakes. %ome scientists hae hypothesi;ed that these electromagneticfields change in a certain way 1ust before an earthquake. %eismologists are also studyinggas seepage and the tilting of the ground as warning signs of earthquakes. or the mostpart howeer they can(t reliably predict earthquakes with any precision.

%o what can we do about earthquakesD The ma1or adances oer the past 83 years haebeen in preparedness -- particularly in the field of construction engineering. )n 4


15/21

$hoto courtesy #%&%

9ridge columns cracked !y the

;oma -rieta, alif# earthquake of $'8'#

Another component of preparedness is educating the public. The #nited %tates &eological%urey/#%&%0 and other goernment agencies hae produced seeral brochures e!plainingthe processes inoled in an earthquake and giing instructions on how to prepare yourhouse for a possible earthquake as well as what to do when a quake hits.

$hoto courtesy #%&%

1he great %an &rancisco fire of $'0. was initiated !y a(owerful earthquake# 1he earthquake )i!rations and

catastro(hic fire destroyed most of the city,lea)ing /30,000 (eo(le homeless#

)n the future improements in prediction and preparedness should further minimi;e theloss of life and property associated with earthquakes. +ut it will be a long time if eerbefore we(ll be ready for eery substantial earthquake that might occur. ust like seereweather and disease earthquakes are an unaoidable force generated by the powerfulnatural processes that shape our planet. All we can do is increase our understanding of thephenomenon and deelop better ways to deal with it. To learn more about earthquakescheck out the #%&% Web site or any of the other sites listed in the ?inkssection.

Earthquake -ictures
http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.usgs.govhttp://science.howstuffworks.com/earthquake.htm/earthquake8.htmhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.usgs.govhttp://science.howstuffworks.com/earthquake.htm/earthquake8.htm


16/21

FAG#H)C C&)A$&etty )mages

Earthquakes can !e caused !y underground e2(losions orcolla(sing structures as well as !y natural mo)ements of the

earth:s (lates#

A& %WA%T)FAA$&etty )mages

An earthquake can shatter our (ers(ecti)e of the ground wewalk on as rock solid#


17/21

%AA, IA**#6A$&etty )mages

An earthquake:s destructi)e (ower )aries de(ending on the

com(osition of the ground in an area and the design and(lacement of manmade structures#

$hoto by 9hina $hotos&etty )mages

Highly de)elo(ed areas !uilt on loose ground material cansuffer se)ere damage from e)en a relati)ely mild earthquake#


18/21

A)6E AG#)A$&etty )mages

Earthquakes can (reci(itate other natural disasters, such asa)alanches and landslides#

Foichi Famoshida&etty )mages

%urface wa)es are res(onsi!le for most of the damageassociated with earthquakes, !ecause they cause the most

intense )i!rations#


19/21

E,A T)%A,)A$&etty )magesEarthquakes that cause (ro(erty damage are usually rated 3 or

higher on the *ichter %cale#

AA6) I#E%H)A$&etty )mages

It:s im(ortant to design structures that can withstand the forceof an earthquake, es(ecially in quake(rone areas#


20/21

A,EF +E*A$&etty )mages

%eismologists ha)e had more success (redicting locations ofaftershocks than of initial earthquakes#

EWE? %A6A,A$&etty )mages

1he loss of home and !elongings in an earthquake can !ecatastro(hic for quake sur)i)ors#

%ee moreearthquake pictures.

;ots


21/21

6ore &reat ?inks

#%&% earthquake information

#%&% Earthquakes for Fids %ite

ational &eophysical ,ata 9enter

9enter for Earthquake esearch and )nformation

The Worldwide Earthquake ?ocator American ed 9ross ,isaster &uide
http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://quake.wr.usgs.gov/info/basics.htmlhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.gov/4kids/http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.ngdc.noaa.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.ceri.memphis.edu/http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.geo.ed.ac.uk/quakes/quakes.htmlhttp://howstuffworks.com/framed.htm?parent=earthquake.htm&url=http://www.redcross.org/services/disaster/0,1082,0_500_,00.htmlhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://quake.wr.usgs.gov/info/basics.htmlhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://earthquake.usgs.gov/4kids/http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.ngdc.noaa.govhttp://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.ceri.memphis.edu/http://science.howstuffworks.com/earthquake.htm/framed.htm?parent=earthquake.htm&url=http://www.geo.ed.ac.uk/quakes/quakes.htmlhttp://howstuffworks.com/framed.htm?parent=earthquake.htm&url=http://www.redcross.org/services/disaster/0,1082,0_500_,00.html

how earthquakes work

Documents