Overview Seismology - II 2

8/17/2019 Overview Seismology - II 2

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EarthquakeEarthquake SeismologySeismology


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Literature – Text BooksLiterature – Text BooksShearer, Introduction to Seismology, CambridgeUniversity Press, 1990.

Wysession and Stein, An introduction to seismology,earthua!es and earth structure, "lac!#ell Scienti$ic

%ennett, &he Seismic Wave$ield, I'II, CambridgeUniversity Press

(ay and Wallace, )odern *lobal Seismology, AcademicPress, 199+.

*ubbins, Seismology and Plate &ectonics, CambridgeUniversity Press, 1990.

A!i and ichards, -uantitative Seismology, Academic

Press, 00.


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IntroductionIntroduction

earthquake: a trembling or shaking of theearthquake: a trembling or shaking of theground caused by the sudden release ofground caused by the sudden release of

energy stored in the rocks beneath the earth’senergy stored in the rocks beneath the earth’s

surfacesurface

seismic a!es: the a!es of energy "roducedseismic a!es: the a!es of energy "roduced

by an earthquakeby an earthquake

elastic rebound theory: the sudden release ofelastic rebound theory: the sudden release of

"rogressi!ely stored strain in rocks that"rogressi!ely stored strain in rocks thatresults in mo!ement along a faultresults in mo!ement along a fault


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#auses of Earthquakes#auses of Earthquakes

mo!ement along faultsmo!ement along faults

ex"losi!e !olcanic eru"tionsex"losi!e !olcanic eru"tions

mineral transformations ithin themineral transformations ithin thedongoing rock as "ressure colla"sesdongoing rock as "ressure colla"ses

one mineral into a denser formone mineral into a denser form

$uclear ex"losion$uclear ex"losion


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Aspects ofAspects of seismology?seismology?

SeiSeissmologmologyy is science dealing with all aspects ofis science dealing with all aspects ofearthquakesearthquakes::

OBSERVATIONAL SEISMOLOGYOBSERVATIONAL SEISMOLOGY Recording earthquakesRecording earthquakes (i(icroseiso!og"#croseiso!og"# $ata!oguing earthquakes$ata!oguing earthquakes O%ser&ing earthquake e''ectsO%ser&ing earthquake e''ects

( (aaccroseiroseiso!og"so!og"##

ENGINEERING SEISMOLOGYENGINEERING SEISMOLOGY

Estiation o' seisic haard and riskEstiation o' seisic haard and risk Aseisic %ui!dingAseisic %ui!ding

) )*+YSI$AL, SEISMOLOGY*+YSI$AL, SEISMOLOGY

Stud" o' the -ro-erties o' the Earth,s interiorStud" o' the -ro-erties o' the Earth,s interior Stud" o' -h"sica! characteristics o' seisicStud" o' -h"sica! characteristics o' seisic sourcessources

E.*LORATIONAL SEISMOLOGY (A--!ied seisic ethods#///E.*LORATIONAL SEISMOLOGY (A--!ied seisic ethods#///


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SeismologySeismology

• Multidisciplinary science, links physics with other geosciencesMultidisciplinary science, links physics with other geosciences

(geolog(geologyy, geogra, geogra phy phy))

• InternaInternattionalional sciencescience

• Large span of amplitudesLarge span of amplitudes

( ~ 10( ~ 10!! " 10 " 1011 m)m)

• ##eer r yy large span of wa$elarge span of wa$e

periods periods ( ~ 10( ~ 10%% " 10 " 10&& s)s)

• ##ery young scienceery young science

(second half of the(second half of the

1!th century) 1!th century)


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Myths and legendsMyths and legends

Earthquakes occur:Earthquakes occur:• 'hen one of the eight elephants that carry the arth gets

tired (indu)

• 'hen a frog that carries the world mo$es'hen a frog that carries the world mo$es

(Mongolia)(Mongolia)• 'hen the giant on whose head we all li$e,'hen the giant on whose head we all li$e,

snee*es or scratches (+frica)snee*es or scratches (+frica)

• 'hen the attention of the god ashima (who looks after'hen the attention of the god ashima (who looks after

the giant catfish -ama*u that supports the arth andthe giant catfish -ama*u that supports the arth and pre$ents it to sink into the ocean) weakens and -ama*u pre$ents it to sink into the ocean) weakens and -ama*u

mo$es (.apan)mo$es (.apan)

• 'hen the god Maimas decides to count the population in'hen the god Maimas decides to count the population in

/eru his footsteps shake the arth hen nati$es run out of/eru his footsteps shake the arth hen nati$es run out of

their huts and yell2 3I4m here, I4m here56their huts and yell2 3I4m here, I4m here56


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Seismic waves in a homogeneousplanet would be neither refected norreracted. Lines drawn perpendicular tothe wave fronts (rays) would

follow linear paths.

Seismic waves in a dierentiatedplanet would pass through materialthat gradually increases in rigidity withdepth. As a result, wave velocities wouldincrease steadily with depth, and rays

would follow curved path.


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The shadow zone o S wavesextends almost halway aroundthe globe from the earthquake’sfocus.his phenomenon can be

e!plained if the outer core of "arth isliquid. #ecause S waves cannottravel through liquid, they do notpass through the core.


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! wave shadow

zone occurs in the areabetween "#$% and

$%&' from an earthquake’sfocus.he best way to e!plainthe wave shadow one is topostulate that "arth has acentral core through which waves travel relatively slowly.*ay $ +ust misses the core andis received at a station located$&' from the earthquake’s

focus. A steeper ray, such asray -, encounters the boundaryof the core and is refracted. ttravels through the core,isrefracted again at the core’sboundary, and is received at astation fewer than $/' fromthe focus. Similarly, ray & isrefractedand emerges at thesurface $%&' from thefocus.0ther rays that are steeperthan ray $

are severely bent by the core,so that no waves are directlyreceived in the shadow one.1rom shadow ones,seismologists

calculate that the boundary ofthe core is -2 km below the


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The internal structure o &arth is deduced rom variations in the velocity o

seismic waves at depth.The velocity of both waves and S waves increases until theyreach a depth of appro!imately $ km.here the waves are slow until they have traveledto a depth of about -3 km.his low4velocity layer lies within the asthenosphere. #elowthis, the velocity of waves and S waves increases until a depth of about -2 km, whereboth velocities change abruptly. S waves do not travel through the central part of "arth,and the velocity of the waves decreases drastically.his variation is the most strikingdiscontinuity and indicates the boundary between the liquid outer core and the mantle.Another discontinuity in wave velocity, at a depth of 3 km, indicates the surface ofthe solid inner core.


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The low velocity zone is revealed bya drop in the velocities of both waves(shown here) and S waves.his marks aone of low strength in the upper mantlebetween about $ and -3 km below thesurface. he low4velocity one is containedin the asthenosphere and marks part ofthe mantle that is very near its meltingpoint and may be a one of partial melting.

'iscontinuities in seismic wave

velocities may correspond to phasechanges.he blue line shows how wavevelocities change with depth.he other linesshow velocities for various minerals.heuppermost mantle is dominated by olivine.#elow % km a velocity increase impliesolivine is replaced by spinel. At greaterdepths, spinel is probably replaced bymagnesium perovskite. "ach change

increases the density of the mantle.


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The Three %a&or #hemical 'adial (i!isionsThe Three %a&or #hemical 'adial (i!isions

$rust$rust

Mant!eMant!e

$ore$ore

To see how earthquakes really occur, we firstTo see how earthquakes really occur, we firstneed to learn about constitution of the Earth!need to learn about constitution of the Earth!


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The Shallowest LayerThe Shallowest Layer

o the &arth( the )rusto the &arth( the )rust he boundary between the he boundary between the

crust and the mantle iscrust and the mantle ismostlymostly chemicalchemical. he crust. he crustand mantle have di5erentand mantle have di5erent

compositions.compositions.

his boundary is his boundary isreferred to as thereferred to as theMohorovi*i+Mohorovi*i+discontinuitydiscontinuity or 67oho8.or 67oho8.

t was discovered in $2$ byt was discovered in $2$ bythe 9roatian seismologistthe 9roatian seismologistAndri+a 7ohorovi:i;.Andri+a 7ohorovi:i;.

he crust is the most he crust is the mostheterogeneous layer in theheterogeneous layer in the"arth"arth

he crust is on average && he crust is on average &&km thick for continents andkm thick for continents and$ km thick beneath$ km thick beneathoceans< however it variesoceans< however it variesfrom +ust a few km to overfrom +ust a few km to over= km globally.= km globally.


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Middle &arth( TheMiddle &arth( The

MantleMantle "arth’s mantle e!ists from the"arth’s mantle e!ists from thebottom of the crust to a depthbottom of the crust to a depthof -/2$ km (radius of &%/of -/2$ km (radius of &%/km) >km) > ,utenberg,utenbergdiscontinuitydiscontinuity

t is further subdivided into?t is further subdivided into? he uppermost mantle he uppermost mantle

(crust to % km depth)(crust to % km depth) he transition one he transition one

(% > = km depth)(% > = km depth) he mid4mantle he mid4mantle

(= to @-3 km depth)(= to @-3 km depth) he lowermost mantle he lowermost mantle

(@-3 > -/2$ km depth)(@-3 > -/2$ km depth) he uppermost mantle is he uppermost mantle is

composed dominantly ofcomposed dominantly ofolivine< lesser componentsolivine< lesser componentsinclude pyro!ene, enstatite,include pyro!ene, enstatite,

and garnetand garnet

#eno#enoButenbergButenberg


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&arth-s )ore&arth-s )ore

0wing to the great pressure0wing to the great pressureinside the "arth the "arth’sinside the "arth the "arth’score is actuallycore is actually freezingfreezing asasthe "arth gradually cools.the "arth gradually cools.

he boundary between the he boundary between theliquid outer coreliquid outer core and theand thesolid inner coresolid inner core occurs at aoccurs at aradius of about $-- km >radius of about $-- km >Lehman discontinuityLehman discontinuity,,afterafter nge Cehmannge Cehman fromfromDenmark.Denmark.

he boundary between the he boundary between themantle and outer core ismantle and outer core issharp.sharp.

he change in density across he change in density acrossthe core4mantle boundary isthe core4mantle boundary is

greater than that at thegreater than that at the"arth’s surfaceE"arth’s surfaceE

he viscosity of the outer he viscosity of the outercore is similar to that ofcore is similar to that ofwater, it Fows kilometers perwater, it Fows kilometers peryear and creates the "arth’syear and creates the "arth’smagnetic Geld.magnetic Geld.

he outer core is the most he outer core is the mosthomogeneous part of thehomogeneous part of the"arth"arth

he outer core is mostly an he outer core is mostly analloy of iron and nickel inalloy of iron and nickel inliquid form.liquid form.

As the core freeesAs the core freees latentlatentheat heat is released< this heatis released< this heatcauses the outer core tocauses the outer core toconvectconvect and so generates aand so generates amagnetic Geld.magnetic Geld.


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1 Lithosphere

7 +sthenosphere

% Mesosphere

Mechanic

al Layers(


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LitLithhosphereosphere

he he lithospherelithosphere is theis theuppermost 34$ kmuppermost 34$ kmof the "arth.of the "arth.

here is not a strict boundary between the here is not a strict boundary between thelithosphere and the asthenosphere as there islithosphere and the asthenosphere as there isbetween the crust and mantle.between the crust and mantle.

t consists of both crust and upper parts oft consists of both crust and upper parts ofmantle.mantle.

t behaves rigidly, like a solid, over very longt behaves rigidly, like a solid, over very longtime periods.time periods.


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ststhhenosphereenosphere

he asthenosphere e!ists between depths of he asthenosphere e!ists between depths of$4- km.$4- km.

t is the weakest part of the mantle.t is the weakest part of the mantle. t is a solid over short time scales, but behavest is a solid over short time scales, but behaves

like a Fuid over millions of years.like a Fuid over millions of years. he asthenosphere decouples the lithosphere he asthenosphere decouples the lithosphere

((tectonic platestectonic plates) from the rest of the mantle.) from the rest of the mantle.


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Tectonic orcesTectonic orces

he interior of the "arth is he interior of the "arth isdynamic > it cools down and thusdynamic > it cools down and thus

provides energy for convectiveprovides energy for convectivecurrents in the outer core and incurrents in the outer core and inthe astenosphere.the astenosphere.

Additional energy comes fromAdditional energy comes fromradioactive decay...radioactive decay...


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!late tectonics!late tectonics

!LT& T&)T/0)S theory is very young($24ies)

t provides answers to the mostfundamental questions in seismology?

Hhy earthquakes occurI

Hhy are earthquake epicenters notuniformly distributed around the globeI

At what depths are their fociI

1 i l


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Ma1or tectonic platesMa1or tectonic plates


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2ow earth3ua4es2ow earth3ua4es

occur5occur5J "arthquakes occur at 67LTS.

J 1ault is a weak one separatingtwo geological blocks.

J ectonic forces

cause the blocksto move relativeone to another.


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occur5occur5 &lastic rebound&lastic reboundtheorytheory

Th i t b th th & th8Th i t b th th & th8


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The point beneath the &arth8sThe point beneath the &arth8ssurace where the roc4s brea4surace where the roc4s brea4

and move is called theand move is called the

focusfocus

oo

the earth3ua4e. The ocus is thethe earth3ua4e. The ocus is theunderground point o origin ounderground point o origin o

an earth3ua4e.an earth3ua4e.

'irectly above the'irectly above the

ocus9 on the &arth8socus9 on the &arth8s

surace9 is thesurace9 is theepicenter epicenter ..

The relationship between an earth3ua4e-s ocus9 itsepicenter9

and seismic wave ronts

http://cires.colorado.edu/people/jones.craig/EQimagemap/global.htmlhttp://cires.colorado.edu/people/jones.craig/EQimagemap/global.htmlhttp://cires.colorado.edu/people/jones.craig/EQimagemap/global.html


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occur5occur5 &lastic rebound&lastic reboundtheorytheory

J :ecause o riction9 the bloc4s do not slide9 but aredeormed.

J ;hen the stresses within roc4s exceed riction9 rupture

occurs.


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Location o &arth3ua4eLocation o &arth3ua4e

6ocus6ocus"arthquakes are concentrated in its upper = km.

Hithin the =4km range, earthquakes can be groupedaccording to depth of focus.Shallow


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&lastic waves > :ody&lastic waves > :ody

waveswavesLongitudinal waves(Longitudinal waves(J hey are faster than transversal waves and thus hey are faster than transversal waves and thus

arrive Grst.arrive Grst.

J he particles oscillate in the direction of he particles oscillate in the direction ofspreading of the wave.spreading of the wave.

J 9ompressional waves9ompressional wavesJ 4waves4waves

Transversal waves(Transversal waves(J he particles oscillate in the direction he particles oscillate in the direction

perpendicular to the spreading direction.perpendicular to the spreading direction.J Shear waves > they do not propagate throughShear waves > they do not propagate through

solids (e.g. through the outer core).solids (e.g. through the outer core).

J S4wavesS4waves


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Motion produced by various types o seismic

waves


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&lastic waves > :ody&lastic waves > :ody

waveswaves

4waves?4waves?

S4waves?S4waves?


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&lastic waves > Surace&lastic waves > Surace

waveswaves

Surface waves? *ayleigh and Cove wavesSurface waves? *ayleigh and Cove waves heir amplitude diminishes with the depth. heir amplitude diminishes with the depth. hey have large amplitudes and are hey have large amplitudes and are slowerslower

than body waves.than body waves.

hese are dispersive waves (large periods are hese are dispersive waves (large periods arefaster .faster .


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!roperties o waves!roperties o waves

mportant terms aremportant terms are

Amplitude 4 height of wave above the starting pointAmplitude 4 height of wave above the starting point

Havelength 4 the distance between successive wavesHavelength 4 the distance between successive waves

eriod 4 the time between successive waves K $L 1requencyeriod 4 the time between successive waves K $L 1requency

#ody waves 4 .3 to - M#ody waves 4 .3 to - M

Surface waves 4 .3 to .$ M.Surface waves 4 .3 to .$ M.


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Measuring &arth3ua4esMeasuring &arth3ua4es seismograph? a recording device thatseismograph? a recording device that

produces a permanent record of earthproduces a permanent record of earthmotion detected by a seismometer,motion detected by a seismometer,usually in the form of a wiggly lineusually in the form of a wiggly linedrawn on a moving strip of paperdrawn on a moving strip of paper

A seismograph for horiontal motion. 7odern seismographs record earth motion on moving strips of paper.

by a wire from the column and swings like a pendulum when the ground moves horiontally. A pen attache

motion on a movin stri of a er.


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Measuring &arth3ua4esMeasuring &arth3ua4es

A simple seismograph for detecting vertical rock motion. he pen records the ground motion on the seism

stretches and compresses with up and down movement of the spring. 1rame and recording drum move wit

the weight keeps it and the needle relatively motionless.


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MeasuringMeasuring

&arth3ua4es ?cont.@&arth3ua4es ?cont.@ seismogram? the paper record of eachseismogram? the paper record of eachvibrationvibration


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Seismology and theSeismology and the

"arth’s Deep nterior"arth’s Deep nterior

seismogram seismogram


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A seismogram is the record ofground movement detected by a

seismometer and portrayed on atime4versus4amplitude graph .Seismograms show how the ground

moves with the passage of time.


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Seismometers can detect local groundmovement caused by large distant events,as well as by the movement caused by large

and small events close to the seismometer.

7icroseisms can be caused byenvironmental

noise (wind, lightening, rain), motor

vehicles, and


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seismograph has $ sensors that record />S9 &>;9 and vertical motion on $ seismograph has $ sensors that record />S9 &>;9 and vertical motion on $seismograms. The dierent behavior o !9 S9 and surace waves explain howseismograms. The dierent behavior o !9 S9 and surace waves explain howa single seismograph station can have $ dierent seismograms( "@ Thea single seismograph station can have $ dierent seismograms( "@ Thevertical component shows the compressive ! wave bumping up romvertical component shows the compressive ! wave bumping up rom

beneathA it has very little horizontal movementA B@ The shearing S wave hasbeneathA it has very little horizontal movementA B@ The shearing S wave hasmostly side


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This 7S,S graphicThis 7S,S graphicshows typicalshows typicalseismogramsseismogramsrom ordinaryrom ordinaryearth3ua4es9 plusearth3ua4es9 plus

the signals that youthe signals that youmightmightsee rom othersee rom otherground


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SeismogramsSeismograms

recognized at Mountrecognized at MountSt. 2elens(St. 2elens(


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SeismographsSeismographs

Seismographs are devicesSeismographs are devicesthat record ground motionthat record ground motionduring earthquakes.during earthquakes.

he Grst seismographs were he Grst seismographs wereconstructed at the very endconstructed at the very endof the $2th century in talyof the $2th century in talyand Bermany.and Bermany.


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2ow Seismographs ;or4 2ow Seismographs ;or4

$. "arthquakes generate waves that$. "arthquakes generate waves that

travel through the earthtravel through the earth


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Moriontal $ kgHiechert seismograph in

Nagreb


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7odern digital broadband7odern digital broadbandseismographs are capable ofseismographs are capable ofrecording almost the wholerecording almost the whole

seismological spectrum (3seismological spectrum (3M > & s).M > & s). heir resolution of -% bits heir resolution of -% bits

(high dynamic range) allows(high dynamic range) allows

for precise recording of smallfor precise recording of smallquakes, as well asquakes, as well asunsaturated registration ofunsaturated registration ofthe largest ones.the largest ones.


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'etermining the Location o an'etermining the Location o an&arth3ua4e&arth3ua4e travel4time curve? a plot of seismic4travel4time curve? a plot of seismic4

wave arrival times against distancewave arrival times against distance

A travel time curve is used to determine the distance to an earthquake.

Oote that the time interval between the Grst arrival of and S waves

increases with distance from the epicenter.Seismogram P has a &4minute

interval between and S waves corresponding to a distance of -, km

from the epicenter, Q has an interval of / minutes, so the earthquakeoccurred 3,& km away, and N an interval of $- minutes, and is a distance

'etermining the Location o an'etermining the Location o an


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'etermining the Location o an'etermining the Location o an&arth3ua4e&arth3ua4e

Cocating an earthquake. he distance from each of three stations (Denver, St. Rohns, and Cim

determined from seismograms and the travel4time curves shown in Ggure $.2. "ach distanc

for the radius of a circle about the station. he location of the earthquake is +ust o5shore of

#ritish 9olumbia, where the three circles intersect.


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Measuring the Size o anMeasuring the Size o an&arth3ua4e&arth3ua4e intensity? a measure of an earthquake’s e5ect onintensity? a measure of an earthquake’s e5ect on

people and buildingspeople and buildings modiGed 7ercalli scale? scale e!pressing intensities ofmodiGed 7ercalli scale? scale e!pressing intensities of

earthquakes (+udged on amount of damage done) inearthquakes (+udged on amount of damage done) in

*oman numerals ranging from to P*oman numerals ranging from to P magnitude? a measure of the energy released duringmagnitude? a measure of the energy released during

the earthquakethe earthquake *ichter scale? a numerical scale of magnitudes*ichter scale? a numerical scale of magnitudes moment magnitude? an earthquake magnitudemoment magnitude? an earthquake magnitude

calculated from the strength of the rock, surface areacalculated from the strength of the rock, surface areaof the fault rupture, and the amount of rockof the fault rupture, and the amount of rockdisplacement along the faultdisplacement along the fault


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bservationalbservational

SeismologySeismology He are nowHe are now

equipped to startequipped to startrecording andrecording and

locatinglocatingearthquakes. 1or thatearthquakes. 1or thatwe need a seismicwe need a seismicnetwork of as manynetwork of as many

stations as possible.stations as possible. 7inimal number of7inimal number of

stations needed tostations needed tolocate the position oflocate the position ofan earthquakean earthquakee icentre is three.e icentre is three.

#road4band seismological stations in"urope


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bservationalbservationalSeismologySeismologyLocating &arth3ua4esLocating &arth3ua4es

o locate an o locate anearthquake we needearthquake we needprecise readings ofprecise readings of

the times when 4 andthe times when 4 andS4waves arrive at aS4waves arrive at anumber of seismicnumber of seismicstations.stations.

Accurate absoluteAccurate absolutetiming (with atiming (with aprecission of .$ s) isprecission of .$ s) isessential inessential inseismologyEseismologyE


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bservationalbservationalSeismologySeismologyLocating &arth3ua4esLocating &arth3ua4es

Tnowing theTnowing thedi5erence in arrivaldi5erence in arrivaltimes of the twotimes of the two

waves, and knowingwaves, and knowingtheir velocity, we maytheir velocity, we maycalculate the distancecalculate the distanceof the epicentre.of the epicentre.

his is done using the his is done using thetravel4time curvestravel4time curveswhich show how longwhich show how longdoes it take for 4 anddoes it take for 4 andS4waves to reachS4waves to reachsome e icentralsome e icentral


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bservationalbservationalSeismologySeismology

Locating &arth3ua4esLocating &arth3ua4es

Another e!ample ofAnother e!ample ofpicking arrivalpicking arrivaltimestimes


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bservationalbservationalSeismologySeismology

Locating &arth3ua4esLocating &arth3ua4es After we know theAfter we know the

distance of epicentredistance of epicentrefrom at least threefrom at least three

stations we may Gndstations we may Gndthe epicentre like thisthe epicentre like this

here are more here are moresoGsticated methodssoGsticated methods

of locating positionsof locating positionsof earthquake foci.of earthquake foci.

his is a classic his is a classice!ample of ane!ample of an inverseinverse

problem. problem.


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S&0SML,C S&0SML,C L)T0/, &DT2E7F&SL)T0/, &DT2E7F&S

TriangulationTriangulationOeed three di5erent seismicOeed three di5erent seismic

stations to locate anstations to locate anearthquake.earthquake.

7easure the 4S arrival times and7easure the 4S arrival times andconvert these to distance.convert these to distance.

riangulate using radius given by 4 riangulate using radius given by 4S travel times.S travel times.

riangulation for position of an riangulation for position of anearthquake near Oew 7adridearthquake near Oew 7adridusing stations in 9olumbus, Stusing stations in 9olumbus, StCouis and 7emphisCouis and 7emphis


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L ti & th 4Locating &arth3ua4es


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Locating &arth3ua4es


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60DST MT0/60DST MT0/


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Magnitude andMagnitude and

0ntensity0ntensityntensityntensity Mow Strong "arthquake 1eels toMow Strong "arthquake 1eels to

0bserver0bserver

7agnitude7agnitude *elated to "nergy *elease*elated to "nergy *elease Determined from Seismic *ecordsDetermined from Seismic *ecords

*ough correlation between the two for*ough correlation between the two forshallow earthquakesshallow earthquakes

3. 7agnitude and ntensity3. 7agnitude and ntensity


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0ntensity0ntensity

Mow Strong "arthquake 1eels toMow Strong "arthquake 1eels to0bserver0bserver

Depends 0n?Depends 0n?

Distance to UuakeDistance to Uuake BeologyBeology ype of #uilding ype of #uilding

0bserverE0bserverEaries from lace to lacearies from lace to lace 7ercalli Scale4 $ to $-7ercalli Scale4 $ to $-


b ti lb ti l


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bservationalbservationalSeismologySeismologySome statisticsSome statisticsMagnitudeMagnitude &ects /umber&ects /umber

per yearper year

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>less than -less than - Oot felt by humans. *ecorded by instrumentsOot felt by humans. *ecorded by instruments

only.only. OumerousOumerous-- 1elt only by the most sensitive.1elt only by the most sensitive.Suspended ob+ects swingSuspended ob+ects swing V$ V$

&& 1elt by some people. ibration like a1elt by some people. ibration like apassing heavy vehiclepassing heavy vehicle $ $

%% 1elt by most people. Manging ob+ects swing.1elt by most people. Manging ob+ects swing.Dishes and windows rattle and may breakDishes and windows rattle and may break $- $-

33 1elt by all< people frightened.1elt by all< people frightened.9himneys topple< furniture moves9himneys topple< furniture moves $ %$ %

anic. #uildings may su5er substantialanic. #uildings may su5er substantialdamagedamage $$

=4/=4/ Hidespread panic. 1ew buildings remainHidespread panic. 1ew buildings remainstanding. Carge landslides< Gssures in groundstanding. Carge landslides< Gssures in ground --

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9omplete devastation. Bround waves9omplete devastation. Bround waves

@-@-

b ti lb ti l


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bservationalbservationalSeismologySeismologySome statisticsSome statistics"quivalent"quivalent7agnitude7agnitude "vent"vent "nergy (tons O) "nergy (tons O)

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

-.-. Carge quary blastCarge quary blast $$

-.3-.3 7oderate lightning bolt7oderate lightning bolt 33

&.3&.3 Carge ligtning boltCarge ligtning bolt =3=3

%.3%.3 Average tornadoAverage tornado 3 $3 $

.. Miroshima atomic bombMiroshima atomic bomb - -

=.=. Cargest nuclear testCargest nuclear test &- &-

=.==.= 7t. Saint Melens eruption7t. Saint Melens eruption $ $

/.3/.3 Trakatoa eruptionTrakatoa eruption $ $

2.32.3 9hilean earthquake $29hilean earthquake $2 &- &-

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bservationalbservationalSeismologySeismologySome statisticsSome statistics

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bservationalbservationalSeismologySeismologySome statisticsSome statistics

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bservationalbservationalSeismologySeismologyMacroseismologyMacroseismology 7A9*0S"S70C0BQ deals with e5ects of7A9*0S"S70C0BQ deals with e5ects ofearthquakes on humans, animals, ob+ects andearthquakes on humans, animals, ob+ects andsurroundings.surroundings.

he data are collected by Geld trips into the he data are collected by Geld trips into theshaken area, andLor by questionaires sent there.shaken area, andLor by questionaires sent there.

he e5ects are then e!pressed as earthquake he e5ects are then e!pressed as earthquakeO"OSQ at each of the studied places.O"OSQ at each of the studied places.

ntensity is graded according tontensity is graded according to macroseismicmacroseismicscalesscales > 7ercalli49ancani4Sieberg (> 7ercalli49ancani4Sieberg (M)SM)S),),7edvedev4Sponheuer4Tarnik (7edvedev4Sponheuer4Tarnik (MSF MSF ), 7odiGed), 7odiGed7ercalli (7ercalli (MMMM), "uropean 7acroseismic Scale), "uropean 7acroseismic Scale((&MS&MS).).

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bservationalbservationalSeismologySeismologyMacroseismologyMacroseismology&uropean Macroseismic Scale ?&MS GH@&uropean Macroseismic Scale ?&MS GH@

&MS&MS '&60/0T0/'&60/0T0/ S2DT '&S)D0!T0/S2DT '&S)D0!T0/

00 /ot elt/ot elt Oot felt, even under the most favourable circumstances.

0000 Scarcely eltScarcely elt ibration is felt only by individual people at rest inhouses, especially on upper Foors of buildings.

000000 ;ea4;ea4 he vibration is weak and is felt indoors by a few people.eople at rest feel a swaying or light trembling.

0I0I LargelyLargely he earthquake is felt indoors by many people, outdoors by

veryobservedobserved few. A few people are awakened. he level of vibration is not

fright4 ening. Hindows, doors and dishes rattle. Mangingob+ects swing.

II StrongStrong he earthquake is felt indoors by most, outdoors by few.7any sleeping people awake. A few run outdoors.

#uildings tremble throughout. Manging ob+ects swing considerably.

&MS&MS '&60/0T0/'&60/0T0/ S2DT '&S)D0!T0/S2DT '&S)D0!T0/

>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


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>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>

I0I0 SlightlySlightly 1elt by most indoors and by many outdoors. 7any people indamagingdamaging buildings are frightened and run outdoors. Small ob+ects fall.

Slight damage to many ordinary buildings e.g. Gne cracks

in plaster and small pieces of plaster fall. I00I00 'amaging'amaging 7ost people are frightened and run outdoors. 1urniture isshifted and ob+ects fall from shelves in large numbers. 7anyordinary buildings su5er moderate damage? small cracks inwalls< partial collapse of chimneys.

I000I000 2eavily2eavily 1urniture may be overturned. 7any ordinary buildings su5er

damagingdamaging damage? chimneys fall< large cracks appear in walls and afew buildings may partially collapse.

0J0J 'estructive'estructive 7onuments and columns fall or are twisted. 7anyordinary buildings partially collapse and a fewcollapse completely.

JJ IeryIery 7any ordinary buildings collapse. destructivedestructive

J0J0 'evastating'evastating 7ost ordinary buildings collapse..

J00J00 )ompletely)ompletely ractically all structures above and below groundare

devastatingdevastating heavily damaged or destroyed.

bservationalbservational


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bservationalSeismologySeismology

MacroseismologyMacroseismology*esults of macroseismic surveysare presented on isoseismalmaps.

0soseismals are curves

connecting the places with sameintensities.

D0 O0 90O1XS" O"OSQ

AOD 7ABOXD"E Rust appro!imately, epicentralintensity is? Io K M Y -

0ne earthquake has +ust one


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Saest K Most 'angerousSaest K Most 'angerous

:uildings:uildings Small, Hood4frame Mouse 4 SafestSmall, Hood4frame Mouse 4 Safest

Steel41rameSteel41rame *einforced 9oncrete*einforced 9oncrete Xnreinforced 7asonryXnreinforced 7asonry

Adobe(Adobe(earthen bric4earthen bric4 made frommade fromearth and straw and dried by the sun)earth and straw and dried by the sun)4 7ost Dangerous4 7ost Dangerous

%. "arthquakes don’t kill people, buildings%. "arthquakes don’t kill people, buildings

kill peoplekill people


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Magnitude andMagnitude and

0ntensity0ntensityntensityntensity Mow Strong "arthquake 1eels toMow Strong "arthquake 1eels to

0bserver0bserver

7agnitude7agnitude *elated to "nergy *elease*elated to "nergy *elease Determined from Seismic *ecordsDetermined from Seismic *ecords

*ough correlation between the two for*ough correlation between the two forshallow earthquakesshallow earthquakes



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0ntensity0ntensity

Mow Strong "arthquake 1eels toMow Strong "arthquake 1eels to0bserver0bserver

Depends 0n?Depends 0n?

Distance to UuakeDistance to Uuake BeologyBeology ype of #uilding ype of #uilding

0bserverE0bserverEaries from lace to lacearies from lace to lace 7ercalli Scale4 $ to $-7ercalli Scale4 $ to $-



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S&0SML,C S&0SML,C M,/0T7'&M,/0T7'&

ScaleScale*icher scale*icher scale

#ased on the log to the base ten#ased on the log to the base tenof the amplitudes of aof the amplitudes of astandard seismometer $standard seismometer $

km from the earthquakekm from the earthquakecenter.center.

1or every ten fold increase in1or every ten fold increase inamplitude the *ichteramplitude the *ichtermagnitude increases by one.magnitude increases by one.

Mowever the energy release goesMowever the energy release goesup by a much large factorup by a much large factor

Uuick and easy to useUuick and easy to use

S&0SML,CS&0SML,C


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S&0SML,C S&0SML,C M,/0T7'&M,/0T7'&/omograph o the Dichter scale/omograph o the Dichter scale

*ead the di5erence in travel*ead the di5erence in traveltime between and Stime between and Swaves and plot.waves and plot.

*ead the ma!imum*ead the ma!imumamplitude of the S waveamplitude of the S waveand plot.and plot.

Draw a line between the twoDraw a line between the two

points and gives thepoints and gives themagnitude.magnitude.



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M,/0T7'&M,/0T7'&&arth3ua4es in the world each year.&arth3ua4es in the world each year.

"arth is shaken by millions"arth is shaken by millionsof quakes each year.of quakes each year.

7ost are too small to be7ost are too small to be

felt.felt.

o be damaging most o be damaging mosthave to be above .have to be above .

Above = they can causeAbove = they can cause

ma+or damage.ma+or damage.

Oote the decrease inOote the decrease infrequency as thefrequency as themagnitude increasesmagnitude increases



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M,/0T7'&M,/0T7'&)omparison o &arth3ua4es)omparison o &arth3ua4es

9omparison of Oorthridge, Coma9omparison of Oorthridge, Comarieta, San 1rancisco andrieta, San 1rancisco andSumatra -% earthquakes.Sumatra -% earthquakes.

9ompute seismic moment and9ompute seismic moment andthen convert to 7omentthen convert to 7oment7agnitude7agnitude

7oment is more reliable7oment is more reliablemeasure for largemeasure for largeearthquakes as it measuresearthquakes as it measures

directly the amount of straindirectly the amount of strainenergy released byenergy released bymovement along rupturemovement along rupturesurface.surface.

,D7/' MT0/,D7/' MT0/


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,D7/' MT0/,D7/' MT0/0M!DT/T )/)&!TS0M!DT/T )/)&!TScceleration and Desonancecceleration and Desonance

#uildings built to handle vertical forces such as weight but#uildings built to handle vertical forces such as weight butnot horiontal forces.not horiontal forces.

Acceleration compared to gravity 2./ mLsecAcceleration compared to gravity 2./ mLsec--

Acceleration 4Acceleration 4 .$g weak buildings break up.$g weak buildings break up

$g nothing stands up$g nothing stands up

*esonance 4*esonance 4

ground has natural frequencyground has natural frequency

buildings have natural frequencybuildings have natural frequency

$ story .$sL & story & secs L .$sec per$ story .$sL & story & secs L .$sec perstorystory

,D7/' MT0/,D7/' MT0/


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,D7/' MT0/,D7/' MT0/0M!DT/T )/)&!TS0M!DT/T )/)&!TS&arth3ua4e 0ntensity&arth3ua4e 0ntensity

Hant quantitative method of measuring e5ect ofHant quantitative method of measuring e5ect ofearthquakes.earthquakes.

Xse descriptive 7ercalli scaleXse descriptive 7ercalli scale

7a+or variables7a+or variables 4 7agnitude4 7agnitude

4 Distance from hypocenter4 Distance from hypocenter

4 1oundation materialsLslopes4 1oundation materialsLslopes

4 #uilding style4 #uilding style

4 Duration of shaking4 Duration of shaking

,D7/' MT0/,D7/' MT0/


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,D7/' MT0/,D7/' MT0/&DT2E7F& 0/T&/S0TC &DT2E7F& 0/T&/S0TC Mercalli ScaleMercalli Scale

"arthquake magnitude asses energy released by earthquakes."arthquake magnitude asses energy released by earthquakes.

"arthquake intensity measures the e5ect on people and buildings"arthquake intensity measures the e5ect on people and buildings

Xsed to asses historical recordsXsed to asses historical records

&DT2E7F& 2D'S&DT2E7F& 2D'S

n addition to ground shaking and surface faultingn addition to ground shaking and surface faulting


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n addition to ground shaking and surface faulting,n addition to ground shaking and surface faulting,earthquake haards includeearthquake haards include

SubmergenceSubmergence

ground motionground motion GreGre landslideslandslides liquefaction (a special type of ground failure)liquefaction (a special type of ground failure)

permanent displacement of the land surfacepermanent displacement of the land surface aftershocks? small earthquakes that follow the main shockaftershocks? small earthquakes that follow the main shock 1ires and Foods caused by breakage of water lines or dam1ires and Foods caused by breakage of water lines or dam

failures are also important.failures are also important.


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TS7/M0TS7/M02; )D&T&'2; )D&T&'

#iggest and most feared wave, harbor wave 4 Rapanese#iggest and most feared wave, harbor wave 4 Rapanese

Due to sub sea movements that disturb water mass, mostDue to sub sea movements that disturb water mass, mostoften earthquakes.often earthquakes.

Analogous to throwing a stone into a pool, ripples 4 tsunamiAnalogous to throwing a stone into a pool, ripples 4 tsunamiwaveswaves

eriod $ 4 min, Havelength $ 4 / km, amplitude $eriod $ 4 min, Havelength $ 4 / km, amplitude $

4- m, velocity -& mLsec (3 mph)4- m, velocity -& mLsec (3 mph)

At coast waves slow down and pile up and come on shoreAt coast waves slow down and pile up and come on shorelike a very rapidly rising tide.like a very rapidly rising tide.

TsunamisTsunamis


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TsunamisTsunamis tsunamis (seismic sea waves)?tsunamis (seismic sea waves)?

huge ocean wave produced byhuge ocean wave produced bydisplacement of the sea Foordisplacement of the sea Foor


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;orld 'istribution o;orld 'istribution o&arth3ua4es&arth3ua4es


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&arth3ua4es&arth3ua4es 9ircum4aciGc belt? the most important9ircum4aciGc belt? the most important

concentration of earthquakes which encircles theconcentration of earthquakes which encircles therim of the aciGc 0cean. Appro!imately /Z ofrim of the aciGc 0cean. Appro!imately /Z ofthe world’s shallow4focus quakes, 2Z of thethe world’s shallow4focus quakes, 2Z of theintermediate4focus quakes, and nearly $Z ofintermediate4focus quakes, and nearly $Z ofthe deep4focus quakes occur there.the deep4focus quakes occur there.

7editerranean4Mimalayan belt? a ma+or7editerranean4Mimalayan belt? a ma+orconcentration of earthquakes which runsconcentration of earthquakes which runsthrough the 7editerranean Sea, crosses thethrough the 7editerranean Sea, crosses the7iddle "ast and the Mimalayas, and passes7iddle "ast and the Mimalayas, and passesthrough the "ast ndies to meet the circum4through the "ast ndies to meet the circum4

aciGc belt north of AustraliaaciGc belt north of Australia

;orld 'istribution o;orld 'istribution o


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;orld 'istribution o;orld 'istribution o&arth3ua4es&arth3ua4es

;orld 'istribution o &arth3ua4es;orld 'istribution o &arth3ua4es


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#enio5 ones? distinct earthquake one that#enio5 ones? distinct earthquake one thatbegins at an oceanic trench and slopesbegins at an oceanic trench and slopes

landward and downward into the earth at anlandward and downward into the earth at anangle of about &angle of about &oo to to oo

island arc? #enio5 ones that slope under aisland arc? #enio5 ones that slope under acontinent or a curved line of islandscontinent or a curved line of islands

Strategies o &arth3ua4eStrategies o &arth3ua4e


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Strategies o &arth3ua4eStrategies o &arth3ua4e!rediction!rediction

Cengthy MistoricalCengthy MistoricalData #aseData #ase

MistoricalMistorical*ecords*ecords

aleoseismologyaleoseismology

Short4termShort4term

redictionrediction recursorsrecursors

Cong4term redictionCong4term rediction Seismic BapsSeismic Baps *isk Cevels*isk Cevels

7odeling7odeling Dilatancy 4Dilatancy 4

Di5usionDi5usion

Stick 4 SlipStick 4 Slip Asperities (kinks)Asperities (kinks) 9rack ropagation9rack ropagation

=. redicting earthquakes is not yet=. redicting earthquakes is not yetpossiblepossible


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&arth3ua4es at 'ivergent :oundaries&arth3ua4es at 'ivergent :oundaries

di t b d i h l tdi t b d i h l t


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divergent boundaries? where platesdivergent boundaries? where platesmove away from each othermove away from each other

the rock motion that is deducedthe rock motion that is deducedfrom Grst4motion studies showsfrom Grst4motion studies showsthat the faults here are normalthat the faults here are normal

faults, parallel to the rift valleyfaults, parallel to the rift valley

&arth3ua4es at Transorm :oundaries&arth3ua4es at Transorm :oundaries transform boundaries? wheretransform boundaries? where


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transform boundaries? wheretransform boundaries? whereplates move horiontally past eachplates move horiontally past each

otherother Grst4motion studies indicate strike4Grst4motion studies indicate strike4

slip motion on faults parallel to theslip motion on faults parallel to the

boundaryboundary

#road band of earthquakes along a system of parallel faults

Oarrow band of shallow4focus earthquakes shown as stars along single fault


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&arth3ua4es at )onvergent&arth3ua4es at )onvergent:oundaries:oundaries convergent boundaries? where platesconvergent boundaries? where plates


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convergent boundaries? where platesg pmove toward each othermove toward each other

two general types 4 one marked by thetwo general types 4 one marked by thecollision of two continents, the othercollision of two continents, the othermarked by subduction of the ocean Foormarked by subduction of the ocean Foorunder a continentunder a continent

&ngineering&ngineeringi lS i l


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"arthquakes are the only naturaldisasters that are mostlyharmless to humansE he onlydanger comes from buildings

designed not to withstand thelargest possible earthquakes inthe area.

&ngineering seismology

provides civil engineersparameters they need in order toconstruct seismically safe andsound structures.

&ngineering seismology is a mit, urkey, $222mit, urkey, $222

&ngineering&ngineering


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n order to estimate the parameters,seismologists need?

9omplete earthquake catalogues thate!tend well into the past,

nformation on the soil structure andproperties at the construction site, as well

as on the path between epicentre and thesite,

*ecords of strong earthquakes and smallevents from near4by epicentral regions,



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g gg gSeismologySeismology

9omplete and9omplete andhomogeneoushomogeneousearthquake cataloguesearthquake catalogues

are of paramountare of paramountimportance in seismicimportance in seismichaard studies.haard studies.

Seismicity of 9roatia afterthe 9roatian "arthquake

9atalogue that lists over

&ngineering Seismology&ngineering Seismology


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SSoil amplicationoil amplication

AmpliGcation of seismic wavesin shallow soil deposits maycause e!tensive damage evenfar away from the epicentre. tdepends on? hickness of soil above

the base rock, Density and elastic

properties of soil, 1requency of shaking,

he strength of

Spectral ampliGcation alonga proGle in hessaloniki ,

Breece

0ntensity and :edroc4 'epth in0ntensity and :edroc4 'epth in


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0ntensity and :edroc4 'epth in0ntensity and :edroc4 'epth inSan 6rancisco9 "G#NSan 6rancisco9 "G#N


OO!hysical-!hysical-


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yySeismologySeismology

0ur knowledge about0ur knowledge aboutthe structure of thethe structure of the"arth deeper than"arth deeper than

several km was gainedseveral km was gainedalmost e!clusively usingalmost e!clusively usingseismological methods.seismological methods.

Seismologists useSeismologists useseismic rays to look intoseismic rays to look intothe interior of the "arththe interior of the "arthin the same way doctorsin the same way doctors

use P4raysuse P4rays

TS7/M0TS7/M0


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TS7/M0TS7/M02; )D&T&'2; )D&T&'

#iggest and most feared wave, harbor wave 4 Rapanese#iggest and most feared wave, harbor wave 4 Rapanese

Due to sub sea movements that disturb water mass, mostDue to sub sea movements that disturb water mass, mostoften earthquakes.often earthquakes.

Analogous to throwing a stone into a pool, ripples 4 tsunamiAnalogous to throwing a stone into a pool, ripples 4 tsunamiwaveswaves

eriod $ 4 min, Havelength $ 4 / km, amplitude $eriod $ 4 min, Havelength $ 4 / km, amplitude $

4- m, velocity -& mLsec (3 mph)4- m, velocity -& mLsec (3 mph)

At coast waves slow down and pile up and come on shoreAt coast waves slow down and pile up and come on shorelike a very rapidly rising tide.like a very rapidly rising tide.

TS7/M0TS7/M0/TL:L& TS7/M0 0/ D&)&/T T0M&S/TL:L& TS7/M0 0/ D&)&/T T0M&S


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/TL:L& TS7/M0 0/ D&)&/T T0M&S/TL:L& TS7/M0 0/ D&)&/T T0M&S

7any tsunami’s in the aciGc.7any tsunami’s in the aciGc.

7ainly from sea bottom motion during earthquake.7ainly from sea bottom motion during earthquake.

elocity K [(g ! D) where D is the depth of the ocean inelocity K [(g ! D) where D is the depth of the ocean inmeters. K -& mLsecmeters. K -& mLsec

7oves quite slowly relative to and S waves7oves quite slowly relative to and S waves

TS7/M0TS7/M0


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&DLC ;D/0/, SCST&M&DLC ;D/0/, SCST&M

, S and surface waves move at between $ 4 3.3 kmLsec., S and surface waves move at between $ 4 3.3 kmLsec.

7uch faster than tsunami velocity of -& mLsec. #ody waves arrive7uch faster than tsunami velocity of -& mLsec. #ody waves arrivemuch sooner than the tsunami waves.much sooner than the tsunami waves.

Mave enough time to determine sie and position of earthquake beforeMave enough time to determine sie and position of earthquake beforethe tsunami arrives.his is the basis of the early warning systemthe tsunami arrives.his is the basis of the early warning system ..


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&arth3ua4es( 6acts and 6iction&arth3ua4es( 6acts and 6iction 6iction(6iction( "arthquakes usually happen in the morning."arthquakes usually happen in the morning.

h k h i b h h d d h


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6act(6act( "arthquakes happen in both the day and the"arthquakes happen in both the day and thenight. here is no pattern.night. here is no pattern.

6iction(6iction( here is such a thing as \earthquake weather.\ here is such a thing as \earthquake weather.\

6act(6act( here is no connection between earthquakes and here is no connection between earthquakes andweather. *emember, earthquakes happen deep in theweather. *emember, earthquakes happen deep in theearth, far away from the weatherEearth, far away from the weatherE

6iction(6iction( "arthquakes are on the increase."arthquakes are on the increase.6act(6act( t may seem like we’re having more earthquakest may seem like we’re having more earthquakesbecause there are more reporting stations, but the truthbecause there are more reporting stations, but the truthis we’re not.is we’re not.

6iction(6iction( He can prevent earthquakes from happening.He can prevent earthquakes from happening.6act(6act( Oo. Qou can protect yourself by doing things toOo. Qou can protect yourself by doing things tosecure buildings, like your home, but earthquakes can’tsecure buildings, like your home, but earthquakes can’tbe prevented 44 or predicted.be prevented 44 or predicted.


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Overview Seismology - II 2

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Transcript of Overview Seismology - II 2