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Ouake!Epicenters& Magnitude
StudentGuide
OVERVIEW
youwill useseismograms
In thisactivity,
includinghowto
and othertoolsto learnmoreaboutearthquakes
locatean epicenter
usinga techniquecalledtriangulation.Youwillalsohavean opportunity
to determine
the approximate
magnitude
of severalearlhquakes
obtainedfromseismograms.
usinginformation
OBJECTIVES
Duringthis activity,you will:
.
Examineseismograms of the same earthquakerecordedat three differentstationsand identifyPwave and S-wave data.
.
Determinethe S-P wave lag time for each seismogram.
.
Determineseismograph station distancesusing either lhe Earthquake P-wave and S-wave Travel
Time graph or the Distance for Epicenter VS. TravelTime for Local Earthquakes gra ph.
.
Determinethe location of two earlhquakeepicentersusingthe triangulationtechnique.
.
Determinethe magnitude of an earthquakeusingdistanceand amplitudedata and the Nomogram
for Richter Magnitude.
MATERIALS
Map of California
Pencil
Compass
Colorpencils
StudentGuide(pagesS1-S9,\ru1-W7)
BACKGROUND
WhatCausesan Eafthquake?
The Earthsystemcan generallybe thoughtof in termsof the threeprimarystatesof matter,solids,liquids,
and gases.The atmosphere
is an envelopof g.ases
the Earth.The hydrosphere
containsthe
thatsurrounds
waternecessary
for life.The lithosphereis the rockyshellcoveringthe surfaceof the Earth.However,
rather
thanone continuous
shell,this rockycoveringis brokenintoabout20 piecesof differentsizes.Thesepieces
are referredto as plates
The Earth'splatesare in motion.Platemotionsproduceearthquakes
as platesare pushedtogether,pulled
apart,or slidepasteachother. Unlesstheyare movingin the samedirectionandwiththe same speed,
conlactbetweenplatesalongplate marginscausesstressas a resultof frictionto buildup in the regionof
contact.As stressbuilds,cracks or
fracturesdevelop.When the stress
exceedsthe frictionalforce between plates,energyis released as
rocks on eitherside of the f raclure
move.The fractureis now called a
fault.A fault is a fracturealong
which there is rock movement.
See Figure.1.
Figure 1
:,.ii;.';.:+o;.
-:,,ij;r; l,ij1.:r
p.s 1
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The strengthof an earthquakeis directlyproportional
to
the amountof energyreleased as the rocksmove alonga
fault.The site of energyrelease withinEarth'scrust is
referredto as the focus. The pointon the Earlh'ssurface
directlyabovethe focus Cscalledthe epicenter. See
Figure 2.
Figure 2
Energy releasedat the focus of an earthquakeradiatesin
all directionsand is transmittedas waves.Stations
locatedaroundthe world have specializedmachines
called seismographs that recordearthquakewavesthat
reach these station.See Figure3.
Figure 3
SeismographsrecordeaGhquakewaves
receivedat the seismicsta.tionproducinga
physicalrecordcalleda seismogram. See
Figure4. A seismogramis a seriesof horizontal
lines.Eachhorizontalline represents
a 15like
is
read
a
book,
from top
minuteintervaland
to bottomand left to right- Groundmotionas the
resultof earthquakewaves appearsor is
recordedon the seismogramas a seriesof
to the
zigzagor verticallinespe rpendicular
timeline.
Figure
Types of Waves
As indicatedearlier,energyis releasedat the siteof an earthquake
andthe energyis transmittedin all
directions
throughthe crust in the formof energywaves.Theseenergywavesare classifiedintotwo general
categories:
bodywavesand surfacewaves.Bodywavestravelthroughthe crustand t owardthe interiorof
the Earth.Surfacewaves onlytravelalongthe surfaceof the Earth.Eachwavecategory-bodyandsurface-is madeup of two distincttypesof waves.
Body Waves
Bodywavesin the forms of primary(P) and secondary
(S)wavesprovidedatacritical{or determining
the
distanceof a seismograp
h stationfroman epicenter.
areshown
Summaries
o{ P andS-wavecha racteristics
in Figures5 and 6 below.
Figure 5: P-wavesare longitudinalor compression
(push-pull)energywaves that travelparallelto the
directionof transmission-These are the first
earthquakewaves receiwedat an eafthquake
recordingstation.Travelingat speeds ranging
between6 and 13 km/sec, P-wavesare transmitted
throughboth solidsand liquids.
Figure 6: S-wavesare transverseor shearenergy
wavesthat travelperpendicular(likean ocean
wave)to the directionof transmission.These are the
secondwaves receivedat an earthquakerecording
station.Travelingat speeds ranging between3.5
and 7.5 km/sec,S-wavesare only transmitted
throughsolids.
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crust
These body wave characteristics
lead scientiststo inferthe
structureof Earth'sinterior.See Figure7,
Surface Waves
Becausethey travelalong Earth'ssurface,surfacewaves in the
forms of Love and Raleighwaves are often the most damaging
earthquakewaves.Summariesof Love and Rayleighwave
characteristics
are shownin FiguresB and 9 below.
Figure 7
Figure 8: Love wavesare shearingwaves that travel
horizontalto the Earth'ssurface.Namedfor the
Englishmathematician,
A.E.H.Love,who createda
mathematical
modelof this seismicwave type in
1911, Love waves travelat speedsrangingf rom two
to six kilometersper second.
Figure 9: Rayleighwaves are rollingwavesthat
causethe surfaceof the Earthto appearto ripple,
heave,or surge.Narned for the Englishscientist,
Lord Rayleigh,who predictedthe existenceof this
seismicwave type in 1885,Rayleighwavestravelat
speedsrangingfromrone to five kilometersper
second.
PA R T 1 : R E A D IN GA S E IS MOGRAM
Activity 1: Workingwith P-wavesand S-waves.
lineon a
A seismogram
is a recordof groundmotion.Zerogroundmotionwouldappearas a horizontal
frommany
is mostunlikely.Vibrations
seismogram.
However,
a perfectlyhorizoniallineon a seismogram
at a seismographstationand appearas minor
sourcesincludingpassingcarsandtrucksare alsoreceived
bumpsor disturbances
on a seismogram.
lmageyou are seatedat a
In contrast,earthquake
wavesappearas muchlargerbumpsor disturbances.
tableslowlydrawinga horizontal
lineon a largesheetof paperwhenyourfriend walksby and shakesthe
the paper hadmoved.The sizeof the
table.Changesin yourlinedrawingsuggestthatthe 'ground'beneath
line.Seismograms
to your horizontal
tableshakewouldmostlikelybe reflectedin the sizeof the disturbance
causedby earthquakewavesappearas signifiare linedrawingsthat recordgroundmotion.Disturbances
cantchangesin the linedrawingof a seismogram.
Becauseprimarywaves travelfasterthanall other
is a recordof the primarywaveemitted
disturbance
on a seismogram
seismicwaves,the firstsignificant
The secondarywavearrivesafterthe primarywave.Howlongit takesfor
fromthe focusof an earthquake.
the secondarywave to arriveis directlyproportional
to the distancebetweenthe seismographstationand
the eafthquake'sepicenter.Due to the way these
waves are transmitted-perpendicular
to the direction of propagation-thereis often greaterground
motion associatedwith this type of body wave
resultingin a much a largerdisturbance.SampleP
and S-wavesare indicatedin Figure10.
Figure
p.s 3
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Step 1: Locate and mark the P-wavesand S-waveson the seismogramsfor stationsA, B, and C o n page
w2.
Step 2: Determinethe arrivaltimesof P-wavesand S-waves.
Step 3: Draw a verticallinefromthe peakof the P-waveandthe peakof the S-waveto timescaleabove
each seismogram.
Step 4: Estimatetheapproximate
positionsat whichyourvertically
drawnlinesintersect
the times cales.
R emember,
lhereare60 secondsin a minute.Tryto estimateas accurately
as possible.
Add your
rninuteandseconddeterminations
for eachwaveto the startingtimeof 4 hours23 minuteszero
seconds(04:23:00).
Forexample,a verticallinedrawnfromthe P-waveat StationA intersectsthe
scale at approximately
5 minutes40 seconds.Adding5 minutes40 secondsto the starttirne of
produces
4:23:OO
a P-wavearrivalat StationAof 4:28:40.Thistimehasbeenrecordedfor you in
Table1 on W1.
Determinethe S-wavearrivaltime for StationA. Recordthis time in the appropriatespace in Table1.
Determine the P-waveand S-wavearrivaltimesfor StationsB and C. Recordthese data in Table 1
on W1.
Step 5: Determinethe lag time betweenthe arrivalof the P-waveand the arrivalof the S-wave.
Lag time is the differencebetweenthe arrivalof the P-waveand the arrivalof the S-wave.To determ ine lag time for each seismogram,subtractP-wavearrivaltime from S-wavearrivaltime. An
example is shown below.Note:These are not actualdata from this investigation.
This is sirnply an
example.
S-arrivaltime= 10:03:05 10:02:65(noticethat60 secondshadto be borrowedfrom
= 10:02:10- 10:02:10the minutes
P-arrivaltime
to do thesubtraction)
ANSWER= 55 seconds
Determinethe lagtimesfor the seismograms
on pageW2. Recordthe lagtimefor eachseismogramin the
appropriatespacein Table1 on pageW1.
Activity 2: Determiningdistancefrom seismographstation to epicenter
As indicated earlier,there is a direct relationshipbetweenlag time and distance.The EarthquakeP-wave
and S-wave Travel Time graph on page W3 can be used to estimate distanceswhen distances (krn) between the epicentersand seismographstationsare large.
Example1: lf a P-wavearrives5 minutesand 40 secondsafteran earthquake
happens,howfar is the
seismographstationfromthe epicenter?
Locate the place on the Earthquake P-wave and S-wave Travel Time graph (page W3), whe re the
time delay betweenthe P-wave and S-wave (alonga line perpendicularto the x-axis) is 5 minutes and
40 seconds. Read verticallyfrom those pointsdown to the x-axis.The distancefrom the seismograph
stat ion to the epicenteris approximately4000 km. Recordthe distancein the appropriatesp ace in
Table 1 on W1.
Tip: Studentscan mark a piece of scrap paper so it representsthe correcttime lag by placing it along
the y-axis.Then slide the scrap paper along the curves until the gap matchesthe gap between the S
and P curyes.Drop verticallydown to thex-axisand read the distance.
Determinethe distancesbetweenstationsB and C andthe epicenterusingthe datafromTable1 and the
EarthquakeP-waveand S-waveTravelTimegraphon pageW3. Recordthesedistancesin the appropriate
spacesin Table1.
p.s 4
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Activity 3: Finding the epicenter
fromthe focus.So whilea seismogram
from a singlestationprovides
Seismicwavesradiatein all directions
information
aboutdistancefromthe stationto an epicenter,
by itself, it cannotbe us ed lo locatean epicenter.
ln otherwords,determining
a distanceof 3000km meansa 30OO
km radiusin any direction.
Seismologists
usea techniquecalledtriangulationto pinpointthe locationof an epicenter.Thistechnique
stationto an
usesdatafromat least threeseismograph
froma seisrnograph
stations.Oncethe distance
epicenterhasbeendetermined,
a compassis usedto drawa circlearoundeachstationwherethe radiusof
Thisprocess is repeatedusingdata
the circleis scaledto the mapdistancefromthe stationto the epicenter.
fromat leasttwo more stations.
The locationof the epicenter
of the earthquake
that producedthe datais
approximately
wherethe threecirclesintersect.
See Figure11.
Step 1: Convertthe distance data in kilometersin
Table 1 to cent imeters.
o Determinethe map scaleof the map on page W4.
To do this, measure the lengthof the map scale
bar shown on the rnap in centimeters.
o Set up a ratio using the map scalethat you determinedand your distance data from Table1.
. Solvefor X
Example:lf the map scaleis 1 cm = 500 km and
the distancebetween the seismographstationand
epicenteris 2000 ksn, then
=
1cm
X
500 km
2000 km
Figure11
Station A
Station B
o
Epicenter
Statitn C
o
2000cmokrn = 4cm
500 lffi
Convertthe distance data from kilometersto
centimetersfor StaEionsA. B. and C.
.
Recordthe mapdistancesin centimeters
for eachstationin the lastcolumnin Table 1.
Step 2: Set one holeof, the safetycompassto the distance(cm)in Table1 for StationA.
StationA on the
Step 3: Placethe intersection
of the crosshairs
of the compasson the pointrepresenting
mapon pageW4.
on the compass.WhilepressStep 4: Inserta sharppencilpointintothe holenextto the set measurement
ingfirmlyon the crosshairs
overthe stationlocation,drawa completecircle aroundStationA. The
radiusof the circleshouldequalthe calculated
mapdistance(cm)fromStationA to the epicenter
fromThble1. (Hint:Thisis easierif you removethe mapfromyourStudentGuideand drawon the
mapas a singlesheetof paperwithoutotherpapersunderit.)
Step 5: Repeatsteps2 thru4 generating
circlesaroundStationB and StationC on the mapon pageW4
usingappropriatedistancedatafromTable1.
Note:Tl'rethreecirclesmaynot
Step 6: Locatethe epicenteron the mapwherethe threecirclesintersect.
Thisresultsfroma numberof
intersectat a s-nglepointbut mayforma small,closedtriangle.
whenthis
errors.Generally,
factorsincluding measurement,
and (or)calculation
timeapproximation,
usethe
occurs,the epicenteris estimated
to be at the centerof the triangle.However,seismologists
that help themto locatean
triangulation
techniquewiththe aid of specialcomputerprograms
earthquake's
epicentermoreaccurately.
Step 7: Recordthe latitudeand longitude
of the epicenterin Table1.
p.S5
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PARTII: MAGNITUDE
Magnitude
is a rneasureof the strengthof an earthquake.
Scientists
as a tool
usemeasuresof magnitude
for comparing
earthquakes.
TheMercalliscaleand Richterscalearetwo commonmagnitude
scales.The
Magnitude/lntensity
tableon page57 providesa generaloverviewof thesetwo scalesin termsof observablephenomena,relativeamountsof energyreleased,
of
andfrequency
of occurrencesof earthquakes
differentmagnitudes.
The MercalliIntensity Scale
Thisis a qualitative scaledeveloped
by ltalianvolcanologist
GiuseppeMercalliin 1902.Thisscaleuses
phenomenaexperienced
observable
by peopleto estimateearthquake
strength.Becauseit is basedon
whatis seenand feltas wellas theamountof damagecaused,an estimationof thestrengthof any given
earthquake
using thisscalemay,at times,be eitheroverestimated
Excessive
damage
or undereslimated.
resultingin an overestimation
of the intensity
of an earthquake
maybe the resultof oneor severalfactors
suchas buildingdesign,distancefromthe epicenter,
andtypeof surfacematerial(rock,soil,sand)that
structures
are built on. In addition,
the MercalliScaleis difficultto applyin rural or scarcelypopulated
areas.
The modifiedMercallilntensity
Scalewasdevelopedin 1931by twoAmericanseismologists,
HarryWood
and FrankNeumann.Thisscaleis stillan arbitraryscaleand is moremeaningfulfor a nonscientist.
The Richter Scale
Thisis a quantitativescaledeveloped
by Americanseismologist
CharlesRichterandGermanseismologist
BenGutenbergin 1935.The Richterscaleis calculated
fromthe amplitude
of the S-waveand is basedon a
logarithmic
scale. Thismeansthateachwholenumbervalueon the Richterscale represents
a ten-fold
increaseabovet he previouswholenumbervalue.Forexample,a magnitude
is 10 times
5 earthquake
strongerthana magnitude4 earthquake,
100timesstrongerthana magnitude3 earthquake,
and 1000
timesstrongerth an a magnitude
2 earthquake.
Nomogram
A nomogramis one toolthatcanbe usedto determine
the magnitude
of an eadhquake.lf the distance
betweenthe seismographstationandepicenter
(mm)of the largestwave,oftenthe Sandthe amplitude
wave,are known,thenthe magnitude
of an earthquake
The nomogram
was developed
can be determined.
usingthe Charles Richter'sequations.Magnitudes
determinedusing a nomogramare Richtermagnitudes.
See Figure13.
Figure 13
=--=----------T=-N
I
:l
AMPLIIT.JDF
Determinethe magnitudesof earthquakesindicatedby
the seismograms for Las Vegas, Nevada and Tokyo,
Japan shown on page W2.
Step 1: ldentify the P-waveand S-wavein each
seismogram.
-9
=t*{*r*l
I
qnn
100
400
Ai
50
300
Step 2: Determine the lag time for each seismogram.
Record these data in Table2 on page W1 .
QN
200
10
5
100
Step 3: Determine the distanceof each seismograph
stationf rom their respectiveepicentersusing
the Distance from Epicenter VS. Travel Time
for Local Earthquakesgraph on page W6.
Note:This is a magnifiedportionof the
distance-timegraphthat you used earlier.
Record distance data in the appropriate
spaces in Table2 on page W1 .
2
bU
1
40
v.t
20
.1
5
DISTANCE
(km)
S- PTIM E
0
r\GAGNITUDE
Ab,IPLITUDE
(mm)
( s)
Example:
The amplitudeoJ the strongestwave is 23 mm.
The S-P lag time is 25 s.
This earthquakehad a magnitudeof 5.
p.s6
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Step4: Usingthe scale(rnm)shownon the rightsideof each seismogram,
determine
the amplitude
(beginning
at O mm) of the S-wavein eachseismogram.Recordthesedatain T-able 2 on pageW1.
Step5: ldentifyand plotthe distance(km)fromthe LasVegasstationto the epicenter
on
on the nomogram
pageW5.
Step 6: ldentifyand plotthe amplitudeof the S-wavereceivedat the LasVegasstationon the nomogram
on pageW5.
points
Step7: Usethe straightedge of the safetycompassor a rulerto connectthedistanceand amplitude
crossingthe magnitudescalein betweenthetwo scales.Recordthe Richtermagnitudeof the Loma
Prietaearthquakeas indicatedby data receivedat the Las Vegasstationin the ap propriatespacein
Table2 on pageW1.
Step8: Usethe tableon page 57 to determine
the MercalliIntensity
of the LomaPrietaearthquake.
Step9: Repeatsteps5 thru 8 to determine
the Richterand MercalliIntensity
of the Kolb, Japan earthquake
produced
thatresultedin the seismogram
stationat the Tokyo,Japanrecording
PARTlll: CALIFORNIAACTIVITY
youwill pull logetherwhatyou'velearnedup to this pointas youlocatethe epicenterand
In thisactivity,
determine
the magnitude
o{ an earthquake
thatoccurredin California
on 10August2001.
Theseismograms
usedin this activityare reproductions
of actualseismograms
froman actual earthquake
event.
Step 1: Usingthe coordinatesshown,plotand labeltherecordingstationson the 11"x 1 7" mapof
California.
Latitude
Station
Longitude
HOP S
3 8 "5 9 ' N
123'4'W
MODS
41' 53' N
120'17'W
KCC
37"56'N
119"19'W
Step2: Examinethe seismogramson pageW7 for the bulletedinformation
shownbelow.Recordall datafor
eachseismogram
in Table3 on pageW5.
.
.
.
.
ldentifyP-waveand S-wavearrivaltimes
for each seismogram.
Determine
lag tirne.
Determine
graphdistance usingthe appropriate
Usingthe map scale shownon the California
from
map, convertdistancemeasurernents
kilometersto centimeters.
Step 3: Usethe distancerneasurements(cm)and the safety compassto drawepicentercircles aroundthe
appropriate
recording stationusingthe sameprocedurethatyou usedin Partl.
Step4: ldentifythe latitudeand longitude
thatoccurredon 10 August
of the epicenterof the earthquake
2001.Recordthese coordinates
in the appropriatespacesin Table3.
P.SB
sK04725-00
Step 5: To determine magnitudeof this earthquake:
. Selectone seismogram
(HOPS,KCC,or MODit'syourchoice)
. Determinethe distancebetweenthe recording
stationandthe epicenter;thisis alreadyrecorded
in Table3.
. Plot thatdistanceon the nomogram
on pageW5.
. Extendthe horizontal
justbeforethe Plineor equilibrium
linethat is presenton the seismogram
lineactsas your
Thishorizontal
wave arrives.Extendthishorizontal
linethroughthe seismogram.
on pageW2.
eq uilibriumor zerolinesimilarto the zerolineindicatedin the seismograms
. Measurethe distance,in millimeters,
fromthe zerolineto thetallestS'wavepeak.
. Plot thisdistanceon the nomogram
on pageW5.
. Connectthe distancepointwiththe amplitude
point.The pointwh erethe linecrossesthe
of this
magnitudescaleis the magnitude
Recordthe Richtermagnitude
of thisearthquake.
ea rthquakein the appropriate
spacein Table3.
. Use the charton page57 to determine
the magnitude
of thisearthquakeeventin termsof the
MercalliIntensityscale.Recordthe Mercallimagnitude
in the appropriatespacein Table3.
thatoccurredin
Step6: Researchthe name,locationof the epicenter,
of the earthquake
and magnitude
Californiaon 10August2001. Writeat leastone paragraph
describingthisevent.Don'tforgetto cite
your references.
p.se
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