Engineering Geology_Earthquakes: Causes and Effects

An Earthquake is sudden motion or trembling of the earth, caused by the abrupt release of energy that is stored in rocks.

1. Earthquakes
Lecture Notes by :
Francis W.

2. what is earthquake ?
AnEarthquakeissuddenmotionortremblingoftheearth,
causedbytheabruptreleaseofenergythatisstoredinrocks.
Thetermearthquakereferstoanyevent,naturalorartificial,
thatcausesseismicwaves.
AnEarthquakeisanaturalvibrationoftheground(orthe
Earth’sCrust)producedbyforcescalledearthquakeforces
orseismicforces.
ThescientificstudyofearthquakesiscalledSeismology.
Seismicwavesarerecordedoninstrumentscalled
seismographs.
Thetime,locations,andmagnitudeofanearthquakecanbe
determinedfromthedatarecordedbyseismographstations.

3. what is earthquake ?
Earthquakes are natural
disasters that have been
affecting the world for
millions of years. They are
caused by the movement of
tectonic plates, which make
up the Earth's crust.

4. what is earthquake ?
Earthquakes can range from
being barely noticeable to
extremely powerful and
destructive. In this article,
we will delve into the details
of earthquakes, including
what causes them, how they
are measured, and what can
be done to prepare for and
reduce the impact of
earthquakes.

5. the four main causes of earthquakes
collapse of rock
formation
underground
explosions
slipping of
tectonic
plates
21 3 4
volcanic eruptions

6. what causes earthquakes??
Earthquakes occur when
two tectonic plates grind
against each other,
causing a release of
energy in the form of
seismic waves. This
energy can cause the
ground to shake, which
is what we experience as
an earthquake.
Hottectonic plates are
huge sections of the
Earth's crust that move
slowly over time, often
colliding with each other
at their edges (plate
margins). When two
plates collide, the edge
of one plate can get stuck
and build up pressure
until it finally gives way,
releasing the energy in
the form of an
earthquake.

7. what causes earthquakes??
Inadditiontoplatetectonics,earthquakescanalso
becausedbyhumanactivitiessuchasunderground
nucleartesting,thepumpingofwaterintooroutof
theground,andtheextractionofoilorgasfromthe
ground.Thesehumanactivities-causedearthquakes,
knownasinducedearthquakes,canbejustas
powerfulasearthquakescausedbyplatetectonics.

8. types of earthquakes

10. types of earthquakes
There are many different types of earthquakes: tectonic,
volcanic, and explosion. The type of earthquake depends
on the region where it occurs and the geological make-up
of that region.

11. types of earthquakes

12. types of earthquakes
These occur when rocks in
the earth's crust break due
to geological forces
created by movement of
tectonic plates.
Tectonic
earthquakes

13. types of earthquakes
Tectonic
earthquakes

14. types of earthquakes
occursin conjunction with
volcanic activity.
Volcanic
earthquakes

15. types of earthquakes
volcanic
earthquakes

16. types of earthquakes
result from the explosion
of nuclear and chemical
devices.
Explosion
earthquakes

17. types of earthquakes
Explosion
earthquakes

18. types of earthquakes
are small earthquakes in
underground caverns and
mines.
Collapse
earthquakes

19. types of earthquakes
collapse
earthquakes

20. Classification of earthquakes

21. Seismic waves
•SeismicwavesarewavesofenergythattravelthroughtheEarth'slayers,andarearesultofearthquakes,
volcaniceruptions,magmamovement,largelandslidesandlargeman-madeexplosionsthatgiveoutlow-
frequencyacousticenergy.
•Seismicwavesarestudiedbygeophysicistscalledseismologists.
•Seismicwavefieldsarerecordedbyaseismometer,hydrophone(inwater),oraccelerometer.
•The propagation velocity of the waves depends on density and elasticity of the medium.
•Velocity tends to increase with depth and ranges from approximately 2 to 8 km/s in the Earth's crust, up to
13 km/s in the deep mantle.

22. Anatomy of an Earthquake

23. Anatomy of an Earthquake
•Althoughthegroundbeneathusseemssolid,it’sactuallyinconstantmotion.Weusuallydon’tseeit,but
wecanexperienceitthroughearthquakes.
•Earth'scrustisbrokenupintoirregularpiecescalledtectonicplates.Therearesevenmajorplatesand
manysmallerones,allmovinginrelationtoeachother.Thelithospherereferstothecrustandupper
mantlethatmakeuptheseplates.
•Astectonicplatesmovepasteachotheralongfaultzones,theysometimesgetstuck.Pressurebuilds,and
whentheplatesfinallygiveandslip,energyisreleasedasseismicwaves,causingthegroundtoshake.
Thisisanearthquake.
•ThefocusistheplaceinsideEarth’scrustwhereanearthquakeoriginates.
•ThepointontheEarth’ssurfacedirectlyabovethefocusistheepicenter.
•Whenenergyisreleasedatthefocus,seismicwavestraveloutwardfromthatpointinalldirections.
•Therearedifferenttypesofseismicwaves,eachonetravelingatvaryingspeedsandmotions.It'sthese
wavesthatyoufeelduringanearthquake.

24. Anatomy of an Earthquake
▪Fault:AfractureintherocksthatmakeuptheEarth’scrust
▪Focus(Hypocenter):ThepointwithintheEarthwhereanearthquakerupturestarts
▪Epicenter:ThepointatthesurfaceoftheEarthabovethefocus
▪Plates:MassiverocksthatmakeuptheouterlayeroftheEarth’ssurfaceandwhosemovementalong
faultstriggersearthquakes
▪Seismicwaves:Wavesthattransmittheenergyreleasedbyanearthquake

25. Types of seismic waves
Amongthemanytypesofseismicwaves,onecanmakeabroaddistinctionbetweenbodywavesandsurfacewaves.
oBodywavesareseismicwavesthattravelthroughtheEarth’sinteriorspreadingoutwardsfromthefocus
inalldirections
oSurfacewavestravelacrossthesurface.Surfacewavesdecaymoreslowlywithdistancethandobody
waves,whichtravelinthreedimensions.Surfacewavescausesmorepropertydamagethanbodywaves
becausesurfacewavesproducemoremovementandtravelmoreslowly,sotheytakelongertopasse.g.
BodyWaves
▪Porprimarywaves
▪Sorsecondarywaves
SurfaceWavesorL(long)waves
▪TheLovewaves
▪TheRayleighwaves

26. Body waves
P(PrimaryWaves)
oPushandPullwaves.
oFastestofseismicwaves.
oAndarelongitudinalincharacteri.e.particlesvibratein
thedirectionofpropagation.
oTravelsfasterinrigidrocks.Thesearelongitudinalwaves
havingshortwavelength
oTheytravelveryfasterandreachseismicstationfirst
oTheirvelocityis1.7timesgreaterthans-waves
oTheypassthroughsolid,liquid,gaseousmedium.
oTypicalvaluesforP-wavevelocityinearthquakesarein
therange5to8km/s.
Acompressional (or longitudinal) wave in which rock
vibrates back and forth parallel to the directions of wave
propagation.

27. Body waves
S(secondaryWaves)
•Shearwaves
•Thesewavesaretransverseincharacterlikelight
i.e.particlesvibrateatrightanglestodirectionof
propagation.
•Swavesdonotpropagatethroughliquids
•Theytravelonlyinsolidmedium.
•Velocitytendstoincreasewithdepthandranges
fromapproximately2to8km/sintheEarth'scrust,
upto13km/sinthedeepmantle.
These are waves that travel in directions at right angles
(i.e. transverse) to the directions of propagation of the wave.

28. Surface waves
RayleighWaves
•Displacementofparticleisofcomplexnature.
•Partlybeinginthedirectionofpropagationand
partlyperpendiculartodirectionofpropagation.
•Tendtodistorthorizontalsurfaceintoazigzag
shape.

29. Surface waves
Lovewaves
▪Displacementofparticleispracticallyhorizontal
i.e.indirectionofpropagation.
▪Lovewavescreaterupture(breaking)orshearing.
▪Itmovesthegroundfromsidetosideina
horizontalplanebutatrightanglestothedirection
ofpropagation.
▪ThehorizontalshakingofLovewavesis
particularlydamagingtothefoundationsof
structures.

32. Seismic
Zones

33. Aftershocks
•Afurtherearthquake,knownasanaftershock,happensafterthemainshock.
•Themainsourcesoftheseaftershocksincludethecrustsurroundingtherupturedfaultlineasitadjuststo
theimpactsofthemainshock,rapidchangesintensionamongstrocks,aswellasthestressfromthefirst
earthquake.
•Abuildingthathasalreadysuffereddamagefromtheoriginalearthquakemightstillsustainmoredamage
fromanaftershock,despitethefactthattheyarealwayslesserinsize.
•Whenanaftershockisgreaterthanthatofthemainshock,themainshockthatoriginallyoccurredis
reclassifiedasaforeshockandtheaftershockisreclassifiedasthemainshock.
•Asthecrustneartheshiftedfaultplaneadaptstothemainshock’seffects,aftershocksarecreated.

34. Earthquake swarms
•Anearthquakeswarmisaseismiceventwherenumerous
earthquakesoccurinalocalareaoveranextendedperiod
withoutaclearpatternofamainshockandaftershocks,
ofteninvolvingmultiplequakesofsimilarmagnitude.
•Inso-called'earthquakeswarms',numerousearthquakes
occurlocallyoveranextendedperiodwithoutaclear
sequenceofforeshocks,mainquakesandaftershocks.

35. Earthquake measurement & detection
oWhentheEarthtrembles,earthquakesspreadenergyintheformofseismicwaves.
oAseismographistheprimaryearthquakemeasuringinstrument.Theseismographproducesadigital
graphicrecordingofthegroundmotioncausedbytheseismicwaves.
oThedigitalrecordingiscalledaseismogram.
oAnetworkofworldwideseismographsdetectsandmeasuresthestrengthanddurationoftheearthquake’s
waves.
oTheseismographproducesadigitalgraphicplottingofthegroundmotionoftheevent.

36. Measurement of earthquake magnitude
Anearthquakehasonemagnitudeunit.Themagnitudedoesnotdependonthelocationwheremeasurementis
made.Since1970,theMomentMagnitudeScalehasbeenusedbecauseitsupportsearthquakedetectionall
overtheEarth.
TheRichterscale
From1935until1970,theearthquakemagnitudescalewastheRichterscale,amathematicalformulainvented
byCaltechseismologistCharlesRichtertocomparequakesizes.
Richter'sequationsarestillusedforforecastingfutureearthquakesandcalculatingearthquakehazards.
MomentMagnitudeScale
Today,earthquakemagnitudemeasurementisbasedontheMomentMagnitudeScale(MMS).MMSmeasures
themovementofrockalongthefault.Itaccuratelymeasureslargerearthquakes,whichcanlastforminutes,
affectamuchlargerarea,andcausemoredamage.
TheMomentMagnitudecanmeasurethelocalRichtermagnitude(ML),bodywavemagnitude(Mb),surface
wavemagnitude(Ms).

37. Measurement of earthquake magnitude
Earthquake Magnitude Classes
•Earthquakes are classified into categories based on their magnitude, ranging from minor to great.
•These categories, known as magnitude classes, serve as a standard for measuring earthquake intensity.
•The classification begins with 'minor', referring to earthquakes with magnitudes between 3.0 and 3.9,
where the shaking is typically noticeable but rarely causes damage. It ends with 'great', which describes
earthquakes with magnitudes exceeding 8.0, often resulting in significant destruction and widespread
impact.
•This system provides a clear framework for understanding the severity of earthquakes and their potential
effects.

38. RicheterScale
•The Richter magnitude scale (also Richter scale) assigns a magnitude number to quantify the energy
released by an earthquake.
•The Richter scale, developed in the 1930s, is a base-10 logarithmic scale, which defines magnitude as the
logarithm of the ratio of the amplitude of the seismic waves to an arbitrary, minor amplitude.
•In 1935, the seismologists Charles Francis Richter and Beno Gutenberg, of the California Institute of
Technology, developed the (future) Richter magnitude scale, specifically for measuring earthquakes in a
given area of study.
•The Richter scale was succeeded in the 1970s by the Moment Magnitude Scale (MMS). This is now the
scale used by the United States Geological Survey to estimate magnitudes for all modern large
earthquakes.
•An Earthquake of magnitude 5 may cause damage within radius of 8km, but that of magnitude 7 may
cause damage in a radius of 80km, and that of 8 over a radius of 250km.

39. Richter Scale Of Earthquake Energy
great
8 ≥
major
7-7.9
strong
6-6.9
moderate
5-5.9
light
4-4.9
minor
3-3.9

40. Richter Scale Of Earthquake Energy
1–1.9 2–2.9 3–3.9 4–4.9 5–5.9 6–6.9 7–7.9 8–8.9 9+
Micro Minor Light Strong Major Great

43. Measurement Of Earthquake Intensity
•Asecondwayearthquakesaremeasuredisbytheirintensity.
•EarthquakeIntensitymeasurementisanon-the-grounddescription.
•Themeasurementexplainstheseverityofearthquakeshakinganditseffectsonpeopleandtheirenvironment.
•Intensitymeasurementswilldifferdependingoneachlocation’snearnesstotheepicenter.
•Therecanbemultipleintensitymeasurementsasopposedtoonemagnitudemeasurement.
TheModifiedMercalliScale
BasedonGiuseppeMercalli'sMercalliintensityscaleof1902,themodified1931scaleiscomposedofincreasing
levelsofintensitythatrangefromobservablequakeimpactsfromlightshakingtocatastrophicdestruction.Intensityis
reportedbyRomannumerals.
Anearthquakeintensityscaleconsistsofaseriesofkeyresponsesthatincludespeopleawakening,movementof
furniture,damagetochimneysandtotaldestruction.

45. Earthquake intensity vs. magnitude
Intensity:Isameasureoftheamountofearthshakingthathappensatagivenlocationinanearthquake.
Magnitude:Isameasureofthesizeoftheseismicwavesortheamountofenergyreleasedatthesourceofthe
earthquake.
TheRichterscaleandtheMomentMagnitudescalebothmeasureearthquake’smagnitude.
TheModifiedMercalliScaleisbasedonearthquakeintensity.

47. Seismograph/ Seismometer

49. Engineering consideration of earthquakes
EngineeringconsiderationsforearthquakesinUganda,likeinanyseismicallyactiveregion,arecrucialtoensurethe
safetyandresilienceofinfrastructureandcommunities.UgandaislocatedwithintheEastAfricanRiftSystem,making
itsusceptibletoseismicactivity.
Keyengineeringconsiderations:
1.SeismicHazardAssessment:
○Thefirststepistoassesstheseismichazardoftheregion.Engineersmustanalyzehistoricalearthquake
data,faultlines,andgeologicalconditionstounderstandthepotentialmagnitudeandfrequencyof
earthquakesindifferentareasofUganda.Thisinformationhelpsdeterminethelevelofseismicdesign
criteriaforstructures.
Example:Engineerswouldusedatatoidentifyhigh-riskareas,suchasthewesternpartofUgandaneartheEast
AfricanRift,andassesstheseismichazardtheretoinformbuildingcodesandstandards.

50. Engineering consideration of earthquakes
2.BuildingCodesandStandards:
○Developingandenforcingseismicbuildingcodesandstandardsiscrucial.Thesecodesspecifythe
minimumdesignrequirementsfornewconstructionandretrofittingofexistingstructurestowithstand
earthquakes.Ugandaadopteditsownbuildingcodesandstandards,whichmustbefollowedinconstruction
projects.
Example:Engineerswouldusedatatoidentifyhigh-riskareas,suchasthewesternpartofUgandaneartheEast
AfricanRift,andassesstheseismichazardtheretoinformbuildingcodesandstandards.
3.SoilConditions:
oUnderstandingthesoilconditionsisessentialbecausedifferenttypesofsoilamplifyordampenseismic
wavesdifferently.Engineersneedtoconductsoilinvestigationstoensurethatfoundationdesignaccounts
forsoilliquefactionandsettlement.
Example:Inareaswithlooseorwater-saturatedsoils,engineersmightrecommenddeepfoundationsorground
improvementtechniquestomitigatetheeffectsofliquefaction.

51. Engineering consideration of earthquakes
4.RetrofittingExistingStructures:
oManyexistingbuildingsandinfrastructureinUgandawereconstructedwithoutconsideringseismicforces.
Retrofittingolderstructuresisachallenge,butit'sessentialtomakethemearthquake-resistant.
Example:Engineersmaystrengthenolderbuildingsbyaddingshearwalls,bracingsystems,orbaseisolatorsto
improvetheirseismicperformance.
5.LifelineInfrastructure:
oCriticalinfrastructurelikebridges,dams,powerplants,andhospitalsmustbedesignedandconstructedto
withstandearthquakestoensurecontinuedoperationduringandafteraseismicevent.
Example:Engineerswoulddesignbridgeswithflexiblejointsandretrofitdamstopreventfailureduringan
earthquake,minimizingdisruptiontotransportationandwatersupply.

52. Engineering consideration of earthquakes
6.PublicAwarenessandEducation:
oPublicawarenessandeducationprogramsarecrucialtoinformresidentsaboutearthquakerisksandsafety
measures.Thisincludeseducatingpeopleaboutevacuationplans,emergencykits,andsafebuilding
practices.
Example:Localauthoritiesandengineersmightorganizeworkshopsanddrillstoeducatethepubliconearthquake
preparednessandresponse.
7.EmergencyResponsePlans:
oEngineersplayaroleindevelopingemergencyresponseplanswithlocalauthorities.Theseplansshould
includeproceduresforevacuations,searchandrescueoperations,andmedicalassistance.
Example:Engineersmayassistinidentifyingsafeassemblyareasandroutesforevacuationsinearthquake-prone
regions.

53. Devastating Effects of Earthquakes
Damage to buildings
Damage to infrastructure
Landslides and Rocks slides
Earthquakes can trigger tsunamis
Leads to liquefaction
Can result in floods

54. Devastating Effects of Earthquakes
Damage to buildings
Damage to infrastructure

55. Devastating Effects of Earthquakes
Landslides and Rockslides
Can result in floods

56. Devastating Effects of Earthquakes
Earthquakes can trigger tsunamis
Leads to liquefaction

57. some of deadliest earthquakes
Indonesia, Indian
ocean
China
Pakistan
Haiti

58. some of deadliest earthquakes

59. examples of earthquakes
8.8
Magnitude of
the main shock
Ecuador-Colombia
Earthquake(1906)

60. examples of earthquakes
9.5
Magnitude of
the main shock
Valdivia Earthquake (1960)

61. Safety tips when earthquake occurs
02
Cover
If there is no shelter nearby, get down near an
interior wall or next to low-lying furniture that won’t
fall on you, and cover your head and neck with your
arms and hands.
01
Drop
DROP down onto your hands and knees before the earthquake
knocks you down. This position protects you from falling but
allows you to still move if necessary.
Hold On
until the shaking stops. Be
prepared to move with your
shelter if the shaking shifts it
around.
03

63. What is Tsunami?
Atsunamiisaseriesofoceanwavesthataregeneratedbylargemovementsorotherdisturbancesonthe
ocean'sfloor.
Suchdisturbancesincludevolcaniceruptions,landslidesandunderwaterexplosions,butearthquakesarethe
mostcommoncause.
Tsunamiscanoccurclosetotheshoreortravelthousandsofmilesifthedisturbanceoccursinthedeepocean.
ThewordTsunamiisderivedfromaJapanesewordtranslatinginto“harborwaves”.

66. Causes of Tsunami
Tsunamisarecausedbypowerfulunderwaterearthquakes,volcaniceruptions,orlandslides.
Whentheseeventsoccurunderwater,asignificantamountofenergyisreleased,generatingenormous
waves.
Certainpartsoftheoceanfrequentlyexperienceearthquakesduetothemovementoftectonicplates.
Mostearthquakeshappennearplateboundaries,whereplatesmoveover,along,orawayfromeachother.
Tsunamiscanpushvastquantitiesofwatertothesurface.
ThePacificOceanisparticularlypronetotsunamisbecauseofitshighlevelofunderseageological
activity.
Inthecaseofavolcaniceruption,theoceanfloormayrapidlyriseseveralhundredfeet.Thisupward
movementdisplaceslargevolumesofwater,creatingmassivewaves.

67. Causes of Tsunami
Tsunamisarealsocalledaseismicseawavesbecausetheyaremostcommonlycausedbyearthquakes.
Becausetsunamisarecausedmainlybyearthquakes,theyaremostcommoninthePacificOcean'sRingof
Fire-themarginsofthePacificwithmanyplatetectonicboundariesandfaultsthatarecapableofproducing
largeearthquakesandvolcaniceruptions.

68. Causes of Tsunami
Inorderforanearthquaketocauseatsunami,itmustoccurbelowtheocean'ssurfaceorneartheoceanandbe
amagnitudelargeenoughtocausedisturbancesontheseafloor.
Oncetheearthquakeorotherunderwaterdisturbanceoccurs,thewatersurroundingthedisturbanceis
displacedandradiatesawayfromtheinitialsourceofthedisturbance(i.e.theepicenterinanearthquake)ina
seriesoffastmovingwaves.
Notallearthquakesorunderwaterdisturbancescausetsunamis-theymustbelargeenoughtomovea
significantamountofmaterial.Inaddition,inthecaseofanearthquake,itsmagnitude,depth,waterdepthand
thespeedatwhichthematerialmovesallfactorintowhetherornotatsunamiisgenerated.

69. Movement of Tsunami
Once a tsunami is generated, it can travel thousands of miles at speeds of up to 500 miles per hour (805 km per
hour). If a tsunami is generated in the deep ocean, the waves radiate out from the source of the disturbance and
move toward land on all sides.
These waves usually have a large wavelength and a short waveheight so they are not easily recognized by the
human eye in these regions.

70. Movement of Tsunami
●Asthetsunamimovestowardshoreandtheocean'sdepthdecreases,itsspeedslowsquicklyandthe
wavesbegintogrowinheightasthewavelengthdecreases.Thisiscalledamplificationanditiswhenthe
tsunamiisthemostvisible.Asthetsunamireachestheshore,thetroughofthewavehitsfirstwhich
appearsasaverylowtide.Thisisawarningthatatsunamiisimminent.Followingthetrough,thepeakof
thetsunamicomesashore.Thewaveshitthelandlikeastrong,fasttide,insteadofagiantwave.Giant
wavesonlyoccurifthetsunamiisverylarge.Thisiscalledrunupanditiswhenthemostfloodingand
damagefromthetsunamioccursasthewatersoftentravelfartherinlandthannormalwaveswould.

71. Assignment
a)Discussthecausesandeffectsofearthquakes.
b)ReferencinganotablehistoricalearthquakeeventinEastAfrica,thatisthe1966Toroearthquake,orthe
1966Rwenzoriearthquake,thatoccurredonMarch20at01:42UTC(04:42Ugandalocaltime).The
earthquakewaslocatedneartheborderbetweenUgandaandtheDemocraticRepublicoftheCongo
(DRC),tothesouthofLakeAlbert.TheearthquakehadamagnitudeofMw6.8andamaximum
perceivedintensityofVIII(Severe)ontheMercalliintensityscale.
Discussitsimpactontheregion,bothgeologicallyandintermsofinfrastructure.

72. References
https://en.wikipedia.org/wiki/Earthquake
https://www.usgs.gov/faqs/what-a-fault-and-what-are-different-types
https://www.lisanenglish.info/2023/02/understanding-earthquakes-causes.html
https://geologyscience.com/natural-hazards/10-most-powerful-earthquakes-in-earth-
history/