3
GEOLOGIC HAZARD
3.1 Geotechnical Setting
ThedowntownAnchoragestudyareaisageotechnicallyunstableareathatunderwentsignificant
movementinthe1964AlaskaEarthquake.Theareaisunderlainbyalayerofgraveloutwash
approximately50feetthick,underlainbyalayerofunstableBootleggerCoveClay,asshowninFigure
1.2.Inasignificantearthquakesuchasoccurredin1964,theunstableclaylayerlosesstrengthalonga
plane,andwitheachcycleofearthquakeshakingtheoverlyingblockofsoilmovestowardsthefree
surface(thebluff).Inthevolumebehindtheblockofsoil,thegroundcollapsesintothevoid,forminga
graben.
Theevaluationdescribedherewasperformedforseismicriskzones1(Lowest)through5(VeryHigh),
discussedindetailintheprojectmemorandumdefiningseismicdisplacementsfortheprojectstudyarea
(datedJune12,2009;seeAppendixD).Figure1.2showsacrosssectionofZone5includingpotential
subsidence,blocklateralmovement,andpressureridges.Theseismicriskzoneshadbeenpreviously
mappedinaccordancewithexpectedpermanentgrounddeformation(PGD)infutureearthquakes.The
“SeismicDisplacements”memorandumquantifiedtheexpectednetdisplacementofsoilonthesurface
ineachzonebypercentage,wherethedisplacementwasavectorcombininghorizontalandvertical
movement.Inmostcases,thepredominantmovementisexpectedtobehorizontal,butintheareas
overthegrabens,thepredominantmovementisexpectedtobesubsidence,andoverthepressure
ridges,uplift.Themethodologyusedinestimatingthemovementscombinedexpectedmovements
overtheentirezone,andispresentedasapercentageofthezoneexpectingthenetmovement.This
displacementestimateservesasinputintothebuildingdamagerelationshipsandlossestimatesinthis
memorandum.
Thecharacterizationofthemagnitudeandtypeofpotentialgroundmovementduringmajor
earthquakesineachofthefourlocalgroundfailurehazardzonesanditsresultingimpactonbuildings
andlifelinesisanintegralpartoftheseismicriskassessmentfortheAnchoragedowntownarea.This
sectiondescribesthelevelsofpotentialseismicdisplacementusedintheDowntownAnchorageRisk
Assessment.UsingascenarioconsistentwiththeIBCdesigngroundmotionsandthestrengthof
materialsderivedfromnumeroussources,theprobablerangeofseismicdisplacementshavebeen
developed.
3.2 Definition and Delineation of Slopes
Thestudyareahasbeendividedintofivecategories(SeismicHazardZones)basedonseismicriskas
outlinedinthe1979HardingLawsonstudyandshowninFigure1.1.
x
Zone5ͲVeryHigh–AreasofpreviousseismicallyͲinducedlandslides.Includeszonesoftension
cracks above the head wall scarp, toe bulge, and pressure ridge areas. Although portions of
thesepreviousslidesmayremainrelativelyundisturbedfromfuturestrongshaking,theseslides
willbethemorelikelysiteoffutureseismicallyͲinducedsliding.
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x
Zone4ͲHigh–FineͲgrained,surficialandsubsurfacedepositswithinthevicinityofsteepslopes;
includesareaaboveandbelowtheslope.HighlysusceptibletoalltypesofseismicallyͲinduced
ground failure, including liquefaction, translational sliding, lurching, land spreading, cracking,
andsubsidence.
x
Zone 3 Ͳ Moderate – FineͲgrained surficial and subsurface deposits, including the Bootlegger
CoveClayandothersilt,clay,andpeatdeposits.Mayexperiencegroundcrackingandhorizontal
groundmovementduetolandspreadingorlurching,andsubsidenceduetoconsolidation.
x
Zone2ͲModerateͲLow–MixedcoarseandfineͲgrainedglacialdepositsinlowlandareas,thick
depositsofchannel,terrace,floodplain,andfanalluvium.Mayhaveverylowsusceptibility;may
experience minor ground cracking, localized settlement due to consolidation, and perhaps
liquefactionorlurchingoflocalizedsaturatedzonesoffineͲgrainedmaterial.
x
Zone 1 Ͳ Lowest – Includes exposed bedrock, thin alluvium and colluvium over bedrock. May
experience minor ground cracking and acceleration of normal mass wasting process in
unconsolidatedmaterialsuchasrockfallsandsnowavalanches.(Noneinstudyarea.)
TheexistingtopographywasprovidedbythePlanningDepartmentoftheMunicipalityofAnchorageand
wasinputintoaGeographicInformationSystem(GIS).Thedowntownareawasthenlayeredwiththe
seismiczonesdescribedabove.Fourfootcontoursofthetopographywereusedtodescribeslope
geometriesthroughoutthestudyarea.
Twentycrosssectionsweredrawninthestudyareatocharacterizearepresentativerangeoftheslopes
foundwithineachzone.Anoverlayofthestudyareawithseismiczonesandcrosssectionlinesis
providedbelowinFigure3.1.
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Figure 3.1 Overlay of Downtown Anchorage with Seismic Zones and Cross-Sectional Lines.
Eachofthetwentycrosssectionswasstudiedand61slopeswithinthesecrosssectionswereanalyzed.
Thenumberofslopesanalyzedwithineachofthezoneswas:
x
x
x
x
Zone5:40slopes
Zone4:15slopes
Zone3:1slope
Zone2:5slopes
Assuggestedinthedescriptionsofthezonesabove,Zones4and5areexpectedtoproducethegreatest
numberofslopeswithlargeseismicallyͲinducedpermanentdisplacements.Thesetwozonesalsohave
thehighestconcentrationofslopeswithintheirboundaries.Zone3hasonlysmallacreagewithinthe
studyareaandislocatedinaflatplain.Forthisreason,onlyoneslopewasanalyzed.Zone2is
relativelyflatwithsmall,undulatinghills.
3.3 Anticipated Slope Displacements Due to Seismic Shaking
Inthisstudy,thepotentialseismicdisplacementforeachslopewasestimatedusingtheBrayand
Travasarou(2007)simplifiedseismicslopedisplacementprocedure.Thisprocedurerequiresasinputthe
slopegeometryandsoilpropertiestoestimatetheslope’scharacteristicdynamicresistance(i.e.,its
yieldcoefficient)anditsdynamicresponsecharacteristics(i.e.,thepotentialslidingblock’sfundamental
period).TheseismicdemandwasdefinedusingtheInternationalBuildingCode(2006)forthelikelylevel
ofearthquakeshaking.
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Inmanyofthecrosssections,multiplesteepslopesthatcouldproduceshallowlandslidesexistedwithin
themassofapotentiallylargetranslationalslide.Giventhiscomplexity,allslopes,bothshallowand
translational,wereindependentlyevaluatedtocalculateanticipateddisplacements.
Foreachcrosssection,iftheseismicdisplacementsforthelargetranslationalslidesweregreaterthan
theanticipatedseismicdisplacementsfortheshallowslideslocatedwithinthelargerslidemass,the
displacementsforthelargerslidewereutilizedinthesubsequentanalysis.Thesmallerslopesonthe
slidemasswereassumedtoessentially“goalongfortheride.”Alternatively,ifthelargetranslational
slidedoesnotmobilize,itisprobablethattheshallowerlandslideswilldominatetheseismic
displacements,andtheshallowerslideswerethusutilized.
TheslopesandtheircalculatedseismicdisplacementswerethenplacedontheHardingLawsonmaps.
Usingengineeringjudgment,theplanviewsquarefootageforthepotentialdisplacementswas
calculatedforeachseismicregion.Theresultsoftheseismicslopedisplacementanalysesarepresented
inFigure3.2.
Zone5istheregionforwhichthelargestpotentialseismicdisplacementsarepredicted.Over80%of
theareaofZone5wouldlikelyexperiencemorethaneightfeetofseismicslopedisplacementduring
thedesignlevelofearthquakeshakinginAnchorage.Theselarge,translationalslidesaretypically
throughtheBootleggerCoveClay.Shallowerslopes,mainlycomprisedof30to50feetofsandsand
gravels,areexpectedto“goalongfortheride”withinthelargerslidingmass.However,thereareareas
withinZone5whichmayhave6to12inchesorlessdisplacement,orevennegligibleestimatedseismic
displacement.Thissuggeststhatsomerefinementofthegeneralizedseismiczonationofthe1979
HardingLawsonmapsmaybepossible.
Zone4hasthelargestsquarefootageincomparisontotheotherzones.ResultsforZone4showthat
calculatedseismicdisplacementscanbelargewithalmost20percentoftheregionpossiblydisplacing
morethanfourfeetforthedesignearthquakescenario.Theseslidesarelarge,translationalslideswithin
theBootleggerCoveClayandarewithoutatoebermorpressureridge.Shallowerpotentialslope
failurescomprisedofthesandsandgravelswithinthelargerslidingmassareexpectedto“goalongfor
theride”withinthelargerslidingmass.Approximately3percentofZone4willlikelydisplacebetween6
and12inchesforthedesignevent.TheseareasaregenerallydirectlybehindZone5.Amovementof
about6inchesiscommensuratewithobservationsofmovementbehindthescarpsfromthe1964
earthquake(Long,1973;Hansen,1965).However,aconsiderablepercentageoftheZone4ismore
stablewithinareassignificantlybehindZone5orwithintheareasofthetoeberm/pressureridge.
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Displacement
(Inches)
Figure 3.2 Displacements by Percentage of Area within each Zone.
Zone3hasonlyonegentleslopewithinthestudyarea.Itisanticipatedthatthisslopewilldisplaceless
than1inchduringthedesignevent.Additionally,thisshallowslideislessthan1percentoftheplanarea
ofZone3withinthestudyarea.Thus,Zone3doesnotpresentasignificantseismicslopedisplacement
hazardwithinthestudyarea.
Zone2ischaracterizedbylargesquarefootageanddoesnotcontainsignificantopenfacesorsteep
slopes.Therelativelyfewslopesinthisareaaremoderateandmaymoveontheorderof6to12inches
duringthedesignevent.Theseslopesrepresentlessthan10percentofthelandmassofZone2within
thestudyarea.
TheFigure3.2resultsareprobabilitybased.Thatis,theyincludepercentagesoftheareaswhere
grabensandpressureridgesformaswellasareaswherelittleornomovementoccurs.Forexample,in
HazardZone5,abuildingcouldbesubjectedtolessthanoneinchofmovement,or,itcouldexperience
greaterthan10feetofmovement.Thelocationofgrabenandpressureridgeformationinfuture
earthquakesisunknown,soitisassumedtheycouldoccuranywherewithinHazardZone5,ortoa
lesserdegreewithinHazardZone4.
Theorientationsofthecalculatedseismicdisplacementsarealignedroughlywiththeslopetopography.
Gentleslopesthatdisplaceonahorizontalbaseslidingplanewilldisplacelargelyhorizontally.Steep
slopesoninclinedbaseslidingplanesmaydisplaceverticallyasmuchastheydisplacehorizontally.
Intermediatecaseswillincludebothhorizontalandverticalcomponentsofdisplacement.
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Inadditiontousingslopetopographytoestimatethelikelyorientationofthecalculatedseismic
displacementvector,observationsofslopemovementsinthestudyareaduringthe1964Alaska
earthquakewereconsidered.Forexample,theLStreetSlidemoved14feethorizontallyand10feet
vertically(abouta1.5H:1Vratio).The4thAvenueslidemovedbetween19feetto11feethorizontally
and10feetvertically,orapproximatelya2H:1Vto1H:1Vratioofdisplacements.Basedona
comparisonwithexistingtopographyandtheseobservationsofpastperformance,theserelationships
mayprovidesomeinsightintopotentialrelativeamountsofhorizontalandverticalmovementswithin
eachzone:
x
x
x
x
x
Zone5withanopenface:1.5H:1V
Zone4withanopenface:1.5H:1V
Zone4withoutanopenface:3H:1V
Zone3:primarilyhorizontalmovementwithminorverticalmovement
Zone2:primarilyhorizontalmovementwithminorverticalmovement
Thedisplacementsdevelopedinthissectionarethenusedtoestimateprototypicalbuildingdamagein
thescenarioearthquake.
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