Figure 1.1 Map of Downtown Anchorage Seismic Zones

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Jerome Maxwell
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1 INTRODUCTION 1.1 Project Objective ThevulnerabilityofDowntownAnchoragetoseismicallyinducedgroundfailure,particularlyinthe1964 AlaskaEarthquake,iswelldocumented.

1 1 INTRODUCTION 1.1 Project Objective ThevulnerabilityofDowntownAnchoragetoseismicallyinducedgroundfailure,particularlyinthe1964 AlaskaEarthquake,iswelldocumented.ThedowntowngeologichazardismappedinFigure1.1. 1Low 2LowMod 3Moderate 4High 5VeryHigh Figure 1.1 Map of Downtown Anchorage Seismic Zones Inrecognitionofthishighvulnerability,theAnchorageDowntownComprehensivePlan(Plan)proposes to ensureseismicallysafedevelopment asoneofthesevenprimarygoalstoguidedowntown sland useanddevelopmentpatterns. 1 ThePlanstatesthatsafedevelopmentshall: Minimizethelifesafetyriskstobuildingoccupantsandeconomicvulnerabilityofproperty ownersandthecommunityasawholeforanyfuturedevelopmentproposedinareaswith potentiallyhighorveryhighgroundfailuresusceptibility. Toimplementseismichazardreductiongoals,thePlanrequiresaresponsetoseismichazardsconsisting oftwotasks: 2 1 Anchorage Downtown Comprehensive Plan Adopted December 11, 2007 Assembly Ordinance (pg. 41) 2 LU -16 and LU17 1

2 1) Conductaseismichazardriskassessmentstudy;and 2) Enactaseismicoverlayzonetoaddresslanduseanddevelopmentinareaswithgreatest potentialforgroundfailure. Thisprojectconductedtheseismichazardriskassessmentandhasdevelopedadraftseismicoverlay zone.thisisanimportantstudybecausethereisahighpotentialforseismicallyinducedgroundfailure inseismichazardzones4and5thatwouldlikelyresultincasualtiesandbuildingdamageintherepeat ofanearthquakesimilarto1964alaskaearthquake. Othergovernmentalbodieshavedevelopedregulationsandordinancestoaddressgeologicandseismic hazards.themostsignificantisthealquistprioloearthquakefaultzoningactincalifornialimitingthe typeofdevelopmentthatcanoccurwithinthevicinityofactiveearthquakefaults.thislawhasbeenin placesince1972.thecityofseattleregulatesthedevelopmentofareasinproximitytoenvironmentally criticalareassuchascreeksandshorelines,geologichazardareaspronetoliquefactionandlandslide, andfloodproneareas Theriskassessment,undertakeninresponsetotheComprehensivePlanmandate,usesHazardsU.S. (HAZUS),anationallyapplicablestandardizedsoftwarebasedmethodologydevelopedbyFEMAto estimatepotentialeconomiclossesandhumancasualties.akeyelementofthedowntownanchorage HAZUSmodelwastoapplyparametersbasedonadesignearthquaketoanalysisoftenprototypical buildingsthatarerepresentativeofusesandbuildingtypesidentifiedindowntownanchorage.the modelinputdescribesthebuildingsintermsofstructuralandoccupancycategoriesderivedfrom HAZUS.The10prototypebuildingsdefinedforthestudyresultedin32basicprototypicalvariantswith theirassociatedstructuralconfigurations.eachvariantwasmodeledintwoways:1)withashallow foundation,and2)withamatfoundation,bringingthetotalnumberofmodeledbuildingstosixtyfour Alaska Earthquake Groundfailuresinthe1964AlaskaEarthquakeserveasareminderofthegeotechnicalvulnerabilityof downtownanchorage.themagnitude9.2subductionearthquakehitalaskaongoodfriday,march27, 1964,thesecondlargestrecordedearthquakeinhistory.Shakinglastednearlyfiveminutes.Ground failures,collapsingbuildings,andatsunamiresultedin131deaths.indowntownanchorage,theshaking resultedinthefailureofthebootleggercoveclayformation,causingbothverticalandhorizontal geotechnicaldisplacementsexceeding10feet.largeblocksofearthmovedtowardsthewaterleaving grabensbehind(photos1.1,1.2,and1.3)andpushingupapressureridgeinfront(photo1.4).figure1.2 showsaschematicofthemovement.theareaswiththemostsignificantmovementaremappedin SeismicZone5(Figure1.1). 2

Grabenssubsiding underbuildings Movementof largeblockofsoil Pressureridgespushing upunderbuildings Figure 1.2 Cross section of soil strata in L Street slide area in 1964 Earthquake.

3 Grabenssubsiding underbuildings Movementof largeblockofsoil Pressureridgespushing upunderbuildings Figure 1.2 Cross section of soil strata in L Street slide area in 1964 Earthquake. (Moriwaki et al, 1985) Photo 1.1 Collapse of Fourth Avenue near C Street, due to earthquake caused landslide. Before the earthquake, the sidewalk at left, which is in the graben, was at street level on the right. The graben subsided 11 feet in response to 14 feet of horizontal movement. Source USGS. 3

that was not involved in the landslide. Source USGS. 3 A graben formed at the head of the L Street landslide in Anchorage during the earthquake.

4 Photo 1.2 The marquee of the Denali Theater, which was in the graben of the Fourth Avenue landslide in Anchorage, subsided until it came to rest on the sidewalk in front of the theater, which was on ground that was not involved in the landslide. Source USGS. Photo 1.3 A graben formed at the head of the L Street landslide in Anchorage during the earthquake. The slide block, which is virtually unbroken ground to the left of the graben, moved to the left. The subsidence trough sank 7 to 10 feet in response to 11 feet of horizontal movement of the slide block. The volume of the trough is theoretically equal to the volume of the void created at the head of the slide by movement of the slide block. A number of houses seen in this photograph were undercut or tilted by subsidence of the graben. Source USGS. 4

5 Photo 1.4 House displaced by compressional ridge formed at toe of L Street landslide. Source USGS. Theobjectiveofthisprojectistoquantifytheriskofsimilardamageinafutureearthquake,andto developaseismicoverlayzonetoaddresslandusedevelopmentinthearea. 1.3 Project Approach and Report Overview ThereportisdividedintoeightsectionsfollowedbyAppendixA,theDraftGroundFailureSusceptibility HazardOverlayDistrict,andsixadditionalattachments,thesixTechnicalMemorandadeliverablesthat weresubmittedduringthecourseoftheproject. Section1Introduction thissection Section2EarthquakeScenario coversthebackgroundforselectionoftheearthquake scenariousedfortheproject. Section3GeologicHazard quantifiestheexpectedgrounddeformationthatwouldresult fromsoilfailureinthescenarioearthquake. Section4PrototypicalBuildings liststhetenprototypicalbuildingsselectedforanalysisofthe expectedperformanceinfutureearthquakes,includingadescriptionofadditionaldataprovided foreach. Section5RiskAssessmentApproach describestheapproachusedtoestimatelossestothe prototypicalbuildingsduetoshaking,groundfailure,andthecombinationofthetwo. Section6Results,Analysis,andRecommendations presentstheriskanalysisresultsinterms ofcasualtiesandeconomiclosses.thesensitivityofunderlyingfragilityassumptionsisexplored, andrecommendationsaremadetoreducetherisk. 5

6 Section7DraftZoningOrdinanceDevelopment thegeneralapproachisdescribed,andthe DraftGroundFailureSusceptibilityHazardOverlayDistrictisincludedinAppendixA. Section8References 1.4 Project Team Theprojectteamconsistedof:DonaldBallantyne,HopeSeligson,andPaulSummersofMMI Engineering,JenniferDonahueandJonathanBrayofGeosyntec,JanePreussandGeorgeWilliamsonof PlanwestPartners,CharlesKircherofCharlesKircherAssociates,KeithMobleyofNorthern Geotechnical,andvariousstafffromUSKH. TheMunicipalityofAnchorageprojectmanagerwasDavidTremontwithdirectionandreviewprovided bythegeotechnicaladvisorycommission. 6

Downtown Anchorage Seismic Risk Assessment & Land Use Regulations to Mitigate Seismic Risk

Prepared for: The Municipality of Anchorage Planning Department and the Geotechnical Advisory Commission Downtown Anchorage Seismic Risk Assessment & Land Use Regulations to Mitigate Seismic Risk Prepared