PlanningforEarthquakesinthe WasatchFront SaraLiechty MastersProject DepartmentofCityandRegionalPlanning UniversityofNorthCarolinaChapelHill PlanningforEarthquakesintheWasatchFront by SaraAnnGrowLiechty AMastersProjectsubmittedtothefaculty oftheUniversityofNorthCarolinaatChapelHill inpartialfulfillmentoftherequirementsforthedegree ofMasterofRegionalPlanning intheDepartmentofCityandRegionalPlanning. ChapelHill 2003 Approvedby: ______________________ Dr.RaymondJ.Burby TableofContents ExecutiveSummary ............................................................................................................1 Part1:TheNeedforSeismicSafetyPlanning.................................................................... 3 Part2:PolicyOptionsforImprovingSeismicSafety.......................................................12 ExistingBuildings.........................................................................................................12 FutureDevelopment......................................................................................................19 Part3:ExistingPolicies .................................................................................................... 29 Part4:Recommendations.................................................................................................. 54 Glossary.............................................................................................................................61 References .........................................................................................................................65 ListofTables Table1:HousingBuiltPriortoIncorporationofSeismicStandardsinBuildingCode.... 8 Table2:DevicestoImproveSeismicSafetyofExistingBuildings ................................18 Table3:DevicestoEnsureSeismicSafetyofFutureDevelopment ...............................27 Table4:SummaryofExistingBuildingPolicies.............................................................30 Table5:SummaryofExistingPoliciesforFutureDevelopment .................................... 31 ListofFigures Figure1:UtahFaultMap.................................................................................................... 4 Figure2:SaltLakeCountyLiquefactionMap.................................................................... 6 ExecutiveSummary ThisdocumentaddressessomeofthestrategiesoutlinedinObjective3(Improve theSeismicSafetyofBuildingsandInfrastructure)oftheStrategicPlanforEarthquake SafetyinUtah(1995).Specificallythisreportinvestigatestheproblemofexisting buildingsinneedofseismicretrofittingandtheseismicvulnerabilityoffuture developmentintheWasatchFront.Itisdividedintofourpartsthatoutlinetheproblem, mitigationmethods,existingpolicies,andrecommendationsforfutureactiononthepart oftheUtahSeismicSafetyCommission,theUtahStateLegislature,andotherstate agencies. Part1investigatestheneedforseismicsafetyplanningalongtheWasatchFront. Theearthquakeriskissignificant.Thepotentialdamageofamajorearthquakeis examinedintermsofthelikelihoodofanearthquake,geologicconditionsthatincrease potentialdamage,existingandfuturelanduse,andtheprevalenceofhighlyvulnerable unreinforcedmasonrybuildings.Part1alsoexaminesthereasonsbuildingowners decidetoinvestinretrofittingorignoretheproblem,includingawarenessofan earthquakerisk,cost,andwhethertheriskcanbepassedofftosociety. Part2identifiesdifferentdevicesthatmaybeusedtopromoteretrofittingof seismicallyvulnerablebuildingsaswellaslanduseplanningtechniquestomitigatethe vulnerabilityoffuturedevelopmenttoearthquakedamage.Theeffectiveness,feasibility, equity,andefficiencyofthesedevicesareexamined.Asuccessfulprogramfor retrofittingexistingbuildingsmustalsoincludeadequateincentives.Inaddition, adequateenforcementiscrucialtomanyofthemethodsforretrofittingexistingbuildings andreviewingnewdevelopmentplans. 1 Part3examinesstatelaws,policies,andprogramsthatrequireorencouragethe retrofittingofexistingbuildings.Effortsbymunicipalities,schooldistricts,andother organizationsarealsodiscussed.Existinglaws,policiesandprogramsdonotadequately addresstheproblem.Morestepsmustbetakentoimprovetheseismicsafetyofexisting buildings. Part4containsrecommendationstotheUtahSeismicSafetyCommissionandthe UtahStateLegislature.Specificactionsthatwillimproveseismicsafetyaresuggested. Theseincludededicatedfundingforseismicupgradesofstateownedbuildings, requirementsforbuildingownerstoseismicallystrengthentheirbuildings,aninventory ofseismicallyvulnerablebuildings,incentivesforbuildingowners,andprogramsto increasepublicawareness. 2 Part1:TheNeedforSeismicSafetyPlanning ThethreatofamajorearthquakealongtheWasatchFrontisonethatcannotbe ignored.Approximately75%ofUtah’s1.7millionpeoplelivewithin15milesofthe WasatchFault’sseveralsegments(USGS1995).Geologistshavecomeupwithmany differentestimatespredictingthelikelihoodofanearthquake.Theyestimatethatthe probabilityofamajorearthquakeoverthenext50yearsisbetween5and25percentor more(USSC2000).OnestudypublishedintheJournalofGeophysicalResearchin 1996statedthatthereisa30percentprobabilityofamagnitude7orhigherearthquake duringthenextcentury(Bauman1999).AstudybytheUtahGeologicalSurvey estimatesthatthereisa57percentprobabilityofamagnitude7earthquakealongtheSalt LakesegmentoftheWasatchFaultduringthenext100years(Bauman1999).The UniversityofUtahSeismographStationsestimatesa25percentchanceofamajor earthquakeinthenext50years(UUSS). Whatevertheprobabilityofamajorearthquake,theassessmentmadebyGrove KarlGilbertoftheU.S.GeologicalSurveyin1883stillholdstrue, “Itisuselesstoaskwhenthisdisasterwilloccur.Ouroccupationofthe countryhasbeentoobriefforustolearnhowfasttheWasatchgrows;and indeed,itisonlybysuchdisastersthatwecanlearn.Bythetime experiencehastaughtusthis,SaltLakeCitywillhavebeenshakendown” (USGS2000). ThereisalonghistoryofseismicactivityintheWasatchFront.Thebeautiful mountainsthatcharacterizetheregionarelargelytheresultofseismicactivity.Strong earthquakeshaveoccurredaboutevery350yearsforthepast6,000years(USGS1995), althoughthelastquakewithamagnitudeof7orgreateroccurredabout600yearsago 3 nearProvo(Bauman2001).Seismicactivityisnotrestrictedtothepast.Everyyearan averageof500mildearthquakesoccurintheWasatchFrontregion(UUSS). TheWasatchFaultiswhatiscalledanormalfaultoradipslipfault(UUSS). Whenanearthquakeoccursthefaultslipsinaverticaldirectionwiththemountainsrising relativetothevalleyfloor(UUSS).Duringamagnitude7.5earthquake,vertical displacementofasmuchas10to20feetcouldoccur(UUSS).Thewidestdeformation tendstobeonthedown-droppedsideofthefault(BerkeandBeatley1992).Thezoneof deformationcreatedbyanormalfaultisasymmetricalandmuchwiderthanthe deformationcausedbyastrike-slipfault, suchastheSanAndreasFault(Berke andBeatley1992). Figure1showstheapproximate locationsofallearthquakefaultsinUtah. TheWasatchFaultismadeupofseveral segmentsandencompassestheurban areasofSaltLake,ProvoandOgden.A powerfulearthquakecouldoccuroneach ofthesesegments(USGS1995). Considerabledamagefromstrong groundshakingcouldoccurupto50 milesfromtheepicenterofthe earthquake(UUSS).Apowerful earthquakecouldresultinsoilliquefaction, Figure1:UtahFaultMap 4 landslides,androckfall.Valleyfloorscouldalsobecomepermanentlytilted,which couldleadtosignificantfloodinginurbanareasfromtheGreatSaltLakeandUtahLake (UUSS). ThegeologyoftheWasatchFrontfurthercompoundstheearthquakethreat.The ancientLakeBonnevillecoveredmuchoftheWasatchFrontregion,whichresultedin softlakesedimentbeingdepositedonthevalleyfloors.TheurbanareasoftheWasatch Frontarebuiltuponthissoftlakesediment(USGS1995).Ofparticularconcern,given thegeologyofthearea,areamplifiedgroundshakingandalsoliquefaction,whichis explainedbelow. Thedegreeofgroundshakingthatoccursisnotsimplyaresultofthemagnitude ofanearthquake.Anareaunderlainbysoftlakesedimentwillexperiencegreatersurface effectsthananareaunderlainbyrock(ShedlockandPakiser1994).KyleRollins,aBYU geotechnicalengineer,estimatesthatthegroundshakingduringamajorearthquakeinthe WasatchFrontwouldbecomparabletothegroundshakingmanifestonCalifornia’smost vulnerableareas,suchastheedgeoftheSanFranciscoBay,whichisunderlainbysoft fill(Siegel1994).IvanWongreachedasimilarconclusioninastudyfortheUtah GeologicalSurvey.Hestatedthatamagnitude7earthquakeinSaltLakeCitycould cause“oneofthemostsevereinstancesofgroundshakingeverexperiencedbya metropolitanarea”intheUnitedStates(Siegel1994). Liquefactioncanoccurinanearthquakeofmagnitude5orgreaterwherewater saturatedsandysoilsexist(UGS1997).Whenshaken,thegroundliquefiesandactsasa fluid(UGS1997).Thismaysignificantlydamagebuildingsbycausingthemtosinkor tilt.Slopefailuresmayalsoresult.Liquefiedsoilonevengentleslopesmaymove. 5 Figure2showstheliquefactionpotentialin SaltLakeCounty(UGS1994).Duringa 100-yearperiodareascategorizedas“high” inthismaphavea50percentprobabilityof havinganearthquakestrongenoughtocause liquefaction(UGS1994).Muchofthe downtownareaandthewesternpartofSalt LakeCityarehighlyvulnerableto liquefaction. LanduseinSaltLakeCitymakesits buildingsevenmoresusceptibletodamagein amajorearthquake.AccordingtoKyle Rollins,“SaltLakeCityisbuiltexactlythe wrongwaygivenitsgeology…Thetaller structuresinthedowntownareaare Figure2:SaltLakeCountyLiquefactionMap locatedondeep,softsoildeposits,which aremostlikelytocausedamagetotallbuildings.Manyofthestructuresonstiffshallow sitesontheedgeofthebasinarelow-risebuildings,whicharemostvulnerabletothe groundmotionsproducedinstiffshallowsoils”(Siegel1994).InSaltLakeCity,critical facilitiessuchashospitals,schoolsandhighoccupancybuildingshavebeenlocatedonor nearfaulttraces(BerkeandBeatley1992).WestValleyCityconductedaseismic