Bells Canyon - Utah Geological Survey

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2S URVEY N OTESrelated to earthquake hazardsincludes soil and rock properties aswell as earthquake and fault characteristics.Prior to the advent of GIS,manipulating this information was alaborious, error-prone process. GIStechnologies facilitate data managementand map production.Earthquake Hazards of the CentralCache ValleyIn our first project we mapped earthquakehazards in the central CacheValley of northern Utah. This areacontains Utah’s tenth-largest city(Logan) as well as the smaller communitiesof Hyde Park, Hyrum, Mendon,Newton, North Logan, Millville,Providence, Smithfield, andWellsville. Three active fault zonesnearby (the Wasatch, West Cache, andEast Cache fault zones) pose a significantthreat to the region, which wasthe site of the most damaging earthquakein Utah’s history. A magnitude5.7 earthquake occurred on August 30,1962, near Richmond, about 10 milesnorth of Logan. Over three-fourths ofthe houses in Richmond were damaged,and landslides and rock fallsblocked highways and canals. Severallarge buildings in Logan and Richmondsuffered structural damage, andthe total estimated earthquake losswas about $1 million (1962 dollars).The consequences of a similar earthquaketoday would be much moresevere given the development that hasoccurred in the intervening 39 years,resulting in increased population andproperty values.Earthquake-hazard mapping is mosteffective for hazard reduction whenconducted prior to development.Cache Valley is largely rural but experiencingrapid growth as populationexpands from urban areas. Thus, ourmapping will be available to reducethe exposure of new development toearthquake hazards in the centralCache Valley. However, earthquakehazardmapping requires the interpretationof existing geotechnical data todetermine the characteristics ofunderlying soils and their response toearthquakes. Because of the ruralDevelopment in the central Cache Valley extends westward from Logan (in the foreground) intolargely rural areas at the base of the Wellsville Mountains (in the distance).Earthquake-hazard mapping is mosteffective for hazard reduction whenconducted prior to development.nature of the central Cache Valley,geotechnical data from boreholes areclustered in population centers andare sparse elsewhere. Boreholes arealso typically drilled to shallowdepths, so the meager geotechnicaldata from deeper soils are insufficientto assess certain earthquake hazards.To supplement geotechnical data, werelied upon information derived fromabundant water wells. In contrast tothe limited depth and irregular distributionof geotechnical boreholes,water wells in the central Cache Valleyare typically deeper and are widelyand uniformly distributed. For ourstudy, we analyzed data from 182geotechnical boreholes and 1,032water wells, as well as informationfrom existing surficial-geologic maps.Although results of our study havenot yet been published by the UGS, areport was submitted to the U.S. GeologicalSurvey. A copy of the report(Seismic Hazards Mapping of the CentralCache Valley, Utah—a Digital Pilot Project,by J.P. McCalpin and B.J.Solomon) is available for inspection inthe Utah Department of NaturalResources Library. The reportincludes a discussion of analyticaltechniques, step-by-step proceduresfor GIS mapping of earthquake hazards,and hazard maps at a scale of1:24,000. Mapped earthquake hazardsinclude amplified earthquake groundmotion, surface fault rupture, liquefaction,and earthquake-induced slopefailure. The hazard maps, most developedfrom recently published mathematicalrelationships between characteristicsof geologic materials andtheir observed association with historicalearthquakes worldwide, show therelative degree of hazard.Although the potential for each hazardis largely independent of the others,certain areas of the central CacheValley are at greater risk from combinationsof earthquake hazards. Thegreatest hazard is generally in the centerof the valley where relatively thickclay, deposited by Lake Bonnevillebetween about 14,000 and 24,000years ago, is locally overlain by morerecent saturated, sandy alluvium. Theclay contributes to greater amplificationof certain types of earthquakeground motions, whereas the alluviumcontributes to a higher potentialfor liquefaction and liquefactioninducedlateral spreading. Much ofthe remainder of the valley floor andthe mountains bordering the valleyare subject to low and moderateearthquake hazards, although moresevere hazards exist locally. Most
S URVEY N OTES 3Contorted beds are possible evidence of prehistoric liquefaction in Lake Bonneville deltaic sandsnear the mouth of Green Canyon, North Logan.. . . as development proceeds into currentlyundeveloped areas, extending from bothvalley margins into the center of the valleyand into the mountains, exposure toearthquake hazards will increase.development in the central Cache Valleyis along the valley margins, whichhave generally lower earthquake-hazardpotentials. However, as developmentproceeds into currently undevelopedareas, extending from both valleymargins into the center of the valleyand into the mountains, exposureto earthquake hazards will increase.An Earthquake Scenario for the SaltLake City segment, Wasatch FaultZoneOur hazard maps of the central CacheValley reflect the response of susceptiblegeologic materials to earthquakesin the region, regardless of the earthquakesource. Another technique formapping earthquake hazards, however,reflects the risk posed by a specificearthquake. This type of analysisbegins with defining the size andlocation of the earthquake (the scenarioearthquake), calculating its associatedground motions, and mappingthe geologic effects such as grounddeformations due to ground failure(for example, liquefaction and landsliding).The resultant earthquakehazardmaps showing geologic effectscan then be used to generate an estimateof the consequences to a regionof the scenario earthquake. The estimatewill describe the scale and extentof damage and disruption, includingquantitative estimates of casualtiesand costs for repair and replacementof damaged buildings and facilities,losses of function for critical facilities,and the extent of induced hazardssuch as fire, flood, or contaminationby hazardous materials. A similarestimate at a national scale was preparedfor the HAZUS99 report citedabove.In our second project, we are mappinggeologic effects of a scenario earthquakein the populous region of thecentral Wasatch Front, centered uponthe Salt Lake City metropolitan area.This region lies astride the Wasatchfault zone, one of the longest andmost active normal-slip faults in theworld. The fault zone is divided intoten segments, each capable of generatingstrong earthquakes. The Salt LakeCity segment is one of the more activesegments, having generated four surface-faultingearthquakes in the past6,000 years. These earthquakes haveoccurred on average every 1,350years; the most recent one occurredbetween about 1,100 and 1,550 yearsago. Geologic studies indicate thatTHE UGS RESPONSE TO AMAJOR EARTHQUAKEWhen an earthquake occurs, emergencyresponseactivities are complex andinvolve many private and public organizations.The UGS plays a key role inthis emergency response as the lead statescientific agency responsible for understanding,documenting, and respondingto geologic hazards. These activities willbe most effective with advance planning.To that end, the UGS has prepared theUtah Geological Survey Earthquake-Response Plan and Investigation FieldGuide to direct the UGS response to significantearthquakes in Utah. This documentupdates our previous plan andincorporates lessons learned from recentsignificant earthquakes elsewhere.The plan defines criteria for selection ofearthquakes to be investigated, delineatesroles of UGS personnel duringresponse, and outlines investigationobjectives and procedures. The plan isdesigned to facilitate the role assigned tostate geological surveys specified in theprogram of post-earthquake investigationsconducted under the auspices ofthe National Earthquake Hazard ReductionsReauthorization Act of November16, 1990.The plan recognizes the critical need fordocumenting geologic effects of earthquakes,and for advising public officialsmanaging response and recovery.Lessons learned can be incorporated intostandards and practices for future hazardreduction. Providing a timelyforum for information exchange amonggeoscientists and engineers is a key elementof this plan, which includes provisionsfor a “post-earthquake informationclearinghouse” with nightly meetingsduring the response period to help coordinateinformation gathering and availability.the magnitude of a surface-faultingearthquake on the Salt Lake City segmentis 7, the size of our scenarioearthquake. An earthquake of thissize centered in the Salt Lake Cityarea would place more than 1.3 millionpeople at risk, with preliminaryestimates of human fatalities of 7,600and damages of about $12 billion.Significant effects of our scenario
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