Presentation on theme: "Geologic Hazards of Idaho"— Presentation transcript:
1 Geologic Hazards of Idaho Bill PhillipsIdaho Geological SurveyPresentation for Prepared Idaho 2007September 13, 2007
2 Natural Hazards Flooding Urban/Wildland Interface Fires Earthquakes LandslidesSnow AvalanchesDroughtLighteningSevere StormsVolcanic EruptionsWind/TornadoesSource: 2004 Idaho State Hazard Mitigation Plan
3 Natural Hazards Flooding Urban/Wildland Interface Fires Earthquakes LandslidesSnow AvalanchesDroughtLighteningSevere StormsVolcanic EruptionsWind/Tornadoes“Geologic Hazards”Source: 2004 Idaho State Hazard Mitigation Plan
4 Natural Hazards Flooding Urban/Wildland Interface Fires Earthquakes LandslidesSnow AvalanchesDroughtLighteningSevere StormsVolcanic EruptionsWind/Tornadoes“Geologic Hazards”Source: 2004 Idaho State Hazard Mitigation Plan
5 Hazard or Risk? Hazard: “a source of danger.” Risk: “the chance of loss to the subject matter of an insurance contract; also, the probability of such loss.”Source: Websters New Collegiate Dictionary“Risk = Hazard x (People + Property)”This talk is about Hazards
6 Frequency of Idaho Geologic Hazards Earthquake – large event every 15 years (?)Volcanic Eruption – infrequent (but remember May 18, 1980?)Landslides – almost every yearSource: 2004 Idaho State Hazard Mitigation PlanEffectsMagnitudeFrequencyLocalRegionalBigRareVolcanoesEarthquakesLandslidesSmallCommon
7 Earthquakes What causes them? Where are they found in Idaho? What are earthquake effects?Where is the hazard highest?Effects of 1959 Hebgen Lake Earthquake (Montana-Idaho border)
8 Generation of Earthquakes Stress: applied force (N/m2)Strain: permanent deformation (%)Rock strengthBrittle FailureElastic Limitbrittle failureStrainelastic limitStressThe “earthquake cycle”image from
9 CrustDepth for Idaho quakes~16 km or less(Hypocenter)
10 Types of Faults shear extensional compressional Applied Stress Most Idaho “active”faults are Normal Faultsextensionalcompressional
11 Idaho’s active faults are NORMAL FAULTSFault scarp, Borah Peak, Idaho Earthquake (October 1983)
12 Most (but not all!) large Idaho earthquakes occurred on Basin and Range Normal Faults in the Intermountain Seismic Zone
13 Pacific Northwest Earthquakes (not Basin and Range features) Historical Earthquakes in the Intermountain West(source:
14 Large Historical Earthquakes Have Occurred in the Intermountain Seismic Zone Source:
15 Faults and earthquakes also occur in western Idaho. In fact, earthquakescan occur ANYWHERE in Idaho.
16 Effects of Earthquakes Intersection by fault (uncommon)Structural Damage by shaking (common)Distant events can cause damageLandslidesUn-Reinforced Masonry (URM)LiquefactionSeichesBridges, Dams, Power Lines, Communication Facilities, Nuclear Reactors, Gas Lines (Fires)Nonstructural Damage (very common)
19 Borah Peak 1983 Earthquake Effects: IGA Store, Mackay
20 M 6. 5 San Simeon Earthquake, 12/22/03 M 6.5 San Simeon Earthquake, 12/22/03. Non-structural damage to Paso Robles area wineries. / IMG_2980 Photos by Joshua Marrow, Simpson Gumpertz Heger
21 Hazard maps produced by USGS show probability of peak ground acceleration Download these maps (for FREE) at:
22 Google Earth maps showing position of Idaho faults can beseen at
23 Summary-Earthquakes Numerous quakes every year in Idaho Few are damaging but LARGEST historical events in USA outside of Alaska occurred in/near IdahoHazard is highest in SE Idaho along Montana-Wyoming-Idaho-Utah bordersRisk (in terms of property loss and economic disruption) may be greatest in metro Boise area.
24 Volcanic Eruptions Mt St Helens ash in Elmira, Idaho (http://gesswhoto.com/idaho/ells-helens.html)
25 LandslidesHebgen Lake, Montana, Earthquake August (Photo: I.J. Witikin, USGS Photographic Library)
26 Causes of Landslides: Forces acting on a slope WtaW: weight of materialN: normal force acting perpendicular to slopet: shear force acting parallel to slopeS: shear strength (resistance to shear)a: slope angle
27 Factor of Safety Factor of Safety (FS) FS = S / ratio of resisting forces (S) and driving forces ()FS = S / When > S, slope is unstable, FS < 1When S < , slope is stable, FS < 1If FS close to 1, then slope is posed for failure
28 Classification of Mass Wasting Slurries, mudflows, debris flows(closely associated with flood hazards)Images from:
29 Causes of Landslides Increase pore pressure by adding water to slope heavy/high intensity precipitationblocked culvertsIncrease slope angle by excavation/undercuttingLoading of sloperoad constructionSeismic shaking/liquefactionFiresBluegill (Little Salmon) Slide near BlissHighway 95 near Bonners Ferry
30 Hazard Identification: Mapping, Inventory, and Analysis of Landslides Detail of Surficial Geologic Map of the Sweetwater Quadrangle, Nez Perce County, Idaho , by Kurt L.Othberg, Roy M.Breckenridge, Daniel W.Weisz: Idaho Geological Survey DWM-13, 2003.Hazard Identification: Mapping, Inventory, and Analysis of LandslidesGeological maps locate existing landslides and identify conditions thatpredispose hillslopes to failure.
31 BasaltSedimentary interbedSnake River Canyon near Waha, Idaho
32 Lost River Fault ScarpPrehistoric (co-seismic?) landslide near Mackay
33 Earthquake-induced landslides; A, B, D 1959 Hebgen; C 1983 Borah Peak
34 ROCKSLIDE IN WASHINTON STATE, USA: The North Cascades Highway was seriously damaged by the record-breaking October 2003 rainstorm. Flooding eroded the roadway, overwhelmed culvert and drainage systems and caused several large sections of road to buckle and collapse. Within three weeks of the flooding, a massive rockslide crashed into the highway, in the same general area. That slide and its twin slide that came down days later, were so large that they registered on the earthquake monitoring seismic scale in Rockport, for the North Cascades.
35 Debris flow on Middle Fork Salmon River, July 2002 (Wayne Wurtsbaugh photo)Log jam at Pistol Creek onMiddle Fork Salmon, July 2007Landslides in wilderness areasMay affect recreational activities.
36 Mitigation of Landslide Hazards Geomorphic and Geologic Mapping“if slope has failed before, it will fail again”Terrain Analysisneed slopes >20 degreesEngineering Studiescan establish Factor of SafetyLogging PracticesRemoval of culverts and roadsPublic Education, Zoning, Land Usekeep people from living on unsafe slopes
37 Summary: Landslides Very common Linked to: heavy/intense precipitationearthquakesfireslogging roads/logging practicesHigh hazard in steep canyons of central and western Idaho, north IdahoTransportation networkHighway 95 and 55 have high landslide riskResidencesLightning Creek, near Clark Fork, IdahoNovember 200617 inches rain/snow in 1 week~$3 million damage to forest road network
38 Idaho Natural Hazards Highest Earthquake Hazard SE Idaho (except Snake River Plain)Highest Earthquake Risk may be metroBoise area (lots more people + property)Volcanic EruptionsN Idaho—Ash from Cascade VolcanoesSnake River Plain—Basalt lava flowsYellowstone— Supervolcano!LandslidesMountainous areas, esp. roads and bridgesHighways 95 and 55Canyons of central and western IdahoSnake River Plain – canyon edgesDebris flows – Boise Front RangePost-wildfire effectsLogging practices
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