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Geologic Hazards of Idaho

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Presentation on theme: "Geologic Hazards of Idaho"— Presentation transcript:

1 Geologic Hazards of Idaho
Bill Phillips Idaho Geological Survey Presentation for Prepared Idaho 2007 September 13, 2007

2 Natural Hazards Flooding Urban/Wildland Interface Fires Earthquakes
Landslides Snow Avalanches Drought Lightening Severe Storms Volcanic Eruptions Wind/Tornadoes Source: 2004 Idaho State Hazard Mitigation Plan

3 Natural Hazards Flooding Urban/Wildland Interface Fires Earthquakes
Landslides Snow Avalanches Drought Lightening Severe Storms Volcanic Eruptions Wind/Tornadoes “Geologic Hazards” Source: 2004 Idaho State Hazard Mitigation Plan

4 Natural Hazards Flooding Urban/Wildland Interface Fires Earthquakes
Landslides Snow Avalanches Drought Lightening Severe Storms Volcanic Eruptions Wind/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 year Source: 2004 Idaho State Hazard Mitigation Plan Effects Magnitude Frequency Local Regional Big Rare Volcanoes Earthquakes Landslides Small Common

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 strength Brittle Failure Elastic Limit brittle failure Strain elastic limit Stress The “earthquake cycle” image from

9 Crust Depth for Idaho quakes ~16 km or less (Hypocenter)

10 Types of Faults shear extensional compressional Applied Stress
Most Idaho “active” faults are Normal Faults extensional compressional

11 Idaho’s active faults are
NORMAL FAULTS Fault 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, earthquakes can occur ANYWHERE in Idaho.

16 Effects of Earthquakes
Intersection by fault (uncommon) Structural Damage by shaking (common) Distant events can cause damage Landslides Un-Reinforced Masonry (URM) Liquefaction Seiches Bridges, Dams, Power Lines, Communication Facilities, Nuclear Reactors, Gas Lines (Fires) Nonstructural Damage (very common)

17 Borah Peak 1983 Earthquake Effects: Custer Hotel, Mackay

18 Borah Peak 1983 Earthquake Effects: Crushed Car, Mackay

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 be seen 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 Idaho Hazard is highest in SE Idaho along Montana-Wyoming-Idaho-Utah borders Risk (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 Landslides Hebgen Lake, Montana, Earthquake August (Photo: I.J. Witikin, USGS Photographic Library)

26 Causes of Landslides: Forces acting on a slope
W t a W: weight of material N: normal force acting perpendicular to slope t: shear force acting parallel to slope S: 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 < 1 When S < , slope is stable, FS < 1 If 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 precipitation blocked culverts Increase slope angle by excavation/undercutting Loading of slope road construction Seismic shaking/liquefaction Fires Bluegill (Little Salmon) Slide near Bliss Highway 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 Landslides Geological maps locate existing landslides and identify conditions that predispose hillslopes to failure.

31 Basalt Sedimentary interbed Snake River Canyon near Waha, Idaho

32 Lost River Fault Scarp Prehistoric (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 on Middle Fork Salmon, July 2007 Landslides in wilderness areas May affect recreational activities.

36 Mitigation of Landslide Hazards
Geomorphic and Geologic Mapping “if slope has failed before, it will fail again” Terrain Analysis need slopes >20 degrees Engineering Studies can establish Factor of Safety Logging Practices Removal of culverts and roads Public Education, Zoning, Land Use keep people from living on unsafe slopes

37 Summary: Landslides Very common Linked to:
heavy/intense precipitation earthquakes fires logging roads/logging practices High hazard in steep canyons of central and western Idaho, north Idaho Transportation network Highway 95 and 55 have high landslide risk Residences Lightning Creek, near Clark Fork, Idaho November 2006 17 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 metro Boise area (lots more people + property) Volcanic Eruptions N Idaho—Ash from Cascade Volcanoes Snake River Plain—Basalt lava flows Yellowstone— Supervolcano! Landslides Mountainous areas, esp. roads and bridges Highways 95 and 55 Canyons of central and western Idaho Snake River Plain – canyon edges Debris flows – Boise Front Range Post-wildfire effects Logging practices


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