Transcript Geologic Hazards of Idaho

Geologic Hazards of Idaho
Bill Phillips
Idaho Geological Survey
Presentation for Prepared Idaho 2007
September 13, 2007
Natural Hazards
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Flooding
Urban/Wildland Interface Fires
Earthquakes
Landslides
Snow Avalanches
Drought
Lightening
Severe Storms
Volcanic Eruptions
Wind/Tornadoes
Source: 2004 Idaho State Hazard Mitigation Plan
Natural Hazards
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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
Natural Hazards
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Flooding
Urban/Wildland Interface Fires
Earthquakes
Landslides
“Geologic Hazards”
Snow Avalanches
Drought
Lightening
Severe Storms
Volcanic Eruptions
Wind/Tornadoes
Source: 2004 Idaho State Hazard Mitigation Plan
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
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
Regional
Magnitude
Frequency
Big
Rare
Small
Common
Volcanoes
Earthquakes
Landslides
Local
Earthquakes
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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)
Generation of Earthquakes
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Stress: applied force (N/m2)
Strain: permanent deformation (%)
Rock strength
Brittle Failure
Strain
Elastic Limit
The “earthquake cycle”
image from http://www.seismo.unr.edu/ftp/pub/louie/class/100/seismic-waves.html
brittle failure
elastic limit
Stress
Crust
Depth for Idaho quakes
~16 km or less
(Hypocenter)
Types of Faults
Applied Stress
shear
Most Idaho “active”
faults are Normal Faults
extensional
compressional
http://earthquake.usgs.gov/4kids/image_glossary/fault.html
Idaho’s active faults are
NORMAL FAULTS
Fault scarp, Borah Peak, Idaho Earthquake (October 1983)
Most (but not all!) large Idaho earthquakes occurred on Basin and
Range Normal Faults in the Intermountain Seismic Zone
Pacific Northwest Earthquakes
(not Basin and Range features)
Historical Earthquakes in the Intermountain West
(source: http://earthquake.usgs.gov/regional/imw/images/figure2.php
Large Historical Earthquakes Have Occurred in the Intermountain Seismic Zone
Source: http://www.seis.utah.edu/NEHRP_HTM/perseq.htm
Faults and earthquakes also occur in
western Idaho. In fact, earthquakes
can occur ANYWHERE in Idaho.
Effects of Earthquakes
• Intersection by fault (uncommon)
• Structural Damage by shaking (common)
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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)
Borah Peak 1983 Earthquake Effects: Custer Hotel, Mackay
Borah Peak 1983 Earthquake Effects: Crushed Car, Mackay
Borah Peak 1983 Earthquake Effects: IGA Store, Mackay
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
Hazard maps produced by USGS show probability of peak ground acceleration
Download these maps (for FREE) at:
http://earthquake.usgs.gov/research/hazmaps/
Google Earth maps showing
position of Idaho faults can be
seen at www.idahogeology.org
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.
Volcanic Eruptions
Mt St Helens ash in Elmira, Idaho
(http://gesswhoto.com/idaho/ells-helens.html)
Landslides
Hebgen Lake, Montana, Earthquake August 1959. (Photo: I.J. Witikin, USGS Photographic Library)
Causes of Landslides:
Forces acting on a slope
N
S
t
W
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
Factor of Safety
• Factor of Safety (FS)
– ratio of resisting forces (S) and driving forces (t)
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FS = S / t
When t > S, slope is unstable, FS < 1
When S < t, slope is stable, FS < 1
If FS close to 1, then slope is posed for
failure
Classification of Mass Wasting
Slurries, mudflows, debris flows
(closely associated with flood hazards)
Images from: http://www.tulane.edu/~sanelson/geol204/masswastproc
Causes of Landslides
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Increase pore pressure by adding water to slope
– heavy/high intensity precipitation
– blocked culverts
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Increase slope angle by excavation/undercutting
Loading of slope
– road construction
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Seismic shaking/liquefaction
Fires
Bluegill (Little Salmon) Slide near Bliss
Highway 95 near Bonners Ferry
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.
Basalt
Sedimentary interbed
Snake River Canyon near Waha, Idaho
Lost River Fault Scarp
Prehistoric (co-seismic?) landslide near Mackay
Earthquake-induced landslides; A, B, D 1959 Hebgen; C 1983 Borah Peak
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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.
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.
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
Summary: Landslides
• Very common
• Linked to:
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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
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