Geologic Hazards of Idaho

Bill PhillipsIdaho Geological Survey

Presentation for Prepared Idaho 2007September 13, 2007

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

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

“Geologic Hazards”

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

“Geologic Hazards”

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

VolcanoesEarthquakes

Effects

Local

Regional

Magnitude

Small

Big

Common

Rare

Frequency

Landslides

Earthquake – large event every 15 years (?)Volcanic Eruption – infrequent (but remember May 18, 1980?)Landslides – almost every year

Frequency of Idaho Geologic Hazards

Source: 2004 Idaho State Hazard Mitigation Plan

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)

Generation of Earthquakes

• Stress: applied force (N/m2)• Strain: permanent deformation (%)• Rock strength• Brittle Failure• Elastic Limit

image from http://www.seismo.unr.edu/ftp/pub/louie/class/100/seismic-waves.html

Stress

Strainelastic limit

brittle failure

The “earthquake cycle”

(Hypocenter)~16 km or less

Depth for Idaho quakes

Crust

Types of Faults

http://earthquake.usgs.gov/4kids/image_glossary/fault.html

extensional

compressional

shear

Applied Stress

Most Idaho “active”faults are Normal Faults

Fault scarp, Borah Peak, Idaho Earthquake (October 1983)

Idaho’s active faults areNORMAL FAULTS

Most (but not all!) large Idaho earthquakes occurred on Basin and Range Normal Faults in the Intermountain Seismic Zone

Historical Earthquakes in the Intermountain West (source: http://earthquake.usgs.gov/regional/imw/images/figure2.php

Pacific Northwest Earthquakes(not Basin and Range features)

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, earthquakescan occur ANYWHERE in Idaho.

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)

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 showingposition of Idaho faults can beseen 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)

Hebgen Lake, Montana, Earthquake August 1959. (Photo: I.J. Witikin, USGS Photographic Library)

Landslides

Causes of Landslides:Forces acting on a slope

W

N

S

W: weight of materialN: normal force acting perpendicular to slope: shear force acting parallel to slopeS: shear strength (resistance to shear): slope angle

Factor of Safety

• Factor of Safety (FS)– 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

Images from: http://www.tulane.edu/~sanelson/geol204/masswastproc

Slurries, mudflows, debris flows(closely associated with flood hazards)

Classification of Mass Wasting

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

Hazard Identification: Mapping, Inventory, and Analysis of Landslides

Geological maps locate existing landslides and identify conditions thatpredispose hillslopes to failure.

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.

Snake River Canyon near Waha, Idaho

Basalt

Sedimentary interbed

Prehistoric (co-seismic?) landslide near Mackay

Lost River Fault Scarp

Earthquake-induced landslides; A, B, D 1959 Hebgen; C 1983 Borah Peak

A B

C D

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 ForkSalmon River, July 2002(Wayne Wurtsbaugh photo)

Log jam at Pistol Creek onMiddle Fork Salmon, July 2007

Landslides in wilderness areasMay 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:

– 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, IdahoNovember 200617 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 metroBoise 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