Geologic Hazards of Idaho

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
Magnitude
Frequency
Effects
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 http//www.seismo.unr.edu/ftp/pub/louie
/class/100/seismic-waves.html
9
Crust
Depth for Idaho quakes
16 km or less
(Hypocenter)
10
Types of Faults
Applied Stress
shear
Most Idaho active faults are Normal Faults
extensional
compressional
http//earthquake.usgs.gov/4kids/image_glossary/fa
ult.html
11
Idahos 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 http//earthquake.usgs.gov/regional/imw/
images/figure2.php
14
Large Historical Earthquakes Have Occurred in the
Intermountain Seismic Zone
Source http//www.seis.utah.edu/NEHRP_HTM/perseq.
htm
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.
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 http//earthqua
ke.usgs.gov/research/hazmaps/
22
Google Earth maps showing position of Idaho
faults can be seen at www.idahogeology.org
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//gesswhot
o.com/idaho/ells-helens.html)
25
Landslides
Hebgen Lake, Montana, Earthquake August 1959.
(Photo I.J. Witikin, USGS Photographic Library)
26
Causes of LandslidesForces acting on a slope
N
S
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)
• ratio of resisting forces (S) and driving forces
  (?)
• FS S / ?
• When ? gt S, slope is unstable, FS lt 1
• When S lt ?, slope is stable, FS lt 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 http//www.tulane.edu/sanelson/geol2
04/masswastproc
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
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
A
B
C
D
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 gt20 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 IdahoAsh
from Cascade Volcanoes Snake River
PlainBasalt 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