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PowerPoint Presentation - Geophysics 189 Natural Hazards

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Often triggered by earthquakes, volcanoes, rainstorms, human activities ... Triggers: rain, frost, earthquakes. Rock fall - Yosemite 1996 ... – PowerPoint PPT presentation

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Title: PowerPoint Presentation - Geophysics 189 Natural Hazards


1
Landslides, Part 1
La Conchita slide, Jan 2005
2
Outline
  • Definitions
  • Factors important for mass movements
  • Timescales of movement
  • Examples

3
What are mass movements?
  • Material moves downslope under force of gravity
  • Occur throughout U.S. and the world
  • Often triggered by earthquakes, volcanoes,
    rainstorms, human activities
  • In U.S., average 1.5 billion, 25 deaths per year

4
Gravity Key Force
1 lb rock
  • Material on horizontal surface - no problem
    because no motion driven by gravity

Gravity pulls with 1lb towards center
5
Gravity Key Force
  • Downslope component leads to slides
  • Just need some energy or trigger to get it started

1 lb rock
Gravity force split into components perpendicular
to surface, and parallel to surface
6
Factors Influencing Mass Movement
  • Nature of slope materials
  • Steepness of slope
  • Water content
  • Slope stability

7
Angle of Repose
  • Angle of Repose
  • the maximum angle at which a pile of
    unconsolidated particles
  • can rest, increases with grain size

8
Classification of mass movement is based on
dominant material, fluid content, and velocity
of movement.
9
Creep
  • Slowest motion, very widespread
  • Downhill motion of soil and uppermost bedrock
    layer, rates of few mm/year

10
How Creep Works
Surface materials expand and contract, resulting
in net motion downslope
11
Creep - Expansion and Contraction
  • Ways to get expansion
  • Water in pores freezes - expands in volume
  • Abundance of clay minerals - absorb water easily
  • Heating of ground surface
  • Ways to get contraction
  • Thawing
  • Drying
  • Cooling

12
Evidence of Creep
13
Evidence of Creep
14
More Speed!
  • More substantial, higher velocity movements -
    important for hazards
  • Several internal and external factors important
    for increasing the odds of movement

15
  • External Factors
  • Steepen slope
  • Remove support at base
  • Waves, streams, people
  • Add mass to top
  • Sedimentation, human effects

16
Internal Factors
  • Weak materials
  • Water
  • Decrease cohesion
  • Geologic conditions

17
Weak Materials
  • Clays most common problem very abundant
  • Sheetlike structure can absorb, release water
  • Some clay minerals lead to large slope failures
  • Quickclay mix of clay, water, salt that can
    collapse from vibrations (blasts), rain
  • Example Canada

18
Water Important for strengthening and weakening
  • Strengthens - surface tension
  • Weakens by
  • Pore pressure
  • Adding weight
  • Interaction with clays

19
Importance of Water Content
Saturated sand flows easily because of
interstitial water
Surface tension in damp sand increases cohesion
Dry sand is bound only by friction
20
Other water weakening methods
  • Weight - excess water adds mass on the slope
  • Clay interaction - can attach to clays because of
    positive/negative charges on water molecule

21
Decrease in Cohesion
  • Occurs through erosion
  • Rocks expand as they reach surface (less pressure
    than at depth)
  • Open fractures, allows water to enter

22
Geologic Conditions
  • Slopes can be weak due to pre-existing geologic
    conditions
  • Ancient slide surfaces
  • Slope of rock layers relative to slope of
    hillside
  • Structures such as joints, faults, clay layers in
    rocks

23
Rock Dip is Important
Rock layers dip at angles less than hillslope -
conditions ripe for failure
24
Triggers of Movements
  • Immediate cause
  • Rains
  • Earthquakes
  • Thawing cycles
  • Construction
  • Long-term
  • Gravity

25
Types of Faster Movements
26
Falls
  • Hard rock splits along joints, weak zones
  • Detaches from cliff, free falls to ground
  • Can shatter, leading to dust clouds
  • Triggers rain, frost, earthquakes

27
(No Transcript)
28
Rock fall - Yosemite 1996
162,000 ton mass fell off valley wall Large tree
kill, thick dust blanket 1 death
29
Slides
  • Movement above failure surface
  • Can be rotational
  • Curved slip surfaces
  • Or translation
  • Fail along planar surface

30
Rotational Slides
  • Move down and out on curved slip surfaces
    (sometimes called slumps)

31
(No Transcript)
32
Rotational Slide Example
  • Rotational landslide in France

33
Translational Slide
  • Move down and outward on planar surfaces of
    weakness
  • Can be joints, faults, clay layers, etc
  • Can go as
  • Block slides
  • Debris slides
  • Lateral spreading

34
Next Time
  • More slides and flows
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