Download Effects of Earthquakes

Lecture Presentation
Chapter 3
Earthquakes
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Learning Objectives
 Understand how scientists measure and compare
earthquakes
 Be familiar with earthquake processes such as
faulting, tectonic creep, and the formation of seismic
waves
 Know which global regions are most at risk for
earthquakes and why they are at risk
 Know and understand the effects of earthquakes
such as shaking, ground rupture, and liquefaction
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Learning Objectives, cont.
 Identify how earthquakes are linked to other natural
hazards such as landslide, fires, and tsunami
 Know the important natural service functions of
earthquakes
 Know how human beings interact with and affect
the earthquake hazard
 Understand how we can minimize seismic risk, and
recognize adjustments we can make to protect
ourselves
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Introduction to Earthquakes
 There are many earthquakes in any given day
 They are compared based on:
 Magnitude, the amount of energy released
 Intensity, the effects on people and structures
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Table 3.1
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Earthquake Magnitude
 They are mapped according
to epicenter
 Focus is directly below the
epicenter
 Measured by moment
magnitude
 Determined from area of
rupture along, amount of
slippage, and the rigidity of
the rocks
 Richter scale was
previously used
Figure 3.2
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Earthquake Magnitude, cont.
 Both scales are logarithmic
 Based on powers of 10
 Ground displacement for a
magnitude 3 earthquake is
10 times that for a
magnitude 2
 Ground motion is measured
by seismograph
 Related to magnitude,
depth, and geologic setting
Table 3.2
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Table 3.3
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Earthquake Intensity
 Measured by Modified Mercalli Scale
 Qualitative scale (I-XII) based on damage to
structures and people’s perceptions
 Modified Mercalli Intensity Maps show where
the damage is most severe
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Table 3.4
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Shake Maps
 Shake Maps use seismograph
data to show areas of intense
shaking
Figure 3.3
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Earthquake Processes
 Earthquakes are distributed along faults
 Places where rocks are broken and displaced
 All plate boundaries are faults
 Movement along faults are slip rates
 Measured in mm/yr or m/1000yr
 Sudden rupture of rock produces seismic waves
 Release of stored energy
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Figure 3.5
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Fault Types—Strike-Slip
 Crust moves in horizontal direction
Figure 3.6a
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Fault Types—Dip-Slip
 Vertical movement
 Include two walls defined by miners as:
 Footwall where miners put their feet
 Hanging-wall where they put their lanterns
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Fault Types—Dip-Slip, cont.
 Normal fault
 Hanging wall moves down relative to footwall
 Reverse fault
 Hanging wall moves up relative to footwall
 If angle is 45° or less it is a thrust fault
 Blind faults do not extend to the surface
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Figure 3.6b
Figure 3.6c
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Figure 3.7
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Fault Activity
 Active fault
 Moved during the past 10,000 years of the Holocene
Epoch
 Potentially active faults
 Moved during the Pleistocene, but not the Holocene
Epoch
 Inactive
 Not moved during the past 2 million years
 Paleoseismicity of the fault
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Table 3.5
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Tectonic Creep and Slow Earthquakes
 Tectonic creep occurs when movement is
gradual such that earthquakes are not felt
 Can produce slow earthquakes
 Also called fault creep
 Can slowly damage roads, sidewalks and
building foundations
 Can last from days to months
 Magnitudes can be between M 6 and M 7
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Seismic Waves—Body Waves
 Caused by a release of energy from rupture of a
fault
 Travel through the body of the Earth
 P waves, primary or compressional waves
 Move fast with a push/pull motion
 Can move through solid, liquid and gas
 It is possible to hear them
 S waves, secondary or shear waves
 Move slower with an up/down motion
 Can travel only through solids
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Figure 3.9a, b
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Seismic Waves—Surface Waves
 Move along Earth’s surface
 Travel more slowly than body waves
 Move both vertically and horizontally with a rolling
motion
 Are responsible for most of the damage near
epicenter
 Love wave—horizontal ground shaking
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Figure 3.9c
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Earthquake Shaking
 Shaking experience depends on:
 Earthquake magnitude
 Location in relation to epicenter and direction of
rupture
 Local soil and rock conditions
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Distance to Epicenter
 Both types of body waves are emitted from
epicenter of quake
 Seismographs record arrivals of waves to
station site
 Seismogram is the record of the waves
 P and S waves travel at different rates and
arrive at station at different times
 Distance to epicenter can be found by
comparing travel times of the waves
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Figure 3.10c
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Figure 3.10d
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Location of Epicenter
 At least three stations are needed to find exact
epicenter
 Distances from epicenter to each station are
used to draw circles representing possible
locations
 The place where all three circles intersect is the
epicenter
 Process is called triangulation
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Figure 3.11
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Depth of Focus
 Depth of earthquake influences the amount of
shaking
 Focus is the place within the Earth where the
earthquake starts
 Deeper earthquakes cause less shaking at the
surface
 Loss of energy is called attenuation
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Figure 3.12
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Direction of Rupture
 Direction that the rupture moves along the fault
influences the shaking
 Path of greatest rupture can intensify shaking
 Directivity
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Supershear
 Occurs when the propagation of rupture is faster
than the velocity of shear waves or surface
waves produced by the rupture
 Can produce shock waves that produce strong
ground motion along the fault
 May significantly increase the damage from a
large earthquake
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Figure 3.13
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Local Geologic Conditions
 Nature of the ground materials affects the earthquake
energy
 Different materials respond differently to an earthquake
 Depends on their degree of consolidation
 Seismic wave move faster through consolidated bedrock
 Move slower through unconsolidated sediment
 Move slowest through unconsolidated materials with high
water content
 Material amplification
 Energy is transferred to the vertical motion of the surface
waves
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Figure 3.14
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The Earthquake Cycle
 There is a drop in elastic strain after an earthquake and
a reaccumulation of strain before the next event
 Strain is a deformation
 Elastic strain is deformation that is not permanent
 Stage 1: Period of inactivity along a segment of fault
 Stage 2: Period of small earthquakes where the stress
begins to release causing strain
 Stage 3: Foreshocks occur prior to a major release of
stress
 Doesn’t always occur
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The Earthquake Cycle, cont.
 Stage 4: Mainshock and aftershocks where the fault
releases all pent up stress releases the major quake
 Cycle is hypothetical and periods are variable
 Stages have been identified for many large
earthquakes
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Figure 3.19
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Figure 3.20
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Plate Boundary Earthquakes
 Subduction Zones
 Site of the largest earthquakes
 Megathrust earthquakes
 Example: Cascade Mountains
 Convergence between a continental and oceanic plate
 Example: Aleutian Islands
 Convergence between two oceanic plates
 Transform Fault Boundaries
 Example: San Andreas Fault in California, Loma
Prieta earthquake
 Boundary between North American and Pacific plates
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Intraplate Earthquakes
 Earthquakes that occur within plates
 New Madrid seismic zone
 Located near St. Louis, MO
 Historic earthquakes similar in magnitude to West Coast
quakes
 Earthquakes are often smaller than plate boundary
quakes
 Can be large and cause considerable damage due to lack
of preparedness and because they can travel greater
distances through stronger continental rocks
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Effects of Earthquakes
 Ground rupture
 Displacement along the fault causes cracks in
surface
 Fault scarp
 Shaking
 Causes damage to buildings, bridges, dams,
tunnels, pipelines, etc.
 Measured as Ground Acceleration
 Buildings may be damaged due to resonance
 Matching of vibrational frequencies between ground and
building
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Effects of Earthquakes, cont. 1
 Liquefaction
 A near-surface layer of water-saturated sand
changes rapidly from a solid to a liquid
 Causes buildings to “float” in earth
 Common in M 5.5 earthquakes in younger sediments
 After shaking stops, ground re-compacts and
becomes solid
 Elevation changes
 Regional uplift and subsidence
 Can cause substantial damage on coasts and along
streams
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Effects of Earthquakes, cont. 2
 Landslides
 Earthquakes are the most common triggers in mountainous
areas
 Can cause a great loss of human life
 Can also block rivers creating “earthquake lakes”
 Fires
 Displacements cause power and gas lines to break and ignite
 Hard to put out because water lines are often broken
 Disease
 caused by a loss of sanitation and housing, contaminated
water supplies, disruption of public health services, and the
disturbance of the natural environment
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Natural Service Functions
 Water, oil, and natural gas may be rerouted due
to faults
 New mineral resources may be exposed
 Scenic landscapes may form
 Future earthquakes may be reduced due to
release of energy
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Human Interaction with Earthquakes
 Loading Earth’s crust, as in building a dam and
reservoir
• The weight from water reservoirs may create new faults
or lubricate old ones
 Injecting liquid waste deep into the ground through
disposal wells
• Liquid waste disposals deep in the earth can create
pressure on faults
 Creating underground nuclear explosions
• Nuclear explosions can cause the release of stress
along existing faults
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Minimizing the Earthquake Hazard
 Focus of minimization is on forecasting and
warning
 National Earthquake Hazard Reduction
Program Goals




Develop an understanding of the earthquake source
Determine earthquake potential
Predict effects of earthquakes
Apply research results
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Estimating Seismic Risk
 Hazard maps show
earthquake risk
 Probability of a particular
event or the amount of
shaking
Figure 3.29
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Short-Term Prediction
 Pattern and frequency of earthquakes
 Foreshocks
 Deformation of ground surface
 Changes in land elevation
 Seismic gaps along faults
 Areas that have not seen recent quakes
 Geophysical and Geochemical changes
 Changes in Earth’s magnetic field, groundwater levels.
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Earthquake Warnings
 Plan for issuing a
prediction or warning
 Earthquake warning
systems
Figure 3.31
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Community Preparations for Earthquake
Hazard
 Critical facilities must be located in earthquake safe
locations
 Requires detailed maps of ground response
 Buildings must be designed to withstand vibrations
 Retrofitting old buildings may be necessary
 People must be prepared through education
 Insurance must be made available
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Personal Reactions and Preparation
 Do a home inspection to make sure that your home is
structurally sound
 Secure large objects
 Make a personal plan of how to react to a quake
 Leave buildings AFTER shaking stops
 Turn off main gas line
 Move to an open area
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End
Earthquakes
Chapter 3
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