Download What Is an Earthquake?

Chapter 6 Lecture Outline
Foundations of
Earth Science
Seventh Edition
Restless Earth:
Earthquakes, Geologic
Structures, and Mountain
Building
Natalie Bursztyn
Utah State University
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Focus Question 6.1
• What is the mechanism that generates large
earthquakes?
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What Is an Earthquake?
• The sudden movement of one block of rock
slipping past another along a fault
• Most faults are locked until a sudden slip
• Seismic waves radiate out from the focus
(hypocenter), where slip begins
– Earth’s surface directly above the hypocenter is the
epicenter
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What Is an Earthquake?
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What Is an Earthquake?
• Weak earthquakes can be generated by
– Volcanoes, landslides, and meteorite impacts
• Destructive earthquakes occur because
tectonic motion builds up stress
– Friction keeps the fault from slipping
– Slip initiates when stress overcomes friction
– Elastic rebound causes deformed rock to spring
back to undeformed position
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What Is an Earthquake?
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What Is an Earthquake?
• Transform faults have many branches and
fractures
– Offset occurs in distinct segments
• Some segments displace slowly with little shaking during
fault creep
• Some segments produce numerous small earthquakes
• Some segments are locked for hundreds of years and
rupture in large earthquakes
– Earthquakes occur in repetitive cycles
– Rupture propagates in a discrete time period
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What Is an Earthquake?
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What Is an Earthquake?
• Convergent plate boundaries generate
compressional forces
– Mountain building and faulting associated with large
earthquakes
– Subducting plates form a megathrust fault
• Produce the most powerful earthquakes
• Vertical motion underneath the ocean generates tsunamis
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What Is an Earthquake?
• Are there any local faults? If so, what type
are they?
• Have you ever experienced an earthquake? If
so, describe it.
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Focus Question 6.1
• What is the mechanism that generates large
earthquakes?
– Stress (from movement of tectonic plates)
accumulates in a block of crust until it overcomes
frictional resistance and slip occurs
– Seismic waves propagate out from the hypocenter
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Focus Question 6.2
• What are the different types of seismic waves?
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Seismology: The Study of Earthquake
Waves
• The study of earthquake waves is known as
seismology
• Waves are measured by seismometers
– Intertia keeps a weighted arm from moving
while ground motion moves the instrument
– Amplifies ground motion
– Generates seismograms
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Seismology: The Study of Earthquake
Waves
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Seismology: The Study of Earthquake
Waves
• Two main types of seismic waves generated
by earthquakes:
– Surface waves travel in rock layers just below
Earth’s surface
– Body waves travel through Earth’s interior
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Seismology: The Study of Earthquake
Waves
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Seismology: The Study of Earthquake
Waves
• Two types of body waves:
– Primary or P waves
• Push/pull rocks in direction that wave is traveling
• Temporarily change volume of material
• Travel through solids, liquids, and gasses
– Secondary or S waves
• Shake particles at right angles to direction that
wave is traveling
• Change shape of material
• Do not travel through liquids or gasses
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Seismology: The Study of Earthquake
Waves
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Seismology: The Study of Earthquake
Waves
• Two types of surface waves:
– Some travel along Earth’s surface like rolling
ocean waves
– Others move Earth materials from side to side
• Most damaging type of ground motion
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Seismology: The Study of Earthquake
Waves
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Seismology: The Study of Earthquake
Waves
• Speed of travel is very different for each type
of wave
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Focus Question 6.2
• What are the different types of seismic
waves?
– Surface waves travel at Earth’s surface and body
waves travel through the interior
– Surface waves roll or move side to side
– Body waves include P waves and S waves
• P waves travel faster and push/pull material
• S waves move material from side to side
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Focus Question 6.3
• How are seismographs used to locate an
earthquake epicenter?
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Locating the Source of an Earthquake
• P waves travel faster than S waves
• Difference in arrival time is exaggerated by
distance
– Greater interval between P and S wave arrivals
indicates greater distance to epicenter
• Seismographs can be used to triangulate the
epicenter of an earthquake
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Locating the Source of an Earthquake
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Locating the Source of an Earthquake
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Locating the Source of an Earthquake
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Focus Question 6.3
• How are seismographs used to locate an
earthquake epicenter?
– Difference between P wave and S wave arrival time
increases with distance from epicenter
– Multiple seismographs are used to triangulate the
epicenter
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Focus Question 6.4
• What scales are used to measure earthquakes?
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Determining the Size of an Earthquake
• Intensity measures the amount of ground
shaking based on property damage
– Used for historical records
– Modified Mercalli Intensity Scale developed in
California in 1902
• Magnitude is a quantitative measure of energy
released in an earthquake
– Richter scale is related to the amplitude of the
largest seismic wave
– Moment magnitude measures total energy released
based on amount of slide, area of rupture, and
strength of faulted rock
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Determining the Size of an Earthquake
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Determining the Size of an Earthquake
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Focus Question 6.4
• What scales are used to measure earthquakes?
– Intensity scales based on property damage
• Modified Mercalli Intensity Scale
– Magnitude scales based on measure of
energy released
• Richter scale
• Moment magnitude
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Focus Question 6.5
• What destructive forces are triggered by
earthquake vibrations?
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Earthquake Destruction
• Magnitude and other factors determine
degree of destruction
• Area 20-50 km surrounding the epicenter
experiences equal shaking
• Ground motion diminishes rapidly outside of
50 km
• Earthquakes in stable interiors are felt over a
larger area
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Earthquake Destruction
• Earthquake damage depends on:
–
–
–
–
–
Intensity
Duration
Nature of surface materials
Nature of building materials
Construction practices
• Flexible wood and steel-reinforced buildings
withstand vibrations better
• Blocks and bricks generally sustain the most
damage
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Earthquake Destruction
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Earthquake Destruction
• Soft sediment amplifies vibrations
• Vibrations cause loosely packed,
waterlogged materials to behave like a fluid
– During liquefaction stable soil becomes mobile and
rises to the surface
• Vibrations also trigger landslides, ground
subsidence, and fires
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Earthquake Destruction
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Earthquake Destruction
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Earthquake Destruction
• Megathrust displacement lifts large slabs of
seafloor, displaces water, and generates a
tsunami
– Low amplitude wave travels at very high speed in
open ocean
– Amplitude can reach tens of meters in shallow coastal
waters
– Arrival on shore is preceded by a rapid withdrawal of
water from beaches, followed by what appears as a
rapid rise in sea level with a turbulent surface
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Earthquake Destruction
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Focus Question 6.5
• What destructive forces are triggered by
earthquake vibrations?
–
–
–
–
–
–
Ground motion
Liquefaction
Landslides
Ground subsidence
Fires
Tsunamis
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Focus Question 6.6
• Where are the major earthquake belts?
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Earthquake Belts and Plate Boundaries
• Circum-Pacific belt convergent boundaries
experience 95% of earthquakes
– Megathrust faults generate largest earthquakes
• Earthquakes along Alpine-Himalayan belt
because of continental collision
• Weak earthquakes along oceanic ridge
system because tension pulls plates apart
• Transform and strike-slip faults generate
large, cyclical earthquakes
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Earthquake Belts and Plate Boundaries
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Focus Question 6.6
• Where are the major earthquake belts?
– Subduction zones around the Pacific ocean
– Alpine-Himalaya belt
– Oceanic ridge systems
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Focus Questions 6.7
• Why is Earth layered?
• How are seismic waves used to describe
Earth’s interior?
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Earth’s Interior
• Earth has distinct layers:
– Heaviest material at the center, lightest at top
– Iron core, rocky mantle and crust, water ocean,
gaseous atmosphere
• Interior is dynamic
– Mantle and crust are in motion
– Material is recycled from surface to deep interior
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Earth’s Interior
• Temperature increased as material
accumulated to form Earth
– Iron and nickel melted and sank to the center to
produce iron-rich core
– Buoyant rock rose to the surface and formed crust
rich in O, Si, and Al (+ Ca, Na, K, Fe, and Mg)
– Chemical segregation led to iron-rich core, primitive
crust, and mantle
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Earth’s Interior
• Seismic waves are the only way to “see”
inside the interior
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–
–
–
Waves are reflected at boundaries
Refracted through layers
Diffracted around obstacles
Velocity increases with depth as stiffness and
compressibility of rock change
• Can be used to interpret composition and temperature of
rock
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Earth’s Interior
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Focus Questions 6.7
• Why is Earth layered?
– Physical and chemical segregation of molten material
when Earth formed
• How are seismic waves used to describe
Earth’s interior?
– Reflection, refraction, diffraction, and velocity are
used to interpret composition and temperature of rock
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Focus Question 6.8
• What is each of Earth’s layers like?
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Earth’s Layers
• Earth is divided into three compositionally
distinct layers:
– Crust
– Mantle
– Core
• Can be further subdivided into zones based
on physical properties
– Solid or liquid
– Strength
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Earth’s Layers
• Thin, rocky crust is divided into:
• Oceanic crust
– ~7 km thick
– Composed of basalt
– Density ~3.0 g/cm3
• Continental crust
– ~35 – 40 (up to 70) km thick
– Many rock types
– Average density ~2.7 g/cm3
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Earth’s Layers
• Mantle
– Solid layer extending to 2900 km
– 82% of Earth’s volume
• Chemical change at boundary between crust
and mantle
• Uppermost mantle (first 660 km) is peridotite
– Stiff top of upper mantle (+ crust) is lithosphere
• Cool, rigid outer shell
• ~100 km thick
• Weaker portion below is asthenosphere
– Upper asthenosphere is partially melted
– Lithosphere moves independently of asthenosphere
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Earth’s Layers
• Core is an iron-nickel alloy
– Density ~10 g/cm3
– Outer core is liquid
• 2270 km thick
• Generates Earth’s magnetic field
– Inner core is solid sphere
• 1216 km radius
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Earth’s Layers
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Focus Question 6.8
• What is each of Earth’s layers like?
• Crust is thin, solid
– Oceanic (thin, basaltic) and continental (thicker,
granodiorite)
• Mantle is solid
• Crust + upper mantle = lithosphere
• Core is divided into outer liquid and inner solid cores
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Focus Questions 6.9
• What is brittle deformation?
• What is ductile deformation?
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Rock Deformation
• Deformation
– All changes in shape, position, or orientation of a rock
mass
– Bending and breaking occurs when stress exceeds
strength
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Rock Deformation
• Elastic deformation
– Stress is gradually applied
– Rocks return to original size and shape when stress is
removed
– Ductile or brittle deformation occurs when elastic limit
is surpassed
• Strength of a rock is influenced by temperature,
confining pressure, rock type, and time
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Rock Deformation
• Brittle deformation
– Results in fractures
– Common at/near surface
• Ductile deformation
– Solid-state flow at great depths
– Produces a change in the size and shape of a rock
– Some chemical bonds break and new ones form
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Focus Questions 6.9
• What is brittle deformation?
– Fracturing of rock when the elastic limit of a rock is
surpassed that occurs at or near the surface
• What is ductile deformation?
– Change in the size and shape of a rock that occurs at
depth
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Focus Question 6.10
• What are the major types of folds?
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Folds: Structures Formed by Ductile
Deformation
• Folds are wavelike undulations that form
when rocks bend under compression
• Compressional forces result in shortening
and thickening of the crust
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Folds: Structures Formed by Ductile
Deformation
• Anticlines
– Upfolded or arched layers
• Synclines
– Associated downfolds or troughs
• Symmetrical
– Limbs are mirror images
• Asymmetrical
– Limbs are different
• Overturned
– One or both limbs are tilted beyond the vertical
• Recumbant
– Axis is horizontal
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Folds: Structures Formed by Ductile
Deformation
• Circular or elongated upwarping structures
are called domes
– Upwarps in basement rocks deform overlying
sedimentary strata
• Downwarping structures are called basins
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Folds: Structures Formed by Ductile
Deformation
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Focus Question 6.10
• What are the major types of folds?
– Anticlines, synclines, overturned, recumbent
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Focus Question 6.11
• What is the relative motion on normal,
reverse, and strike-slip faults?
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Faults: Structures Formed by Brittle
Deformation
• Faults
– Fractures in the crust with appreciable displacement
– Movement parallel to dip are dip-slip faults
• Rock surface above the fault is the hanging wall
block
• Surface below the fault is the foot-wall block
• Fault scarps
– Long, low cliffs produced by vertical displacement
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Faults: Structures Formed by Brittle
Deformation
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Faults: Structures Formed by Brittle
Deformation
• Normal faults
– Hanging wall moves down relative to footwall
– Accommodate extension of crust
• Fault-block mountains are associated with
large normal faults
– Uplifted blocks are elevated topography called
horsts
– Down-dropped blocks are basins called
grabens
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Faults: Structures Formed by Brittle
Deformation
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Faults: Structures Formed by Brittle
Deformation
• Reverse faults
– Hanging wall moves up relative to footwall
– Thrust faults
• Reverse faults with a dip of less than 45º
– Result from compressional stress
– Accommodate crustal shortening
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Faults: Structures Formed by Brittle
Deformation
• A strike-slip fault
– Exhibits horizontal displacement
– Parallel to strike
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Faults: Structures Formed by Brittle
Deformation
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Focus Question 6.11
• What is the relative motion on normal,
reverse, and strike-slip faults?
– Hanging wall block down relative to footwall
block = normal fault
– Hanging wall block up relative to footwall
block = reverse fault
– Horizontal displacement = strike-slip fault
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Focus Question 6.12
• Where are Earth’s major mountain belts?
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Mountain Building
• Orogenesis is the set of processes that
forms a mountain belt
• Older mountain chains are eroded and less
topographically prominent
• Compressional mountains
– Large quantities of preexisting sedimentary and
crystalline rocks that have been faulted and folded
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Mountain Building
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Focus Question 6.12
• Where are Earth’s major mountain belts?
– Young mountain belts at convergent boundaries
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Focus Question 6.13
• What are the major features of an Andean-type
mountain belt and how are they generated?
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Subduction and Mountain Building
• Subduction of oceanic lithosphere is the
driving force of orogenesis
– Volcanic island arcs build mountains by volcanic
activity, emplacement of plutons, and the
accumulation of sediment from the subducting plate
onto the upper plate
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Subduction and Mountain Building
• Continental volcanic arcs form at Andeantype convergent zones
– Before subduction, sediment accumulates on a
passive continental margin
– Becomes an active continental margin when a
subduction zone forms and deformation begins
– An accretionary wedge is an accumulation of
sedimentary and metamorphic rocks scraped from the
subducting plate
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Subduction and Mountain Building
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Focus Question 6.13
• What are the major features of an Andeantype mountain belt and how are they
generated?
– Continental volcanic arc
– Accretionary wedge
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Focus Question 6.14
• What are the stages of development in a
collisional mountain belt?
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Collisional Mountain Belts
• Cordilleran-type mountain building occurs in
Pacific-like ocean basins
– Rapid seafloor spreading is balanced by rapid
subduction
– Island arcs and crustal fragments (terranes) collide
with a continental margin
• Some terranes may have been microcontinents
• Small terranes are subducted
• Larger terranes are thrust onto the continent
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Collisional Mountain Belts
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Collisional Mountain Belts
• Alpine-type orogenies result from continental
collisions
– Himalayas
– Collision between Indian and Eurasian plates
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Collisional Mountain Belts
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Focus Question 6.14
• What are the stages of development in a
collisional mountain belt?
– Large terranes are thrust onto a continental block
during subduction
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