Download Ch06_Restless Earth Earthquakes

Chapter 6 Lecture Outline
Restless Earth:
Earthquakes, Geologic
Structures, and Mountain
Building
Focus Question 6.1
• What is the mechanism that generates large
earthquakes?
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
What Is an Earthquake?
What Is an Earthquake?
• Weak earthquakes can be generated by
– Volcanoes, landslides, and meteorite impacts
• Destructive earthquakes occur because tectonic
motion builds up stress
– Mechanism for earthquakes was first explained by H. F.
Reid
– Friction keeps the fault from slipping
– Rocks bend and store elastic energy
– Slip initiates when stress overcomes friction
– Elastic rebound causes deformed
rock to spring back to undeformed position
What Is an Earthquake?
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
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
Focus Question 6.2
• What are the different types of seismic waves?
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
Seismology: The Study of Earthquake
Waves
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
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
• Only travel through solids!
• Slower than P waves
• Slightly greater amplitude than P waves
Seismology: The Study of Earthquake
Waves
Seismology: The Study of Earthquake
Waves
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
Seismology: The Study of Earthquake
Waves
• Speed of travel is very different for each type
of wave
Focus Question 6.3
• How are seismographs used to locate an
earthquake epicenter?
Locating the Source of an Earthquake
• P waves travel faster than S waves
• Generally, in any solid material, P waves
travel about 1.7 times 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
Locating the Source of an Earthquake
Locating the Source of an Earthquake
Locating the Source of an Earthquake
Focus Question 6.4
• What scales are used to measure earthquakes?
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 the
amount of energy released at the source 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
Determining the Size of an Earthquake
• 1989 – Loma Prieta Earthquake
• Richter used a logarithmic scale:
Determining the Size of an Earthquake
Focus Question 6.5
• What destructive forces are triggered by
earthquake vibrations?
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
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
Earthquake Destruction
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
Earthquake Destruction
Earthquake Destruction
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
Earthquake Destruction
Tsunami
Focus Question 6.6
• Where are the major earthquake belts?
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
Earthquake Belts and Plate Boundaries
Focus Questions 6.7
• Why is Earth layered?
• How are seismic waves used to describe
Earth’s interior?
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
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
Earth’s Interior
• Seismic waves are the only way to “see”
inside the interior
–
–
–
–
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
Earth’s Interior
Focus Question 6.8
• What is each of Earth’s layers like?
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
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
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
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
Earth’s Layers
Focus Questions 6.9
• What is brittle deformation?
• What is ductile deformation?
Rock Deformation
• Deformation
– All changes in shape, position, or orientation of a rock
mass
– Bending and breaking occurs when stress exceeds
strength
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
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–
–
–
temperature,
confining pressure
rock type
time
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
Stress and Strain Graphs
Focus Question 6.10
• What are the major types of folds?
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
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
Folds: Structures Formed by Ductile
Deformation
Folds
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
Focus Question 6.11
• What is the relative motion on normal, reverse,
and strike-slip faults?
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
Faults: Structures Formed by Brittle
Deformation
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
Faults: Structures Formed by Brittle
Deformation
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
Faults: Structures Formed by Brittle
Deformation
• A strike-slip fault
– Exhibits horizontal displacement
– Parallel to strike
Faults
Focus Question 6.12
• Where are Earth’s major mountain belts?
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
Mountain Building
Focus Question 6.13
• What are the major features of an Andean-type
mountain belt and how are they generated?
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
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
Subduction and Mountain Building
Focus Question 6.14
• What are the stages of development in a
collisional mountain belt?
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
Collisional Mountain Belts
Terrane Formation
Collisional Mountain Belts
• Alpine-type orogenies result from continental
collisions
– Himalayas
– Collision between Indian and Eurasian plates
Convergent Margins: India-Asia Collision II
Collisional Mountain Belts