Chapter 8 Earthquakes and Earth’s Interior
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Transcript Chapter 8 Earthquakes and Earth’s Interior
Chapter 8
Earthquakes and Earth’s
Interior
What is an earthquake?
Earthquake the vibration of Earth
produced by the rapid release of energy
within the lithosphere.
Caused by slippage along a break in the
lithosphere, called a fault.
Fault fractures in Earth where movement
has occurred.
Focus and Epicenter
Focus the point within Earth where an
earthquake starts
Located along a fault beneath the surface
Seismic Waves form of energy
released by earthquakes
Ex: stone dropped into a pond
Epicenter location on the surface
directly above the focus
Faults and Change to Earth’s
Surface
The movement along faults during an
earthquake is a major factor in changing
Earth’s surface
Land can shift up to tens of meters in just
one quake
Pushes up coastlines, mountains, plateaus
Uplifted crust moves up vertically
Displaced crust moves horizontally
The San Andreas Fault
Extends about 1300 km through
California and into the Pacific Ocean
Most studied fault in the world
Each segment of the fault behave
differently
San Francisco earthquake of 1907
Land on Western side moved 4.7 meters
relative to the land on the Eastern side
Cause of Earthquakes
Elastic Rebound Hypothesis
Most earthquakes are produced by the
rapid release of energy stored in rock
that has been subjected to great forces.
When the strength of the rock is
exceeded, it suddenly breaks, releasing
some of its stored energy as seismic
waves.
Elastic Rebound
Elastic Rebound the tendency for the
deformed rock along a fault to spring
back after an earthquake
Similar to what happens after you release
a stretched rubber band
Aftershocks and Foreshocks
Aftershock an earthquake that occurs
sometime soon after a major earthquake
May occurs hours or even weeks after
Usually much weaker than the original
Foreshocks small quakes the come
before a major earthquake
Can happen days or even years before
Measuring Earthquakes
After an earthquake, Earth vibrates like
a bell that has been struck with a
hammer.
Seismic Waves transmit the energy of
these vibrations through the lithosphere,
mantle and core.
Two main types of waves produced:
Body Waves (P Waves and S Waves)
Surface Waves
P Waves
P Waves push-pull waves that push
(or compress) and pull (or expand)
particles in the direction the waves
travel.
Also known as compressional waves
Travel faster than S waves
Can travel through both solids and liquids
S Waves
S Waves shake particles at right
angles to the waves’ direction of travel
Also called transverse waves
Travel slower than P Waves
Cannot travel through liquids
Surface Waves
Surface Waves result of body waves
reaching the surface
Travel more slowly than body waves
Move up-and-down and well as side to
side
Usually much larger than body waves, so
they are the most destructive
Seismographs
Seismograph machine developed to
amplify and record ground motion
A weight is suspended from a support
attached to bedrock
Inertia keeps the weight almost
motionless as the support and bedrock
vibrate during an earthquake
This provides a reference to how much
the earth moved and measures the
severity of the quake
Seismogram
Seismogram a time record of ground
motion during an earthquake produced
by a seismograph
Shows all three types of waves
Stronger the earthquake = larger waves
P waves first, followed by S waves, and
finally surface waves
Richter Scale
Based on the height of the largest wave
Familiar but outdated tool
A tenfold increase in wave height equals
an increase of 1 on the magnitude scale
For example: a 5.0 quake is ten times
greater than a 4.0 quake
Only useful for small, shallow quakes
within 500 km of the epicenter
Only really used by news reports
Moment Magnitude
Moment Magnitude derived from the
amount of displacement that occurs
along a fault
More precise method used by scientists
Only scale that estimates the energy
released by earthquakes
Provide a measure of how much energy
rock can store before it suddenly slips
and releases this energy as a quake
Earthquake Hazards
Causes of Earthquake Damage
Seismic Shaking
Liquefaction
Landslides and Mudflows
Tsunamis
Seismic Shaking
Seismic Shaking the ground vibrations
caused by seismic waves
Interact to jolt and twist structures
Buildings that are not properly reinforced
may crumble and collapse
Generally strongest close to an epicenter
However, can still be strong away from
epicenter in areas with loose soil
Liquefaction
Liquefaction once stable soil suddenly
turns into liquid
Happens in areas where soil and rock are
saturated with water
Liquid cannot support the buildings, and
they may settle and collapse
Underground pipes and storage tanks can
rise to the surface
Landslides and Mudflows
Quakes can trigger different types of
mass movements
Can bury entire towns under millions of
tons of debris
Quakes can cause loose soil and rock on
slopes to become a landslide
Mudflows occur where water content in
the soil is much higher
Tsunamis
Tsunami a wave formed when the
ocean floor shifts suddenly during an
earthquake
Earthquake pushes up a slab of ocean
floor along a fault or and underwater
landslide/eruption occurs
This displaces a large amount of water
Wave begins very small, but increases in
size the closer it gets to land
Reducing Earthquake Damage
Factors that play a role:
Determining the earthquake risk for an
area
Building earthquake resistant structures
Following earthquake safety precautions
Assessing Earthquake Risk
Earthquakes are most frequent along
the boundaries of Earth’s tectonic plates
Study the historical records of quakes
Devices measure uplift
Seismic Gap an area along a fault
where there has not been any
earthquake activity for a long period of
time
Earth’s Layered Structure
Layers Defined by Composition
Consists of three major layers defined
by chemical composition
Crust
Mantle
Core
Crust
Crust the thin, rocky outer layer of
Earth
Oceanic
Roughly 7km thick
Continental
Between 8km and 75km thick
Mantle
Mantle a solid, rocky shell that
extends to a depth of 2890km
Contains over 82% of Earth’s volume
Core
Core a sphere composed mostly of an
iron-nickel alloy
Average density of 13 times denser than
water
Layers Defined by Physical
Properties
Earth can be divided into layers based
on the physical properties of each
Lithosphere
Asthenosphere
Lower Mantle
Outer Core
Inner Core
Lithosphere
Lithosphere the crust and uppermost
mantle that form a relatively cool, rigid
shell
Asthenosphere
Asthenosphere found beneath the
lithosphere, it is composed of rocks
close to their melting point
Lower Mantle
Lower Mantle exists from a depth of
about 660km down to near the base of
the mantle
Inner and Outer Core
Outer Core a liquid layer whose flow
of metallic iron creates the Earth’s
magnetic field
Inner Core a sphere within the outer
core that is solid due to the extreme
pressure