Download 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