Forces Within Earth

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Forces Within Earth
Forces Within Earth

• An earthquake is a shaking and trembling of the
  earths surface that results from the sudden
  movement of part of the earths crust at a fault.
  
• The shaking can last from 30 seconds up to a
  minute.

• In some instances a single earthquake has killed
  more than 100,000 people and destroyed entire
  cities.

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Earthquake Waves
Forces Within Earth

• Most earthquakes are caused by movements along
  faults.

• A fault is a crack or break in the crust where
  movement occurs.
• Irregular surfaces in rocks can snag and lock,
  causing stress to build in the rocks.
• When the rocks reach their elastic limit they
  break, and this produces an earthquake.
• Most faults are between the surface and 70km.

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San Andreas Fault

• -Its 960km long and 32 km deep.

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Focus/Epicenter
Forces Within Earth

• The focus of an earthquake is the point of
  failure of rocks at the depth where an earthquake
  originates.

The epicenter of an earthquake is the point on
Earths surface directly above the focus.
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• More than 1,000,000 earthquakes occur each year.
• Thats an average of about one every thirty
  seconds.
• Most (about 90) are low in magnitude and arent
  even felt.

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Foreshocks and Aftershocks

• A foreshock is a small earthquake that occurs
  days or years before the main earthquake.
• An aftershock is an earthquake that occurs after
  the main earthquake, usually a small one.
• China earthquake Impact and aftermath - China
  earthquake- msnbc.com

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Some Earthquake Hazards
Earthquakes and Society

• Tsunami

• A tsunami is a large ocean wave generated by
  vertical motions of the seafloor during an
  earthquake or underwater landslide.

• These motions displace the entire column of water
  overlying the fault, creating bulges and
  depressions in the water.
• They can travel at speeds of between 500- 800
  km/h
• When the waves enter shallow water they may form
  huge breakers with heights occasionally
  exceeding 30 m.

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• The most devastating tsunami in recorded history
  occurred on Dec. 26, 2004 in the Indian Ocean off
  the coast of Sumatra.
• It radiated out across the Indian Ocean all the
  way to Africa.
• It was a magnitude 9.0 on the Richter Scale.
• It killed over 150,000 people.

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Some Earthquake Hazards
Earthquakes and Society

• The damage produced by an earthquake is directly
  related to the strength or quality of the
  structures involved.

• The most severe damage occurs to unreinforced
  buildings made of stone, concrete, or other
  brittle building materials.
• Wooden structures and many modern high-rise,
  steel-frame buildings sustain little damage
  during an earthquake.

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Nimitz Freeway collapse in 1989
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Some Earthquake Hazards
Earthquakes and Society

• Structural Failure

• In many earthquake-prone areas, buildings are
  destroyed as the ground beneath them shakes.
• Pancaking occurs when the supporting walls of
  the ground floor fail, causing the upper floors
  to fall and collapse as they hit lower floors.
• When shaking caused by a quake has the same
  period of vibration as the natural sway of a
  building, they will sway violently.
• The natural sway of a building is related to
  height longer waves affect taller buildings and
  shorter waves affect shorter buildings.

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Pancaking of a Hotel

• Mexico City 1985 due to 8.1 Magnitude quake

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Some Earthquake Hazards
Earthquakes and Society

• Land and Soil Failure

• Earthquakes may trigger massive landslides in
  sloping areas.
• In areas with fluid-saturated sand, seismic
  vibrations may cause subsurface materials to
  liquefy and behave like quicksand.
• Liquefaction is when soils or sand under a
  structure become saturates with water and are no
  longer able to support structures.

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Liquifaction Example
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Some Earthquake Hazards
Earthquakes and Society

• Fault Scarps

• Fault movements associated with earthquakes can
  produce fault scarps.
• Fault scarps are areas of great vertical offset
  where the fault intersects the ground surface.

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Movement along a fault line
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Earthquake Prediction
Earthquakes and Society

• Earthquake prediction research is largely based
  on probability studies.

• The probability of an earthquakes occurring is
  based on two factors

• The history of earthquakes in an area
• The rate at which strain builds up in the rocks

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Seismic Risk
Earthquakes and Society

• The probability of future quakes is much greater
  in seismic belts than elsewhere around the globe.

• The past seismic activity in any region is also a
  reliable indicator of future earthquakes and can
  be used to generate seismic-risk maps.

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Earthquake Prediction
Earthquakes and Society

• Earthquake History

• Earthquake recurrence rates can indicate that the
  fault involved ruptures repeatedly at regular
  intervals to generate similar quakes.
• Probability forecasts are also based on the
  location of seismic gaps.
• Seismic gaps are sections of active faults that
  havent experienced significant earthquakes for a
  long period of time but will likely have an
  earthquake in the future.

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US Earthquake Risk Map
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Seismic Risk
Earthquakes and Society
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Past Earthquakes in NC
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NC Earthquake 1698-1997
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Section Assessment
Forces Within Earth

• 1. Match the following terms with their
  definitions.
• ___ stress
• ___ strain
• ___ fault
• ___ focus
• ___ epicenter

A. deformation of materials in response to forces
acting upon them B. surface point directly above
an earthquakes point of origination C. actual
point where an earthquake originates D. a
fracture or system of fractures in Earths crust
along which movement occurs E. force per unit
area acting on a material
E A D C B
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End of Section 1
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Earthquake Waves
Forces Within Earth

• Types of Seismic Waves

• The vibrations of the ground during an earthquake
  are called seismic waves.
• Every earthquake generates three types of
  seismic waves.

• Primary waves, or P-waves, squeeze and pull rocks
  in the same direction along which the waves are
  traveling.

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Earthquake Waves

• Types of Seismic Waves

S waves are Secondary waves They travel through
SOLIDS only! They arrrive Second!
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Earthquake Waves

• Surface waves travel along Earths surface,
• moving in two directions as
• they pass through rock.

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Seismic Wave and Earths Interior
Seismic Waves and Earths Interior

• Seismology is the study of earthquake waves.

• The seismic waves that shake the ground during a
  quake also penetrate Earths interior.
• This has provided information that has enabled
  Earth scientists to construct models of Earths
  internal structure.

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Seismometers and Seismograms
Seismic Waves and Earths Interior

• Seismometers, or seismographs, are sensitive
  instruments that detect and record the vibrations
  sent out by earthquakes.

• All seismometers include a frame that is anchored
  to the ground and a mass that is suspended from a
  spring or wire.
• The relative motion of the mass in relation to
  the frame is recorded during an earthquake.

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Seismograph Animation

• http//www.wwnorton.com/college/geo/egeo/flash/8_3
  .swf

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Seismograph and seismogram
Seismic Waves and Earths Interior
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Seismogram example
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Seismographs seismograms
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Seismometers and Seismograms
Seismic Waves and Earths Interior

• A seismogram is the record produced by a
  seismometer.

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Seismometers and Seismograms
Seismic Waves and Earths Interior

• Travel-Time Curves

• Seismologists have been able to construct global
  travel-time curves for the initial P-waves and
  S-waves of an earthquake.

• For any distance from the epicenter, the P-waves
  always arrive first at a seismic facility.

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Locating an Earthquake
Measuring and Locating Earthquakes

• Time of an Earthquake

• The travel time of either wave at the epicentral
  distance of that station can be read from the
  travel-time graph.

• The time of occurrence of the earthquake is then
  determined by subtracting the appropriate travel
  time from the known arrival time of the wave.

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Seismometers and Seismograms
Seismic Waves and Earths Interior

• Travel-Time Curves

• The time separation between the curves for the
  P-waves and S-waves increases with travel
  distance.

• From this separation, the distance from the
  epicenter of a quake to the seismic facility that
  recorded the seismogram can be determined.

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Layers of the Earth
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Clues to Earths Interior
Seismic Waves and Earths Interior

• Earths Internal Structure

• The travel times and behavior of seismic waves
  provide a detailed picture of Earths internal
  structure.

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Clues to Earths Interior
Seismic Waves and Earths Interior

• Earths Internal Structure

• Earths lower mantle is solid and is probably
  composed of simple oxides containing iron,
  silicon, and magnesium.

• The core is probably made of a mixture of iron
  and nickel.

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Clues to Earths Interior
Seismic Waves and Earths Interior

• This disappearance of S-waves has allowed
  seismologists to reason that Earths outer core
  must be liquid.

• Detailed studies of how other seismic waves
  reflect deep within Earth show that Earths inner
  core is solid.

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Clues to Earths Interior
Seismic Waves and Earths Interior

• Seismic waves change speed and direction when
  they encounter different materials in Earths
  interior.

• P-waves and S-waves traveling through the mantle
  follow fairly direct paths.
• P-waves that strike the core are refracted, or
  bent, causing P-wave shadow zones where no direct
  P-waves appear on seismograms.
• S-waves do not enter Earths core because they
  cannot travel through liquids and do not reappear
  beyond the P-Wave shadow zone.

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Clues to Earths Interior
Seismic Waves and Earths Interior
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Objectives
Measuring and Locating Earthquakes

• Compare and contrast earthquake magnitude and
  intensity and the scales used to measure each.

• Explain why data from at least three seismic
  stations are needed to locate an earthquakes
  epicenter.
• Describe Earths seismic belts.

Vocabulary

• magnitude
• Richter scale
• movement magnitude scale
• modified Mercalli scale

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Earthquake Magnitude and Intensity
Measuring and Locating Earthquakes

• Magnitude is the measurement of the amount of
  energy released during an earthquake.

• The Richter scale is a numerical scale based on
  the size of the largest seismic waves generated
  by a quake that is used to describe its magnitude.

• Each successive number in the scale represents an
  increase in seismic-wave size, or amplitude, of a
  factor of 10.
• Each increase in magnitude corresponds to about a
  32-fold increase in seismic energy.

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Earthquake Magnitude and Intensity
Measuring and Locating Earthquakes

• Moment Magnitude Scale

• The moment magnitude scale, widely used by
  seismologists to measure earthquake magnitude,
  takes into account the size of the fault rupture,
  the amount of movement along the fault, and the
  rocks stiffness.

• Moment magnitude values are estimated from the
  size of several types of seismic waves produced
  by an earthquake.

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Earthquake Magnitude and Intensity
Measuring and Locating Earthquakes

• Modified Mercalli Scale

• The modified Mercalli scale, which measures the
  amount of damage done to the structures involved,
  is used to determine the intensity of an
  earthquake.

• This scale uses the Roman numerals I to XII to
  designate the degree of intensity.
• Specific effects or damage correspond to specific
  numerals the higher the numeral, the worse the
  damage.

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Earthquake Magnitude and Intensity
Measuring and Locating Earthquakes
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Earthquake Magnitude and Intensity
Measuring and Locating Earthquakes

• Depth of Focus

• Earthquake intensity is related to earthquake
  magnitude.
• The depth of the quakes focus is another factor
  that determines the intensity of an earthquake.
• An earthquake can be classified as shallow,
  intermediate, or deep, depending on the location
  of the quakes focus.
• A deep-focus earthquake produces smaller
  vibrations at the epicenter than a shallow-focus
  quake.

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Locating an Earthquake
Measuring and Locating Earthquakes

• All epicenter locations, as well as times of
  occurrence, however, can be easily determined
  using seismograms and travel-time curves.

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Locating an Earthquake
Measuring and Locating Earthquakes

• Distance to an Earthquake

• The P-S separation determines the epicentral
  distance, or distance to a quakes epicenter from
  the seismic station that recorded the waves.
• By measuring the separation on a seismogram as
  well as the distance on a travel-time graph at
  which the P-curve and S-curve have the same
  separation, this distance can be determined.

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Locating an Earthquake
Measuring and Locating Earthquakes

• Distance to an Earthquake

• The earthquake could have occurred anywhere on a
  circle around the seismic station.

• The radius of the circle is equal to the
  epicentral distance.
• If the epicentral distances for three or more
  seismic stations are known, the exact location
  of the epicenter can be determined.

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Seismic Belts
Measuring and Locating Earthquakes

• The majority of the worlds earthquakes occur in
  relatively narrow seismic belts that are
  associated with tectonic plate boundaries.

• Almost 80 percent of all earthquakes occur in the
  Circum-Pacific Belt.
• About 15 percent take place across southern
  Europe and Asia.
• Most of the remaining earthquakes occur in narrow
  bands that run along the crests of ocean ridges.
• A very small percentage of earthquakes happen far
  from tectonic plate boundaries and are
  distributed more or less at random.

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Seismic Belts
Measuring and Locating Earthquakes
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Section Assessment
Measuring and Locating Earthquakes

• 1. Match the following terms with their
  definitions.
• ___ magnitude
• ___ Richter scale
• ___ moment magnitude scale
• ___ modified Mercalli scale

A. rates intensity through the type of damage and
other effects of an earthquake B. takes into
account the fault rupture, the amount of movement
along the fault, and the rocks
stiffness C. describes a quake based on its
largest seismic waves D. the amount of energy
released during an earthquake
D C B A
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End of Section 3
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Earths Changing Surfaces
Forces Within Earth

• Most earthquakes occur when rocks fracture, or
  break, deep within Earth.

• Stress is the forces that push and pull on the
  Earths surface.
• As rock of the crust undergo stress, the change
  shape and volume. Also, they move up and down.
• Deformation is the breaking, tilting and folding
  of rocks.

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Stresses
Forces Within Earth

• There are three kinds of stress that act on
  Earths rocks

• Compression is stress that squeezes rocks
  together.

• Tension is stress that pulls a material apart.

• Shearing is stress that twists rock in two
  different directions, causing it to twist.

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Faults
Forces Within Earth

• A fracture is a crack in a rock. (usually caused
  by one of the three stresses)

• A fault is the fracture or system of fractures
  along which movement occurs.

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Hanging Wall and Foot Wall

• The hanging wall is the block of rock above the
  fault. (HW)
• The foot wall is the block of rock below the
  fault. (FW)

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Hanging Wall and Foot Wall
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Normal Fault
Forces Within Earth

• A normal fault is when the hanging wall moves
  down relative to the foot wall. This is due to
  tension.

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Reverse Fault

• A reverse fault is when the hanging wall moves
  up, relative to the foot wall. Due to compression.

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More faults

• A thrust fault is when the hanging wall slides up
  and over the foot wall. Due to compression.

• A lateral fault (also called strike-slip) is when
  shearing causes the rocks to slide past one
  another horizontally.

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Folds

• A fold is a bend in a rock. There are two types.
• An anticline is an upward fold.

• A syncline is a downward fold.

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Chapter Resources Menu
Chapter Resources Menu

• Study Guide
• Section 19.1
• Section 19.2
• Section 19.3
• Section 19.4
• Chapter Assessment
• Image Bank

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Section 19.1 Main Ideas
Section 19.1 Study Guide

• Stress is the force per unit area that acts on a
  material. The deformation of materials in
  response to stress is called strain.

• Reverse faults form as a result of horizontal
  compression normal faults, horizontal tension
  strike-slip faults, horizontal shear.
• P-waves squeeze and pull rocks in the same
  direction along which the waves travel. S-waves
  cause rocks to move at right angles to the
  direction of the waves. Surface waves cause both
  an up-and-down and a side-to-side motion as they
  pass through rocks.

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Section 19.2 Main Ideas
Section 19.2 Study Guide

• A seismometer has a frame that is anchored to the
  ground and a suspended mass. Because of inertia,
  the mass tends to stay at rest as the ground and,
  thus, the frame vibrates during a quake. The
  motion of the mass in relation to the frame is
  registered and recorded.

• Seismic waves are reflected and refracted as they
  strike different materials. Analysis of these
  waves has enabled scientists to determine the
  structure and composition of Earths interior.

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Section 19.3 Main Ideas
Section 19.3 Study Guide

• Earthquake magnitude is a measure of the energy
  released during a quake and can be measured on
  the Richter scale. Intensity is a measure of the
  damage caused by a quake and is measured with the
  modified Mercalli scale.

• Data from at least three seismic stations are
  needed to locate an earthquakes epicenter.
• Most earthquakes occur in areas associated with
  plate boundaries called seismic belts.

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Section 19.4 Main Ideas
Section 19.4 Study Guide

• Earthquakes cause structural collapse,
  landslides, soil liquefaction, fissures, fault
  scarps, uplift or subsidence, and tsunamis.
  Factors that affect the extent of damage done by
  a quake include the type of subsurface as well as
  the quality, height, and structure of buildings
  and other structures involved.

• The probability of an earthquake is based on the
  history of quakes in an area and the rate at
  which strain builds in the rocks.
• Seismic gaps are places along an active fault
  that havent experienced significant earthquakes
  for a long period of time.

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Short Answer
Chapter Assessment

• 6. What is the difference between an earthquakes
  focus and its epicenter?

An earthquakes focus is the point of initial
fault rupture which is often located deep
underground. The epicenter is the point on
Earths surface directly above the focus.
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Chapter 19 Images
Image Bank
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Chapter 19 Images
Image Bank
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Chapter 19 Images
Image Bank
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Chapter 19 Images
Image Bank
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Chapter 19 Images
Image Bank