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Stress is a force that acts on rock to change its volume or ... parts of the earths surface where ... the surface A lot of earthquakes begin ... – PowerPoint PPT presentation

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Title: Earthquakes

  • Chapter 5

An Earthquake is
  • The movement of Earth's plates produces strong
    forces that squeeze or pull the rock in the
    crust. We feel this as shaking and trembling.
  • Stress is a force that acts on rock to change its
    volume or shape

3 Types of Stress
  • There are three different types of stress that
    occur on the crust, shearing, tension, and
  • These forces cause some rocks to become fragile
    and they snap
  • Some other rocks tend to bend slowly like road
    tar softened by the suns heat

  • A fault is a break in the crust where slabs of
    crust slip past each other. The rocks on both
    sides of a fault can move up or down or sideways
  • When enough stress builds on a rock, the rock
    shatters, creating faults
  • Faults usually occur along plate boundaries,
    where the forces of plate motion compress, pull,
    or shear the crust too much so the crust smashes

Strike-Slip Faults
  • Shearing creates this fault
  • In this fault, rocks on both sides of the fault
    slide past each other with a little up and down
  • When a strike-slip fault forms the boundary
    between two plates, it becomes a transform

Normal Faults
  • Tension forces in Earth's crust causes these
    types of faults
  • Normal faults are at an angle, so one piece of
    rock is above the fault, while the other is below
    the fault
  • The above rock is called the hanging wall, and
    the one below is called the footwall
  • When movement affects along a normal fault, the
    hanging wall slips downward
  • Normal faults occur along the Rio Grande rift
    valley in New Mexico, where two pieces of Earth's
    crust are diverging

Reverse Faults
  • Compression forces produce this fault
  • This fault has the same setup as a normal fault,
    but reversed, which explains its name
  • Just like the normal fault, one side of the
    reverse fault is at an angle of the other
  • This fault produced part of the Appalachian
    Mountains in the eastern United States

How Do Mountains Form?
  • The forces of plate movement can build up Earth's
    surface, so over millions of years, movement of
    faults can change a perfectly flat plain into a
    gigantic mountain range
  • Sometimes, a normal fault uplifts a block of
    rock, so a fault-block mountain forms
  • When a piece of rock between two normal faults
    slips down, a valley is created

Mountains Formed by Folding
  • Sometimes, under current conditions, plate
    movement causes the crust to fold
  • Folds are bends in rock that form when
    compression shortens and thickens part of Earth's
  • The crashing of two plates can cause folding and
    compression of crust
  • These plate collisions can produce earthquakes
    because rock folding can fracture and lead to

Anticlines and Synclines  
  • Geologists use the terms syncline and anticline
    to describe downward and upward folds in rock
  • An anticline is a fold in a rock that arcs upward
  • A syncline is a fold in a rock that arcs downward
  • These folds in rocks are found on many parts of
    the earths surface where compression forces have
    folded the crust

  • The forces that elevate mountains can also raise
    plateaus, a large area of flat land elevated high
    above sea level
  • Some form when a vertical fault pushes up a large
    flat piece of rock
  • Like a lasagna, a plateau consists of many
    layers, so it is wider than it is tall

How Earthquakes Form
  • Everyday, about 8,000 earthquakes hit Earth, but
    most of them are too little to feel
  • Earthquakes will always begin in a rock beneath
    the surface
  • A lot of earthquakes begin in the lithosphere
    within 100 km of Earth's surface
  • Focus the point beneath Earth's surface where
    rock that is under stress breaks
  • The focus triggers an earthquake

Seismic Waves
  • Seismic Waves vibrations that travel through
    Earth carrying the energy released during an
  • an earthquake produces vibrations called waves
    that carry energy while they travel out through
    solid material
  • During an earthquake, seismic waves go out in all
    directions to the focus
  • They ripple like when you through a stone into a
    lake or pond

Seismic Waves Ctd.
  • There are three different types of seismic waves
    Primary waves, Secondary waves, and surface waves

Primary Waves
  • Also known as P Waves
  • The first waves to come are these waves
  • P waves are earthquake waves that compress and
    expand the ground like an accordion
  • P waves cause buildings to expand and contract

Secondary Waves
  • Also known as S Waves
  • After p waves, S waves come
  • S waves are earthquake waves that vibrate from
    one side to the other as well as down and up
  • They shake the ground back and forth
  • When S waves reach the surface, they shake
    buildings violently
  • Unlike P waves, which travel through both liquids
    and solids, S waves cannot move through any

Surface Waves
  • When S waves and P waves reach the top, some of
    them are turned into surface waves
  • Surface waves move slower than P waves and S
    waves, but they can produce violent ground
  • Some of them make the ground roll like ocean
  • Other surface waves move buildings from side to

Detecting Seismic Waves
  • Geologists use instruments called seismographs to
    measure the vibrations of seismic waves
  • Seismographs records the ground movements caused
    by seismic waves as they move through the Earth

Mechanical Seismographs
  • Until just recently, scientists have used a
    mechanical seismograph
  • a mechanical seismograph consists of a heavy
    weight connected to a frame by a wire or spring
  • When the drum is not moving, the pen draws a
    straight line on paper wrapped around the drum
  • Seismic waves cause the drum to vibrate during an
  • the pen stays in place and records the drum's
  • The higher the jagged lines, the more severe

Measuring Earthquakes
  • There are many things to know about the measures
    of an earthquake
  • There are at least 20 different types of measures
  • 3 of them are the Mercalli scale, Richter scale,
    and the Moment Magnitude scale
  • Magnitude is a measurement of earthquake strength
    based on seismic waves and movement along faults

The Mercalli Scale
  • Developed in the twentieth century to rate
    earthquakes according to their intensity
  • The intensity of an earthquake is the strength of
    ground motion in a given place
  • Is not a precise measurement
  • But, the 12 steps explain the damage given to
    people, land surface, and buildings
  • The same earthquake could have different Mercalli
    ratings because of the different amount of damage
    in different spots
  • The Mercalli scale uses Roman numerals to rank
    earthquakes by how much damage they cause

The Richter Scale
  • The Richter scale is a rating of the size of
    seismic waves as measured by a particular type of
    mechanical seismograph
  • Developed in the 1930s
  • All over the world, geologists used this for
    about 50 years
  • Electric seismographs eventually replaced the
    mechanical ones used in this scale
  • Provides accurate measurements for small, nearby
  • Does not work for big, far ones

The Moment Magnitude Scale
  • Geologists use this scale today
  • Its a rating system that estimates the total
    energy released by an earthquake
  • Can be used for any kind of earthquakes, near or
  • Some news reports may mention the Richter scale,
    but the magnitude number they quote is almost
    always the moment magnitude for that earthquake

Locating the Epicenter
  • Sine the P waves travel faster than the S waves,
    scientists can use the difference in arrival
    times to see how far away the earthquake
  • It does not tell the direction however.

Determining Direction
  • One station can only learn how far away the quake
  • They would draw a circle at that radius.
  • If three stations combine their data, the quake
    occurred where the three circles overlap.

How Earthquakes Cause Damage
  • The severe shaking provided by seismic waves can
    damage or destroy buildings and bridges, topple
    utility poles, and damage gas and water mains
  • With their side to side, up and down movement, S
    waves can damage or destroy buildings, bridges,
    and fracture gas mains.
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