Earthquakes

1
Earthquakes

• 8th Grade Science
• Mrs. Pientok

2
Forces inside Earth

• When rocks break they move along faults.
• Applied forces cause rocks to undergo elastic
  deformation.
• When elastic limits are passed, rocks break.
• Rock on one side of a fault can move up, down, or
  sideways in relation to rock on the other side of
  the fault.

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Forces inside Earth

• Faults occur because force inside Earth cause
  Earths plates to move placing stress on or near
  the plate edge.
• Rocks will bend, compress, stretch and possibly
  break.
• Earthquakesvibration produced by breaking rock
• Rocks break, move along the fault, return to
  original shapes.
• Rock on one side of a fault can move over, under,
  or past each other along fault lines.

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Forces inside Earth

• Three types of forces act on rockstension,
  compression, and shear.
• Tension forces normal faultscaused by rock
  above the fault moving downward in relation to
  the rock below the fault.
• Reverse faultcompression forces squeeze rock
  above the fault up and over the rock below the
  fault.
• Created by shear forces strike-slip faultrocks
  on either side of the fault move past each other
  without much upward or downward motion.

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Types of Faults

• Normal faulting.  Normal Faulting is the most
  common found in nature.
• Reverse Faulting occurs when compressional stress
  pushes the fault block UP the fault line
  "REVERSE" to gravity.
• A strike-slip fault is a fault on which the two
  blocks slide past one another.

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Seismic Waves

• Waves generated by an earthquake, can move the
  ground forward and backward, up and down, and
  side to side.
• Focusan earthquakes point of energy release.
• Epicenterlocated directly above the focus on the
  surface of the earth.

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Seismic Waves

• Primary waves(P-waves) cause particles to move
  back and forth in the same direction that the
  wave is traveling.
• Secondary waves(S-waves) cause particles of rock
  to move at right angles to the direction of wave
  travel.
• Surface waves move rock particles in a backward,
  rolling motion and a sideways motion.

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Seismic Waves

• Fastest seismic wave
• Compressional wave
• Does not travel through fluids
• Moves rock particles at right angles to the
  direction of the wave
• Travels at the surface
• Most destructive

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Locating the Epicenter

• The different speeds of seismic waves allow
  scientists to determine the epicenter.
• Need at least three locations to pinpoint
  epicenter.
• Primary waves move fastest, cause least
  destruction.
• Secondary waves follow.
• Surface waves move slowest and arrive at the
  seismograph station last. Cause the most
  destruction.

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Measuring seismic waves

• Seismographmeasures seismic waves.
• Consists of a rotating drum of paper and a
  pendulum with a an attached pen.
• The paper record of a seismic event is called a
  seismogram.

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Earths Interior

• Earths structure consist of an inner, mostly
  iron, solid core, surrounded by a mostly iron
  liquid outer core surrounded by the mantle.
• The crust is Earths outer layer, about 5-60 km
  thick.

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Movement of Waves Through Earths Interior

• A seismic waves speed and direction change as
  the wave moves through different layers with
  different densities.
• Density generally increases with depth as
  pressure increase.
• Shadow zones do not receive seismic waves because
  the waves are bent or stopped by materials of
  different densities.
• Changes in seismic wave speed allowed detection
  of boundaries between Earths layers.

13
People and Earthquakes

• Although earthquakes are natural geological
  events, they kill many people and cause a lot of
  damage.
• Seismologistsscientists who study earthquakes.
• Magnitudemeasure of energy released by an
  earthquake determined by the Richter scale and
  based on the height of the lines on a seismogram.

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Measuring Earthquakes

• Two scales are used to measure earthquakes.
• Richter scale and Mercalli scale.
• Richter scale.
• Has no upper limit.
• Most earthquakes have magnitudes too low to be
  felt by humans3.0-4.9 on Richter scale.
• The modified Mercalli intensity scale describes
  earthquake intensity based on structural and
  geologic damage (I-XII).

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Effects of Earthquakes

• Liquefacationshaking from an earthquake can make
  wet soil act like a liquid.
• Tsunamisocean waves caused by earthquakes.
• Caused by a sudden movement of the ocean floor
  pushes against the water.
• Can travel thousands of kilometers in all
  directions.

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Predicting and Planning for Earthquakes

• Earthquakes cannot be reliably predicted.
• Knowing how and where to plan for earthquakes can
  help prevent death and damage.
• Buildings can be constructed to withstand seismic
  vibration.
• Flexible, circular moorings are being placed
  under buildings made of alternating layers of
  rubber and steel.
• The rubber acts like a cushion to absorb
  earthquake waves.

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Earthquake Safety

• Homes can be protected by careful placement of
  heavy objects and securing gas appliances.
• During an earthquake, crawl under a sturdy table
  or desk outdoors stay away from buildings and
  power lines.
• After an earthquake, check for water or gas line
  damage leave immediately if a gas smell is
  present.