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Earthquakes. 8 th Grade Science Mrs. Pientok. 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.

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Earthquakes l.jpg

Earthquakes

8th Grade Science

Mrs. Pientok


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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 Earth’s plates to move placing stress on or near the plate edge.

    • Rocks will bend, compress, stretch and possibly break.

    • Earthquakes—vibration 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 rocks—tension, compression, and shear.

    • Tension forces; normal faults—caused by rock above the fault moving downward in relation to the rock below the fault.

    • Reverse fault—compression forces squeeze rock above the fault up and over the rock below the fault.

    • Created by shear forces; strike-slip fault—rocks 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.

  • Focus—an earthquake’s point of energy release.

  • Epicenter—located 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

  • Seismograph—measures 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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Earth’s Interior

  • Earth’s structure consist of an inner, mostly iron, solid core, surrounded by a mostly iron liquid outer core surrounded by the mantle.

    • The crust is Earth’s outer layer, about 5-60 km thick.


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Movement of Waves Through Earth’s Interior

  • A seismic wave’s 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 Earth’s layers.


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People and Earthquakes

  • Although earthquakes are natural geological events, they kill many people and cause a lot of damage.

    • Seismologists—scientists who study earthquakes.

    • Magnitude—measure 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 humans—3.0-4.9 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

  • Liquefacation—shaking from an earthquake can make wet soil act like a liquid.

  • Tsunamis—ocean 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.


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