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Earthquakes

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

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  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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. 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.

  8. 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

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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).

  15. 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.

  16. 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.

  17. 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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