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Understanding Earthquakes: Causes, Waves, Measurement, and Hazards

Explore the forces behind earthquakes, types of stress on the Earth's crust, wave characteristics, measuring scales, and the risks associated with seismic events. Learn how to locate earthquake epicenters and understand the impact of structural failures and natural phenomena like tsunamis. Discover how seismic belts shape our understanding of earthquake-prone regions and the importance of earthquake preparedness.

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Understanding Earthquakes: Causes, Waves, Measurement, and Hazards

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  1. Chapter 19 Notes Earthquakes

  2. Stress and Strain • Earthquakes occur when stress builds up and causes movement in the Earth’s crust • Compression: squeezing forces • Tension: pulling forces • Shear: opposing parallel forces/twisting • Elastic deformation: initial response to stress • plastic deformation occurs under high stress just before breaking

  3. Faults • Normal fault: produced by tension forces • Reverse fault: produced by compression forces • Strike-slip fault: produced by shear forces

  4. Earthquake Waves • Body Waves • Primary (P-waves): compression waves; move parallel to the direction of wave motion • Secondary (S-waves): transverse waves; move at right angles to the direction of wave motion • Surface waves: move sideways and up and down

  5. Earthquake Waves • P-waves move fastest • Surface waves are slowest, last the longest, and are the most destructive

  6. Earthquake location • Focus: the origin of the EQ waves • Epicenter: “above” the center

  7. Earthquake Measurement • Seismometer: an instrument that records EQ waves • Seismogram: the record of EQ waves

  8. Time travel curves • The difference in arrival time of the P-waves and S-waves can show how far the waves have travelled.

  9. Clues to Earth’s Interior • EQ waves are reflected and refracted in different ways by the different materials inside the Earth

  10. Measuring Earthquakes • Richter Scale: • measures magnitude • energy released • numeric scale (no true upper limit) • Modified Mercalli Scale: • measures intensity • based on the amount of damage • Roman numerals I to XII

  11. Richter Scale

  12. Mercalli Isoseismic Map

  13. Locating an Earthquake • The P-wave and S-wave arrival times determine the distance a station is to an EQ • Three stations are required to locate the epicenter of an EQ

  14. Seismic Belts • Circum-Pacific and Mediterranean-Asian belts

  15. Earthquake Hazards • “Earthquakes don’t kill people, buildings do.” • Structural failures • Brittle building materials cause more damage • Building height oscillations • Land and soil failure • Soil liquefaction • Tsunami • Ocean wave generated by sea floor movement

  16. U.S. Earthquake Risk

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