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Chapter 16 Earthquakes

Chapter 16 Earthquakes. What is an earthquake? Shaking of the ground caused by the sudden release of energy stored in the rocks. When rocks are stressed, potential energy is stored in them.

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Chapter 16 Earthquakes

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  1. Chapter 16 Earthquakes

  2. What is an earthquake? • Shaking of the ground caused by the sudden release of energy stored in the rocks. • When rocks are stressed, potential energy is stored in them.

  3. Rocks can behave plastically upto a certain limit (elastic limit), after which they fail and release the energy. • Energy waves produced by an earthquake are called Seismic Waves.

  4. The generation of an earthquake is explained by Elastic Rebound Theory. • According to this theory, the sudden release of progressively stored strain in rocks causes movement along a fault and an earthquake is generated.

  5. Causes of earthquakes • Movement along a fault • Volcanic eruptions • Plate movement • Subsidence

  6. Earthquake terminology • Focus: the point inside the earth where seismic waves first generate. • Epicenter: the point on ground surface directly above the focus. It may be any geographic locality.

  7. Seismic waves • 2 types • Body waves: traveling inside the earth • Surface waves: travel on the earth’s surface.

  8. Body waves: • P-waves (primary) • S-waves (secondary) • P-wave: is the fastest (4-7 km/sec) and is the first to arrive at a recording station.

  9. P-wave: compressional or longitudinal wave in which rocks vibrate back and forth parallel to the direction of wave propagation.

  10. S-wave: slower (2-5 km/sec) and arrives after the P-wave at a recording station. • It’s a transverse wave in which the rock vibrates perpendicular to the direction of wave propagation.

  11. Surface waves: • slowest waves generated by an earthquake. • They take longer to pass and cause more property damage.

  12. EQ measurement • The instrument used to measure an EQ is called a seismometer.

  13. Seismograph: paper or electronic record of an EQ.

  14. World seismograph stations

  15. Locating EQ Epicenter • Travel time of the seismic waves from the focus to the seismograph station is used to determine the EQ epicenter. • P and S waves gradually separate because they travel at different velocities.

  16. The P-S interval is compared with a standard travel-time curve.

  17. Data from one station can be used to determine only distance to the station and not the direction. • At least three stations are required to determine the location of the epicenter.

  18. Measuring the size of an EQ • Two ways of determining the EQ size; • Intensity • Magnitude

  19. Measuring the size of an EQ • EQ Intensity: it is a measure of how an area has been damaged by the EQ. • Modified Mercalli Scale is used to measure intensity.

  20. EQ magnitude: it’s a measure of the amount of energy released by an EQ. • EQ magnitude is reported on the Richter scale.

  21. It is done by measuring the height (amplitude) of a specific wave. • ML = log10A(mm) + (Distance correction factor)

  22. Magnitude (log Scale) Possible Effects 1-2 Normally only detected by instruments 3 Only faintly felt 4 Faint tremor causing little damage 5 Structural damage 6 Distinct shaking, less well-constructed buildings collapse 7 Most buildings destroyed 8 Major structures destroyed 9 Ground seems to shake-catastrophic

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