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Earthquake Essentials: Understanding the Causes, Waves, and Measurement

Discover the fundamentals of earthquakes, including their causes, seismic waves, measurement techniques, and potential effects. Learn about terms like focus, epicenter, P-waves, S-waves, intensity, and magnitude in this informative guide.

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Earthquake Essentials: Understanding the Causes, Waves, and Measurement

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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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