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Warm Up

Warm Up. When an earthquake occurs , energy radiates in all directions from its source , which is called the ____. a. fault c. seismic center b. epicenter d. focus The hypothesis that explains the release of energy during an earthquake is called the ____.

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Warm Up

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  1. Warm Up • When an earthquake occurs, energy radiates in all directions from its source, which is called the ____. a. fault c. seismic center b. epicenter d. focus • The hypothesis that explains the release of energy during an earthquake is called the ____. a. moment magnitude hypothesis b. elastic rebound hypothesis c. Richter hypothesis d. vibration hypothesis • Most earthquakes are produced by the rapid release of which kind of energy stored in rock subjected to great forces? a. thermal c. chemical b. elastic d. mechanical Answers: 1) d. 2) b. 3) b.

  2. Measuring Earthquakes Chapter 8, Section 2

  3. Seismographs • Seismographs – instruments that record earthquake waves • When waves from an earthquake come into contact with a seismograph, a weight suspended from a support will remain motionless as Earth moves (providing a reference point) and can draw a diagram showing Earth’s motion • Seismograms – the electronically recorded ground motion from a seismograph

  4. Seismographs

  5. Seismograms

  6. Earthquake Waves • Surface Waves – seismic waves that travel along Earth’s surface, most destructive seismic waves • Surface waves travel along the ground and cause the ground and anything resting upon it to move • P waves – push-pull waves; they push (compress) and pull (expand) rocks in the direction the waves travel • S waves – shake the particles at right angles to their direction of travel • Gases and liquids do not transmit s waves, but do transmit p waves • A seismogram shows all three types of waves: the p waves arrive first, then the s waves, followed by the surface waves last

  7. Surface Waves

  8. P Waves

  9. S Waves

  10. Concept Check • Which seismic wave travels fastest? • P waves

  11. Locating an Earthquake • We can determine the distance to an epicenter by finding the difference between the arrival of p waves and s waves , then looking at a travel-time graph we can determine how far away the epicenter is • Travel-time graphs from three or more seismographs can be used to find the exact location of an earthquake epicenter • About 95% of earthquakes occur in a few narrow zones, and most of these occur around the Pacific Ocean

  12. Travel-Time Graph

  13. Finding an Epicenter

  14. Global Distribution of Earthquakes

  15. Concept Check • Where do most earthquakes occur? • Along the edge of the Pacific Ocean

  16. Measuring Earthquakes • Historically, scientists have used two different types of measurements to describe the size of an earthquake – intensity and magnitude • Richter Scale – outdated scale for measuring the magnitude of earthquakes, uses amplitude of the largest seismic wave and uses a logarithmic scale (ten times increase for every increase of 1 on the scale) • Moment Magnitude – derived from the amount of displacement that occurs along a fault zone (surface area of fault) x (avg. displacement along fault) x (rigidity of rock) • Moment magnitude is the most widely used measurement for earthquakes because it is the only magnitude scale that estimates the energy released by earthquakes

  17. Moment Magnitude

  18. Concept Check • Which scale is the most widely used for measuring an earthquake’s magnitude? • Moment Magnitude

  19. Modified Mercalli Scale • Modified Mercalli Scale – rates an earthquake’s intensity in terms of the earthquake’s effects at different locations • It has 12 steps, expressed as roman numerals • An earthquake that can barely be felt is rated as a I • An earthquake that causes near total damage is rated as a XII • The same earthquake can be given different ratings at different locations

  20. Modified Mercalli Scale

  21. Some Notable Earthquakes

  22. Assignment • Read Chapter 8, Section 2 (pg. 222-228) • Do Chapter 8 Assessment #1-29 (pg. 243-244) • For Section 2: Do #’s 4, 5, 9-12, 18, 19, 22, & 24-29

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