Measuring Earthquakes - PowerPoint PPT Presentation

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  1. Measuring Earthquakes Chapter 8, Section 2

  2. 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

  3. Seismographs

  4. Seismograms

  5. 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 • The waves arrive at different times because they travel at different speeds

  6. Surface Waves

  7. P Waves

  8. S Waves

  9. 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

  10. Travel-Time Graph

  11. Finding an Epicenter

  12. Global Distribution of Earthquakes

  13. 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two different types of measurements to describe the size of an earthquake – intensity and magnitude • Richter Scale – outdated scale for measuring the magnitude of earthquakes, uses amplitude of the largest seismic wave and uses a logarithmic scale (ten times increase for every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

  14. Moment Magnitude

  15. 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

  16. Modified Mercalli Scale

  17. Some Notable Earthquakes

  18. Assignment • Read Chapter 8, Section 2 (pg. 222-228) • Do 8.2 Assessment #1-6 (pg. 228)