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Earthquakes

Earthquakes. If the Earth’s a rockin’…. Deformation Elastic deformation (rubber band)—object being deformed stretches building up potential energy Remove stress  energy released object returns to normal Everything has limits Stretching a rubber band until it breaks Rocks behave similarly.

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Earthquakes

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  1. Earthquakes

  2. If the Earth’s a rockin’…. • Deformation • Elastic deformation (rubber band)—object being deformed stretches building up potential energy • Remove stress energy released object returns to normal • Everything has limits • Stretching a rubber band until it breaks • Rocks behave similarly

  3. The Stress Builds

  4. Where do earthquakes occur • Tend to form along plate boundaries • Earth’s surface is divided into ~16 plates • Plates interact by ramming into and sliding past each other

  5. Earthquake Strength • Old system—Mercalli scale • Based on building damage • inaccurate • Richter Scale (ML) • Magnitude 5.0 has amplitude 10x that of 4.0 • Based on height of largest peak on seismograph • Richter less accurate above ML=6.5 • Moment Magnitude (Mw) • Total energy expended during quake • Measured from fault offset, area affected • Each increment is 30 times stronger than the one previous to it

  6. Richter ScalePractical equivelents avg. San Andreas quakes Sumatran Tsunami quake

  7. Earthquakes…globally

  8. Transform Boundary EQs • Transform margin • Plates grind past each other—strike-slip fault • One large fault—numerous smaller ones • San Andreas Fault Zone • 10,000 Eqs/year • Periodic large quakes

  9. Great San Francisco Quake of 1909 • Mw 7.8 • City of San Fran completely destroyed • Fires caused by ruptured gas mains caused 99% of the damage—rebuilding costs comparable to Hurricane Katrina • 3000 people died

  10. Strike-slip motion • 3 ways that the NAM plate slides past the Pac plate • Fault creep—smooth • Hopping—movement occurs as a series of jumps—series of small Eqs • Catastrophic breaks—fault sticks and releases generating one big Eq

  11. Convergent Plate Eqs • Benioff zone—area along top of subducting plate frequented by Eqs • Tend to be large and devastating • Coastal population threat • Tsunamis often generated • “harbor wave” • Huge waves caused by displacement of water

  12. Sumatra Tsunami • 7:59 AM, Dec. 26, 2004 • Mw=9.0 • Subduction Zone • Indian + Burma Plate • 15 m total movement, seafloor rises sev. m • 700 kph (420 mph) • Total deaths: 283,000 (~2.5x pop of McKinney)

  13. Why There? Why So Big? • Indian moving NE at 6 cm/yr • Plate was stuck • For likely 100s of years • Burma Plate bends Burma Plate Indo-Australian Plate

  14. Burma Plate flexes/moves • 1200 km length of subduction zone displaced vertically 15 m • Causes sea level to rise/fall • Big wave generated

  15. Tsunami Like an Ocean Wave • A particle on the surface moves in a circular fashion • Effects of waves disappear at about ½ wavelength • Tsunami wavelength ~ 180 km (much deeper than oceans) • Tsunami drags on ocean floor

  16. Sumatra Tsunami, 2004 • Waves were ~10m (32’) and 24m locally • In Sumatra and Thailand – people live on coast • Homes are usually less than 15’ • Energy carries waves inland, maybe 1-8 km • And then wave goes back out • Time between waves can be >30 minutes • The benefit of a good education

  17. Video 1

  18. Notice: bent waves

  19. Damage Control • People outside are safer (even in tsunami) • Biggest danger is overhead • Neighboring buildings • Unsupported buildings, glass, and just about anything not nailed down • If caught inside: • Lie down next to something that won’t flatten (not in doorways or under table)

  20. Politics and Predictions • Hysteria vs. withholding information • Predictions and chance • The need for short-term predictions • Uncertainty! Tangshan 1976 242,769 deaths No warning

  21. Problems and Precursors • Large earthquakes and changing stress • Large earthquakes can put stress on neighboring faults • Foreshocks • Radon Gas?? • Fracturing of rock allows radon gas to seep into foundations • Earthquake frequency changes, clustering

  22. Long-Term Forecasts • If you knew decades in advance: • Construction projects, insurance • More reliable than short-term predictions • Recurrence interval of past events • Risk Maps

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