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Intraplate Earthquakes

Intraplate Earthquakes

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Intraplate Earthquakes

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  1. Intraplate Earthquakes • New Madrid, Missouri: December 1811, January 1812, February 1812

  2. Intraplate Earthquakes • Eastern U.S. earthquakes less frequent, but more widely felt • Crust is older and transmits stress

  3. Earthquake Size and Characteristics • How big is it? • Answer in terms of perceived effects: intensity • Answer in terms of amount of energy released: magnitude

  4. Earthquake Intensity • Mercalli Intensity Scale developed in 1902 • Based on effects • Local small quake is similar to distant large quake

  5. Felt effects using Mercalli Scale http://www.scec.org/instanet/01news/images/NorthridgeSMap.gif

  6. Felt effects using Mercalli Scale Hypothetical M7.8 earthquake

  7. Landers 1992 earthquake: Felt intensity by zip code

  8. Earthquake Magnitude • Richter Magnitude Scale (ML) derived by Charles Richter in 1935 • Uses maximum amplitude of earthquake waves on seismograph • Logarithmic scale • Richter magnitude less accurate above M6.5

  9. Problems with scales • Moment magnitude is measure of total energy expended during earthquake • Determined from long-period waves • Moment = (shear strength of rocks) x (surface area of rupture) x (slip distance on fault) • Each number is 32X energy • Each 2 numbers =1000X • This is the most common scale for quakes >3.5

  10. Another example of M=1/f

  11. 1995 Kobe earthquake, Japan

  12. 1989 Loma Prieta earthquake, SF Bay Area

  13. Liquifaction Water in sediment causes solid rock to behave like a liquid.

  14. Liquifaction

  15. Secondary Ground Effects • Surface rupture-scarp • Earthquakes often trigger landslides • Can also cause liquefaction • Soils become almost liquid when shaken, solidify when shaking stops • Significant damage to structures atop liquefied sediments • Fires

  16. Surface rupture-scarp

  17. 1906 earthquake surface rupture. 8’ fence offset above http://mnw.eas.slu.edu/Earthquake_Center/1906EQ/1906thumb.html And http://quake.wr.usgs.gov/info/1906/images/fenceoffset_big.html

  18. Australia, 1968, M6.8

  19. Armenia, 1988, M6.9

  20. Borah Peak, ID: 1983 M7.3

  21. California, 1979, M6.9

  22. Landslides caused by 2002 Denali Fault earthquake

  23. 1965 Seattle quake M6.5

  24. Liquifaction Water in sediment causes solid rock to behave like a liquid.

  25. This residential and commercial building sank more than three feet into the partially liquefied soil.

  26. Liquifaction: Niigata, Japan, 1964 :

  27. San Francisco 1906 M8.3 Secondary effects: Fire

  28. Long term probability • Probability of where and when an earthquake will strike used to construct risk map

  29. Earthquake Hazards2% chance in next 50 years

  30. The San Francisco Bay Area

  31. Las Cruces: 15-18% chance of M5 in 50 yrs.

  32. Sample Test Questions