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Earthquake Hazards And Earthquake Risks in the Central US

Earthquake Hazards And Earthquake Risks in the Central US. Or, What Keeps Geologists Awake at Night…. Earthquake Magnitude. How much energy released Logarithmic scale M6 = ~30 x M5 M7 = ~1,000 x M5. Earthquake Intensity. How much energy delivered to any one site

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Earthquake Hazards And Earthquake Risks in the Central US

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  1. Earthquake HazardsAnd Earthquake Risks in the Central US Or, What Keeps Geologists Awake at Night….

  2. Earthquake Magnitude • How much energy released • Logarithmic scale • M6 = ~30 x M5 • M7 = ~1,000 x M5

  3. Earthquake Intensity • How much energy delivered to any one site • Subjective: depends on felt-reportsfrom each location • Many different intensities for same earthquake

  4. Earthquake Depth • Range from shallow to deep (surface to ~800 km) • Central US range 0 to 40 km • Shallow = more energy and intensity at the surface • Deep = less energy and intensity at the surface

  5. Earth’s Crust • Thinner than an apple peel • Floats on viscous mantle • Pieces ‘bump and grind’ along plate edges  plate tectonics • Anomaly: Central US & others

  6. Earthquake Duration • Felt for a few seconds • small earthquake, near epicenter • Felt for several minutes • large earthquake, farther from epicenter • Extreme earthquakes ‘ring the earth’ for hours

  7. Aftershocks & Series • Occur after most larger earthquakes • Become smaller and less frequent over time • Can cause significant damage • Central US: major earthquakes tend to occur in series

  8. Did You Feel It? • April 18, 2008 • 4:36 am (CDT) • Magnitude 5.4 • Depth ~11 km • Epicenter near Bellmont, Ill.

  9. Earthquake Locations • Need three earthquake recordings (seismograms) • Measure distance from each recorder • Common point is approximate epicenter

  10. Earthquake Locations • Regional velocity of earthquake waves is known • Distance from epicenter is estimated • More recordings = better accuracy

  11. Mississippi Embayment • Very clear on maps! • ‘Bedrock trough’ dips & widens to the SW • New Madrid fault zone • ‘Bottom’ of trough • North end of trough • Filled with sediments • Mississippi River follows ‘easiest’ route

  12. New Madrid fault zone • Southeast Missouri & northeast Arkansas • Mississippi Embayment • Old weakness in earth’s crust • Active for hundreds of millions of years • Activity continues now • 8-year ‘monitoring’ is inconsequential

  13. Central US Earthquakes • New Madrid FZ • Three ‘dog-legs’ segments • Wabash Valley FZ • East Tennessee FZ • Ste. Genevieve FZ • ‘Background’ faults everywhere

  14. New Madrid 1811-12 • Founded 1789; heavy forests • Largest town between St. Louis & New Orleans • Frequent floods and swamplands around it • Heavy forests

  15. New Madrid Earthquakes • Winter of 1811-12 • Three earthquakes ~M7+ • 1000s of aftershocks • Wracked land, choked river • Most people left the area

  16. New Madrid Earthquakes • December 16, 1811 • ~mag 7.5 • January 23, 1812 • ~mag 7.3 • February 7, 1812 • ~mag 7.6

  17. Eliza Bryan • Born Pennsylvania 1780 • Arrived New Madrid 1791 • Earthquakes 1811-12 • Chronicled earthquakes 1816

  18. New Madrid Earthquakes • Eliza Bryan account • ‘Violent shocks …’ • ‘Continuous agitation …’ • ‘Sand ... from fissures’ • ‘Twenty foot waves …’ • Evidence still visible today

  19. New Madrid Earthquakes • River recedes from bank • 15- to 20-foot waves • ‘Waters gathered like a mountain …’ • Boats torn from moorings • ‘Water took groves of cottonwood trees’ • Flooded tributary ¼-mile

  20. New Madrid Earthquakes • ‘Retrograde current’ • Fault uplifted land surface downstream • Natural dam • Backflow created Reelfoot Lake • Channel soon reclaimed • Evidence still visible today

  21. New Madrid Earthquakes • Probably hundreds died, mostly on the river • African and Native Americans not counted • Insurance records (!) show losses of lives and insured cargoes

  22. Evidence Still Visible Today • Sandblows

  23. Evidence Still Visible Today • Reelfoot Lake • Northwest Tennessee • Sunklands

  24. New Madrid Earthquakes • Felt area larger than same-size California earthquakes • Rock here is different! • Aftershocks for years • What is odd about this map?

  25. USGS Products • Detailed hazard maps • Memphis, Tenn. • Evansville, Ind. • St. Louis, Mo. • Groundshaking • Liquefaction • Not site-specific!

  26. Phyllis Steckel, RG Earthquake Insight LLC Washington, Mo. In cooperation with the US GEOLOGICAL SURVEYCENTRAL US EARTHQUAKE PROGRAM

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