Earthquake Hazards And Earthquake Risks in the Central US

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Earthquake HazardsAnd Earthquake Risks in the
Central US

• Or, What Keeps Geologists
• Awake at Night.

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Earthquake Magnitude

• How much energy released
• Logarithmic scale
• M6 30 x M5
• M7 1,000 x M5

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Earthquake Intensity

• How much energy delivered to any one site
• Subjective depends on felt-reports from each
  location
• Many different intensities for same earthquake

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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

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Earths Crust

• Thinner than an apple peel
• Floats on viscous mantle
• Pieces bump and grind along plate edges ? plate
  tectonics
• Anomaly Central US others

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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

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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

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Did You Feel It?

• April 18, 2008
• 436 am (CDT)
• Magnitude 5.4
• Depth 11 km
• Epicenter near Bellmont, Ill.

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Earthquake Locations

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

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Earthquake Locations

• Regional velocity of earthquake waves is known
• Distance from epicenter is estimated
• More recordings better accuracy

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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

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New Madrid fault zone

• Southeast Missouri northeast Arkansas
• Mississippi Embayment
• Old weakness in earths crust
• Active for hundreds of millions of years
• Activity continues now
• 8-year monitoring is inconsequential

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Central US Earthquakes

• New Madrid FZ
• Three dog-legs segments
• Wabash Valley FZ
• East Tennessee FZ
• Ste. Genevieve FZ
• Background faults everywhere

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New Madrid 1811-12

• Founded 1789 heavy forests
• Largest town between St. Louis New Orleans
• Frequent floods and swamplands around it
• Heavy forests

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New Madrid Earthquakes

• Winter of 1811-12
• Three earthquakes M7
• 1000s of aftershocks
• Wracked land, choked river
• Most people left the area

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New Madrid Earthquakes

• December 16, 1811
• mag 7.5
• January 23, 1812
• mag 7.3
• February 7, 1812
• mag 7.6

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Eliza Bryan

• Born Pennsylvania 1780
• Arrived New Madrid 1791
• Earthquakes 1811-12
• Chronicled earthquakes 1816

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New Madrid Earthquakes

• Eliza Bryan account
• Violent shocks
• Continuous agitation
• Sand ... from fissures
• Twenty foot waves
• Evidence still visible today

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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

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New Madrid Earthquakes

• Retrograde current
• Fault uplifted land surface downstream
• Natural dam
• Backflow created Reelfoot Lake
• Channel soon reclaimed
• Evidence still visible today

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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

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Evidence Still Visible Today

• Sandblows

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Evidence Still Visible Today

• Reelfoot Lake
• Northwest Tennessee
• Sunklands

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New Madrid Earthquakes

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

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USGS Products

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

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US GEOLOGICAL SURVEYCENTRAL US EARTHQUAKE PROGRAM

• Phyllis Steckel, RG
• Earthquake Insight LLC
• Washington, Mo.
• In cooperation with the