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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 • Subjective: depends on felt-reports from each location • Many different intensities for same earthquake 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 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 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 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 Did You Feel It? • April 18, 2008 • 4:36 am (CDT) • Magnitude 5.4 • Depth ~11 km • Epicenter near Bellmont, Ill. Earthquake Locations • Need three earthquake recordings (seismograms) • Measure distance from each recorder • Common point is approximate epicenter Earthquake Locations • Regional velocity of earthquake waves is known • Distance from epicenter is estimated • More recordings = better accuracy 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 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 Central US Earthquakes • New Madrid FZ – Three ‘dog-legs’ segments • Wabash Valley FZ • East Tennessee FZ • Ste. Genevieve FZ • ‘Background’ faults everywhere New Madrid 1811-12 • Founded 1789; heavy forests • Largest town between St. Louis & New Orleans • Frequent floods and swamplands around it • Heavy forests New Madrid Earthquakes • Winter of 1811-12 • Three earthquakes ~M7+ • 1000s of aftershocks • Wracked land, choked river • Most people left the area New Madrid Earthquakes • December 16, 1811 – ~mag 7.5 • January 23, 1812 – ~mag 7.3 • February 7, 1812 – ~mag 7.6 Eliza Bryan • Born Pennsylvania 1780 • Arrived New Madrid 1791 • Earthquakes 1811-12 • Chronicled earthquakes 1816 New Madrid Earthquakes • Eliza Bryan account – – – – ‘Violent shocks …’ ‘Continuous agitation …’ ‘Sand ... from fissures’ ‘Twenty foot waves …’ • Evidence still visible today 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 New Madrid Earthquakes • ‘Retrograde current’ – Fault uplifted land surface downstream – Natural dam – Backflow created Reelfoot Lake – Channel soon reclaimed • Evidence still visible today 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 Evidence Still Visible Today • Sandblows Evidence Still Visible Today • Reelfoot Lake • Northwest Tennessee • Sunklands New Madrid Earthquakes • Felt area larger than same-size California earthquakes – Rock here is different! • Aftershocks for years • What is odd about this map? USGS Products • Detailed hazard maps – Memphis, Tenn. – Evansville, Ind. – St. Louis, Mo. • Groundshaking • Liquefaction • Not site-specific! Phyllis Steckel, RG Earthquake Insight LLC Washington, Mo. In cooperation with the US GEOLOGICAL SURVEY CENTRAL US EARTHQUAKE PROGRAM