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, :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 Founded 1789; heavy forests Largest town between St. Louis & New Orleans Frequent floods and swamplands around it Heavy forests

New Madrid Earthquakes Winter of Three earthquakes ~M s 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 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!

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