Earthquake HazardsAnd 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!

US GEOLOGICAL SURVEYCENTRAL US EARTHQUAKE PROGRAM

Phyllis Steckel, RG

Earthquake Insight LLC

Washington, Mo.

In cooperation with the