earthquake hazards and earthquake risks in the central us or, what keeps geologists awake at...

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