Transcript Phyllis Steckel, EQ Insight, LLC

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
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–
–
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‘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