Midcontinent
Earthquakes
EPSC 330 Earthquakes and Earth Structure
Emily Fitzhenry 260407158
Alexa Haugan 260422828
Overview of the Presentation
 Introduction to earthquake processes
 Midcontinent earthquakes
 Real-life examples
 New Madrid
 Tangshan
 Sichuan
 Methods of predicting these types of earthquakes
 Implications for the tectonic hazards of Quebec
Elastic Rebound Theory
 Steady relative motion of tectonic plates due to convection of the
mantle
 Elastic strain energy builds up along their edges in the rocks along
fault planes
 When shearing stresses induced exceed shear strength of the rock,
rupture occurs: there is failure (slip) along the fault plane
 Thus, earthquakes are focused along plate boundary faults and tend
to repeat on the same fault segments at periodic time intervals
General Causes of Earthquakes
 Fault movements occurring preferentially along the
edges of tectonic plates
 Magma movement in the shallow subsurface at
spreading centres
 The compressional movements at subduction zones
and continent–continent collisions
- Not a spreading
centre
- Not a
subduction zone
- Well within the
tectonic plate
Probabilistic assessment of seismic hazard in the United States. Parsons (2009), based on Frankel, A. et al. US Geol. Surv. Open-File
Rep. 02-420 (2002). Warm colors show regions with the highest probability of strong shaking.
Midcontinent Earthquakes
 Tectonic loading is collectively accommodated by a
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
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complex system of interacting faults
Commonly, ancient basement faults are hidden under a
thick cover of younger sediments which bury them
Loading rate on a given fault is slow & may not be
constant
Earthquakes can cluster on a fault for a while, then may
suddenly shift
Long-range migration between mechanically coupled
fault systems in the continental interior
Seismic Moment Release
 Should stay around a certain level, with the moment
release rate of individual fault zones being complementary
 The moment release between the Weihe and Shanxi rifts
show increases in one corresponding to decreases in the
other
 Thus, they must be mechanically coupled!
- μ is shear modulus
- A is area of the rupture along the fault
- D is average displacement
New Madrid Fault Earthquakes
 New Madrid Fault: Missouri,
Arkansas, Tennessee, Illinois,
Kentucky
 3 earthquakes greater than
magnitude 7
 Eyewitness accounts:
 “subterraneous thunder….everything
went dark as the skies filled with
sulphurious vapour”
 “all of nature seemed to be running
into chaos…the rivers they boiled like
a pot over coals”
 “the roar I thought would leave us
deaf if we lived”
Probabilistic assessment of seismic hazard in the United States. Parsons (2009), based on Frankel, A. et al. US Geol. Surv. Open-File
Rep. 02-420 (2002). Warm colors show regions with the highest probability of strong shaking.
Tangshan Earthquake
 Occurred early in the morning on Wednesday, July 28, 1976.
 Death toll of over 240,000, making it one of the deadliest
earthquakes of all time.
 Magnitude 7.5
 Aftershocks in the following days continued to cause a lot of
damage.
 Could have been predicted?
Sichuan Earthquake
 Monday May 12, 2008
 Mw 7.9
 Highly unexpected
 About 70,000 deaths were
confirmed, and 4.8 million
people were left homeless
 Damage costs estimated to
be 75 Billion USD
 Tremors were felt as far as
Russia, Indonesia and India
Implications for Earthquake Prediction
 Difficulty: midcontinent
earthquakes are so infrequent,
we do not have many historical
records, and patterns are not
well understood
 Past large earthquakes indicate
where more earthquakes will
occur; this leads to
underestimation of hazards
elsewhere
Approaches to Predicting
Midcontinent Earthquakes
 High-precision global positioning system
measurements
 Detailed paleoseismic studies
 Computer simulation of fault interactions in
midcontinents
 Need to show which faults with past large earthquakes
are shutting off for some time, and which apparently
dormant faults are awakening and quietly building up
stress
St. Lawrence Rift System
 Fault reactivation is believed to occur along faults that
run parallel to the river, formed during the Iatepan rift,
where there was breakup of Rodinia in the early
Paleozoic
 Greatest seismicity occurs where the rift is overprinted
by a meteorite crater, the Charlevoix impact structure
 5 earthquakes of estimated magnitude of 6 or greater
since historical records began
 What is in store for the future? seismic risk in currently
quiescent areas of the rift valley may be
underestimated
Alternate Causes of Midcontinent
Earthquakes?
 Hydroelectric dams, mineral deposit
exploitation and other anthropogenic causes
 Climatic factors, such as melting ice sheets:
 “16,000-10,000 years ago an ice sheet
melted, and steadily rushed water down the
Mississippi River
 This washed away sediments and removed
weight pressing down on the Earth’s crust
 The crust rebounded and bulged slightly up,
causing crushing top layers and compacting
bottom layers
 Force exertion on the preexisting faults
sufficient to trigger earthquakes in the New
Madrid region, culminating with the 1811-1812
events”
Final Thoughts
 We are at the complete mercy of
nature
 Even people in Winnipeg might
not be safe 
 Rescue techniques using modern
equipment can actually make
things worse, and aftershocks can
still strike.
 We tend to overestimate the
hazards in places of previous
large earthquakes, meanwhile we
underestimate the hazards
elsewhere (like in mid-continental
regions!)
References:
1. Baird, A.F., McKinnon S.D., and Godin L. (2010). Relationship between structures, stress and seismicity in the Charlevoix seismic zone
revealed by 3‐D geomechanical models: Implications for the seismotectonics of continental interiors. Journal of Geophysical Research:
Solid Earth, 115
2. Li, Q., Liu, M., and Stein, S. (2009). Spatiotemporal Complexity of Continental Intraplate Seismicity: Insights from Geodynamic
Modeling and Implications for Seismic Hazard Estimation. Bulletin of the Seismological Society of America 99(1), 52–60
3. Li, Q., Liu, M., and Stein, S. (2011). 2000 years of migrating earthquakes in North China: How earthquakes in midcontinents differ from
those at plate boundaries. Lithosphere 3(2),128–132
4. Jacobs, Andrew, Garbled Report on Sichuan Death Toll Revives Pain, New York Times published November 21, 2008
5. New York Times, Sichuan Earthquake, Updated May 6, 2009
http://topics.nytimes.com/topics/news/science/topics/earthquakes/sichuan_province_china/index.html Retrieved March 25, 2013
6. Stein, S., Liu, M., Calais, E., and Li, Q. (2009) Mid-continent earthquakes as a complex system. Seismological Research Letters, v.80,
551-553
7. USGS Photographic Library, Tangshan, China, Earthquake July 28, 1976. “The Chengli Bridge in Tangshan crumpled during the
earthquake”. Portion of Figure 3, page 108, Earthquake Information Bulletin, v.11, no.3 http://libraryphoto.cr.usgs.gov/cgibin/show_picture.cgi?ID=ID.%20Earthquake%20Information%20Bulletin%20306 Retrieved March 26, 2013
8. USGS Earthquakes Hazards Program, Historic Earthquakes, Tangshan, China
http://earthquake.usgs.gov/earthquakes/world/events/1976_07_27.php Retrieved March 26, 2013
9. USGS Earthquakes Hazards Program, Historic Earthquakes, Magnitude 7.9-Eastern Sichuan,
Chinahttp://earthquake.usgs.gov/earthquakes/eqinthenews/2008/us2008ryan/ Retrieved March 25, 2013
10.Wang, H et. al (2010). Balance of seismic moment in the Songpan-Ganze region, eastern Tibet: Implications for the 2008 Great
Wenchuan earthquake. Tectonophysics, 491, 154–164