Midcontinent Earthquakes EPSC 330 Earthquakes and Earth Structure Emily Fitzhenry 260407158 AlexaHaugan 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 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 sulphuriousvapour” • “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: • 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 • 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 • 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 • Jacobs, Andrew, Garbled Report on Sichuan Death Toll Revives Pain, New York Times published November 21, 2008 • 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 • Stein, S., Liu, M., Calais, E., and Li, Q. (2009) Mid-continent earthquakes as a complex system. Seismological Research Letters, v.80, 551-553 • 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/cgi-bin/show_picture.cgi?ID=ID.%20Earthquake%20Information%20Bulletin%20306 Retrieved March 26, 2013 • USGS Earthquakes Hazards Program, Historic Earthquakes, Tangshan, China http://earthquake.usgs.gov/earthquakes/world/events/1976_07_27.php Retrieved March 26, 2013 • USGS Earthquakes Hazards Program, Historic Earthquakes, Magnitude 7.9-Eastern Sichuan, Chinahttp://earthquake.usgs.gov/earthquakes/eqinthenews/2008/us2008ryan/ Retrieved March 25, 2013 • 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