Just over three years ago on the 5th of November 2011, an earthquake erupted in Oklahoma State, a foreshock measuring M5.0 on the Richter scale. That earthquake gave birth to another earthquake less than twenty four hours, a mainshock that measured M5.7 on the richter scale. The energy released by this secondary earthquake then further instigated thousands of aftershocks, one of which measured M5.0, the same size as the original foreshock a couple days earlier.
Only experiencing around 50 earthquakes a year, as recorded by the U.S. Geological Survey (USGS), the state of Oklahoma was never a state known for its earthquakes in the United States, only tornadoes. However public perception changed when, in the year 2011 alone, the USGS recorded 1,500 earthquakes in Oklahoma, and 2,611 for the state in 2013.
Whilst Oklahoma have experienced similar sized earth quakes before, an M4.6 on 9th April 1969, and M5.7 on 2nd May 1952, never before has the state experienced this high a frequency as recorded in 2011.
The M5.0 foreshock occurred in close proximity to active wastewater injection wells utilised by the gas industry for shale gas extraction.
A 2013 study by Keranan et al, linked the M5.0 foreshock to the nearby injection wells, stating that fluid injection of fracking waste water caused a build up of pore fluid pressure underground, which in turn decreased the fault strength, therefore creating the ability to induce an earthquake. They found that the epicentre of the M5.0 foreshock occurred 200m from active injection wells and over half a mile under the surface.
In a new study recently published in the Journal of Geophysical Research 7th March 2014, Sumy et al, show that the original M5.0 foreshock, induced by fluid injection, potentially triggered a cascading failure of earthquakes, known as earthquake swarms, including the M5.7 main-shock and subsequent aftershocks. By analysing the co-seismic Coulomb stress transfers, the rupture geometry of the three M≥5.0 and regional stress orientation, the report outlines the cause and effect relationship between the earthquakes released and the injecting of wastewater fluids from Unconventional Shale Gas Extraction.
The reports add to previous studies that pumping millions of gallons of drilling fluids underground puts stress on fault lines and can cause anthropogenically produced earthquakes.
“The observation that a human-induced earthquake can trigger a cascade of earthquakes, including a larger one, has important implications for reducing the seismic risk from wastewater injection,” said Elizabeth Cochran, USGS seismologist and coauthor of the study.
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2. USGS. (2014). Earthquake Hazards Program – Oklahoma Earth Quake History. Available: http://earthquake.usgs.gov/earthquakes/states/oklahoma/history.php. Last accessed 11/03/2014.
3. Keranen.K, Savage.H, Abers.G, Cochran.E. (2013). Potentially induced earthquakes in Oklahoma, USA: Links between wastewater injection and the 2011 Mw 5.7 earthquake .. Available: http://geology.gsapubs.org/content/early/2013/03/26/G34045.1.abstract. Last accessed 11/03/2014.
4. Sumy.D, Cochran.E, Keranen.K, Wei.M, Abers.G. (2014). Observations of static Coulomb stress triggering of the November 2011 M5.7 Oklahoma earthquake sequence. Available: http://onlinelibrary.wiley.com/enhanced/doi/10.1002/2013JB010612/. Last accessed 11/03/2014.
5. Garcia, S. (2014). 2011 Oklahoma Induced Earthquake May Have Triggered Larger Quake. Available: http://www.usgs.gov/newsroom/article.asp?ID=3819#.UyMUvM1bwy4/. Last accessed 11/03/2014.