PASADENA, CA, March 6, 2014 -- According to a new study involving researchers at the U.S. Geological Survey (USGS), scientists observed that a human-induced magnitude 5.0 earthquake near Prague, Okla., in November 2011, may have triggered the larger M5.7 earthquake less than a day later. This research suggests that the M5.7 quake was the largest human-caused earthquake associated with wastewater injection.
Historically, earthquakes in the central United States have been uncommon. Yet in the year 2011 alone, numerous moderate-sized earthquakes occurred in Colorado, Texas, Oklahoma, Ohio, and Arkansas. Many of these earthquakes occurred near wastewater injection wells, and some have been proven to be caused by human activities.
The 2011 Oklahoma earthquake sequence included the M5.7 earthquake on November 6 that ruptured a part of the Wilzetta fault system, a complex fault zone about 200 km (124 mi) in length near the ceity of Prague. Less than 24 hours prior to the M5.7, a M5.0 foreshock occurred on November 5, which occurred near active wastewater disposal wells and was linked in a previously-published study to fluid injection in those wells. These earthquakes have not been directly linked to hydrofracturing.
The new research suggests that the foreshock, by increasing stresses where M5.7 mainshock ruptured, may have triggered the mainshock, which in turn, triggered thousands of aftershocks along the Wilzetta fault system, including a M5.0 aftershock on November 8. If this hypothesis is correct, the M5.7 earthquake would be the largest and most powerful earthquake ever associated with wastewater injection. All three earthquakes of magnitude 5.0 and greater along the Wilzetta fault exhibited strike-slip motion at three independent locations along the fault, suggesting that three separate portions of the Wilzetta fault system were activated.
The paper, "Observations of Static Coulomb Stress Triggering of the November 2011 M5.7 Oklahoma Earthquake Sequence," was published in the "Journal of Geophysical Research" this week.
See also: "Underground Unrest"