A recently released study has found that artificial earthquakes resulting from the use of disposal wells for produced waters and other oil and gas wastewaters are less intense, and in general about sixteen times weaker, than naturally occurring earthquakes. The peer-reviewed paper included analysis of eleven earthquakes occurring between 2011 and 2013 in Arkansas, Colorado, Ohio, Oklahoma, and Texas that are suspected of being caused by the use of injection wells.
The study was authored by U.S. Geological Survey (USGS) geophysicist Susan Hough, and appears in the October 2014 edition of the Bulletin of the Seismological Society of America. Hough examined user-submitted data from the USGS’ “Did You Feel It?” reporting system, finding that although natural and manmade earthquakes may generate equivalent strength readings on earthquake sensor equipment, people report feeling different intensities of shaking. Hough speculated that the artificial quakes may have less energy because the fault lines are lubricated by the injected wastewater and therefore slip smoothly and with less built-up energy than if they had slipped naturally years later.
Individuals within about six miles of the epicenter reported little difference between the types of earthquakes, but beyond that distance, artificial earthquakes are reported to feel, on average, sixteen times weaker. The disparity is often even larger. Hough cited an August 2011 artificial earthquake in Trinidad, Colorado that was recorded at a 5.3 magnitude, but was reported as feeling like a 4.0 magnitude quake. The Richter magnitude scale used to measure earthquake intensity is a complicated logarithmic scale, so a 4.0 magnitude quake is approximately ninety times weaker than a 5.3 magnitude quake.
The manmade earthquakes in question have not been linked to hydraulic fracturing itself, but rather to the injection of fluids, brines, and wastewater into disposal wells. Two recent studies, one by the University of Texas and the other by Southern Methodist University, found evidence of seismic risk from well injection, but not from hydraulic fracturing itself. Despite the known seismic risks of well injection, only six Class II injection wells, out of nearly 150,000 across the country, have been linked to seismic activity. The risk can be mitigated with wastewater recycling that minimizes injection volumes and careful selection of disposal well sites.