Disposal of Hydrofracking Waste Fluid by Injection into Subsurface Aquifers Triggers Earthquake Swarm in Central Arkansas with Potential for Damaging Earthquake

Only a handful of the thousands of waste disposal wells across the United States have been linked to induced or triggered earthquakes. Still, two well-documented cases—Rocky Mountain Arsenal, Colorado, in the 1960s (Healy et al. 1968) and Paradox Valley, Colorado, in the 1990s (Ake et al. 2005)— demonstrate that fluid injection into the subsurface can trigger earthquakes. The largest event at Rocky Mountain Arsenal was M 5.2, and the largest event at Paradox Valley was M 4.3. The U.S. Environmental Protection Agency provides Underground Injection Control (UIC) regulations administered by the states to protect underground sources of drinking water. However, the UIC does not limit the proximity of waste disposal wells to active seismic zones or to critical facilities ( e.g. , hospitals, schools, or nuclear power plants) based on the potential to induce or trigger earthquakes. Over the last several years, hydraulic fracturing (hydrofracking), a technique used to enhance natural gas recovery, has become widely used in north-central Arkansas (Figure 1). Wastewater, a byproduct of the hydrofracking process, is being injected under pressure into subsurface rocks at eight waste disposal wells (Table 1) in the study area. Since the first waste disposal well became operational in April 2009, the study area has experienced an increase in the rate of magnitude ≥ 2.5 earthquakes, with one in 2007, two in 2008, 10 in 2009, 54 in 2010, and 157 in 2011. The study area has a long history of seismic activity including earthquake swarms in the early 1980s (Chiu et al. 1984) and 2001 (Rabak et al. 2010), so the current earthquake-rate increase may simply reflect another peak in a natural cycle. However, 98% of the recent earthquakes occurred within 6 km of one of three waste disposal wells after the start of injection at those wells. This close spatial and …