Microearthquakes (that is, those with magnitudes below 2) are routinely produced as part of the hydraulic fracturing (or “fracking”) process used to stimulate the production of oil, but the process as currently practiced appears to pose a low risk of inducing destructive earthquakes. More than 100,000 wells have been subjected to fracking in recent years, and the largest induced earthquake was magnitude 3.6, which is too small to pose a serious risk. Yet, wastewater disposal by injection into deep wells poses a higher risk, because this practice can induce larger earthquakes. For example, several of the largest earthquakes in the U.S. midcontinent in 2011 and 2012 may have been triggered by nearby disposal wells. The largest of these was a magnitude 5.6 event in central Oklahoma that destroyed 14 homes and injured two people. The mechanism responsible for inducing these events appears to be the well-understood process of weakening a preexisting fault by elevating the fluid pressure. However, only a small fraction of the more than 30,000 wastewater disposal wells appears to be problematic—typically those that dispose of very large volumes of water and/or communicate pressure perturbations directly into basement faults.
An article in Mother Jones gives further information and explains how small quakes can lead to bigger events much farther away from the original incident.
The scientists looked at three big quakes: the Tohuku-oki earthquake in Japan in 2011 (magnitude 9), the Maule in Chile in 201 (an 8.8 magnitude), and the Sumatra in Indonesia in 2012 (an 8.6). They found that, as much as 20 months later, those major quakes triggered smaller ones in places in the Midwestern US where fluids have been pumped underground for energy extraction.
"[The fluids] kind of act as a pressurized cushion," lead author Nicholas van der Elst of the Lamont-Doherty Earth Observatory at Columbia University explained to Mother Jones. "They make it easier for the fault to slide."
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