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Throughout history, earthquakes have been regarded as particularly disturbing phenomena. They quite literally upset the very ground we stand upon. There’s nowhere to run and no place to hide, and you can’t see them coming. No wonder the ancients considered them portentous, foretelling catastrophes and political upheavals.
Cicero: Good even, Casca … Why are you breathless? And why stare you so?
Casca: Are not you moved when all the sway of earth shakes like a thing unfirm?
—Shakespeare’s Julius Caesar
Almost all earthquakes occur in predictable places, if not at predictable times. Tectonically active regions of the world, particularly along plate boundaries and within deformation zones, are characterized by the steady accumulation of strain along fault lines within the earth’s crust. When the fault slips, that is, when the strength of the shear force exceeds the strength of the contact, an earthquake happens. The Italian peninsula, where poor Casca lived, is home to two major fault lines and numerous smaller ones along the zone where African and European seismic plates are relentlessly colliding. Throw in active volcanoes, and it’s an unfirm place, indeed.
In the U.S., the same trend holds. Almost all earthquakes occur along the long plate boundaries in the far western and intermountain regions. Quakes happen more rarely in the continental interior. With a few exceptions, these tend to be of the almost-unnoticeable variety, less than 3 on the Moment Magnitude (Mw) scale.
Pesky swarms. Recently, seismic events have gotten a lot of attention in the U.S. Mid-continent region because of an unusual flurry of small earthquakes. This is particularly true in parts of Oklahoma, Ohio and Texas. Whatever the cause, the increase in seismic activity is significant: From 1970 to 2000, there were, on average, 20 earthquakes per year greater than 3.0 Mw in the central and eastern U.S. Between 2010 and 2013, this increased to more than 100 per year. While such earthquake “swarms” occurred in the past, nothing historically compares to the recent uptick.
This leads to some obvious questions: Is the recent earthquake activity a direct result of oil and gas industry practices? Are the recent seismic phenomena naturally-occurring, man-made, or some combination of the two? Specifically, is hydraulic fracturing, the most popular target of public alarm and criticism, partly to blame? If oil and gas does come in for responsibility, what should be done about it? These are important questions for the industry to address directly, because unlike other energy issues such as gasoline prices, imports, regulations or balance of trade, earthquakes tend to really grab a person’s attention.
Injection connection. The U.S. Geological Survey is taking the issue seriously. Last January, it released a summary of the latest research into the earthquake spike: “USGS scientists have found that at some locations the increase of seismicity coincides with the injection of wastewater in deep disposal wells. Much of this wastewater is a byproduct of oil and gas production and is routinely disposed of by injection into wells specifically designed for this purpose.”
Hydraulic fracturing by itself was found to be an unlikely culprit. “Studies suggest that the actual hydraulic fracturing process is only very rarely a direct cause of felt earthquakes,” according to the USGS. While fracing works by producing thousands of “microquakes,” these are almost all too tiny to be felt or cause damage.
The fact that wastewater injection is implicated in some seismic events has been known for decades. In the 1960s, wastewater injection at the Rocky Mountain Arsenal near Denver was stopped after a 4.8-Mw earthquake occurred. The largest earthquake every recorded in Oklahoma, a 5.7-Mw event on Nov. 6, 2011, near Prague, was plausibly attributed to wastewater injection. A 2011 study by the University of Texas attributed a swarm of small earthquakes at Dallas/Fort Worth International Airport to a nearby disposal well. The state of Ohio imposed new injection rules after a series of tremblers struck Youngstown. The state can now order pre-drilling seismicity tests to identify the volume and pressure of wastewater fluids that can be injected, and can stop injection on any well it deems to be a seismic hazard.
Hydraulic fracturing, which uses an enormous amount of water, is, therefore, a contributor to the problem, if indirectly. The wastewater this generates must be disposed of safely—that’s a regulatory mandate—and injection into disposal wells is usually the lowest-cost solution. Wells used for injecting fluids, whether for enhanced oil recovery or wastewater, are UIC Class II wells. About 110,000 of them in the U.S. are used for EOR, and another 30,000 for disposal. Only a few of these have ever been associated with seismicity.
Traffic lights. One solution is to use science and common sense in selecting the location of disposal wells. Current regulation of Class II wells focuses on protection of potable aquifers. Future considerations could be the avoidance of known geologic faults, and monitoring the area around a well for seismic changes. If a certain threshold of seismic activity is detected, then injection rate or pressure could be reduced, or suspended completely. (These so-called “traffic-light” systems are in use today in various locations around the world; notably Basel, Switzerland.)
To successfully avoid seismic problems with injection wells, a better knowledge of the hydrogeologic framework is essential, including mapping of faults and small natural earthquakes (including those below the 3.0-Mw threshold). Better, more frequent reporting of injection data, such as volume and pressure, from existing Class II wells would also be invaluable.
Perhaps the best solution would be to greatly reduce the volumes of wastewater in the first place by recycling, treatment and the use of non-toxic chemical additives in water. A safe disposal well is one that is properly designed. The safest disposal well is the one you never have to drill. 
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