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The central United States has undergone a dramatic increase in seismicity over the past 6 years. From 1973-2008, there was an average of 24 earthquakes of magnitude 3 and larger per year. From 2009-2014, the rate steadily increased, averaging 193 per year and peaking in 2014 with 688 earthquakes. So far in 2015, there have been 430 earthquakes of that size in the central U.S. region through the end of May.

There are many questions and misconceptions about what’s happening. How does the observed increase relate to oil and gas production activities? Does this connect to fracking—more formally known as hydraulic fracturing? What exactly is fracking? What are induced earthquakes?

USGS scientists recently published a report that explains what is causing these seismic events and addresses common misunderstandings. The article is published in the Seismological Research Letters journal.

Fluid Injection Can Cause Earthquakes

Injecting fluid underground can induce earthquakes, a fact that was established decades ago by USGS scientists. This process increases the fluid pressure within fault zones, essentially loosening the fault zones and making them more likely to fail in an earthquake. When injected with fluids, even faults that have not moved in historical times can be made to slip and cause an earthquake if conditions underground are appropriate.

There are several purposes for injecting fluid underground. The three main reasons are wastewater injection, hydraulic fracturing and enhanced oil recovery. Within the United States, each of these three activities has induced earthquakes to varying degrees in the past few years. All three types of wells used for these purposes are regulated under the Safe Drinking Water Act with minimum standards set by the U.S. Environmental Protection Agency. Additional regulations vary by state and municipality. Other purposes for injecting fluid underground include enhanced geothermal systems and geologic carbon sequestration.

Wastewater Injection

Water that is salty or polluted by chemicals needs to be disposed of in a manner that prevents it from contaminating freshwater sources. It is usually most economical to isolate and inject it into deep underground wells, below any aquifers that provide drinking water. This process is known as wastewater injection. Wastewater injection wells typically operate for years or decades.

Most wastewater currently disposed of is saltwater found in the same rock formations as oil and gas. This saltwater comes up as a byproduct during the oil and gas production process. It is often in large volumes and is too salty or contains minerals and other chemicals that economically preclude it from being cleaned and released at the surface or reused.

Another component of wastewater is hydraulic fracturing fluid that was not reused. However, it typically makes up less than 10 percent of the wastewater in most locations in the United States.

Hydraulic Fracturing

Hydraulic fracturing, commonly referred to as fracking, injects water at high pressures to crack rock formations, allowing for easier fluid flow and more complete oil and gas extraction. While the hydraulic fracturing process itself is often accompanied by extremely small “microearthquakes,” they are mostly too small to be felt. Hydraulic fracturing operations typically last a few hours, with the biggest operations extending for up to a few days.

Once hydraulic fracturing is completed, drilling engineers extract the fluids remaining in the well. Some of this recovered hydraulic fracturing fluid is used in subsequent fracking operations, while some of it is disposed of in deep wells through wastewater injection.

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Enhanced Oil Recovery

Enhanced oil recovery involves the injection of water, steam or carbon dioxide into the formation from which oil and gas are extracted. These operations try to sweep the oil and gas towards wells that are extracting them. Enhanced oil recovery wells usually operate for years or decades and can produce large volumes of wastewater.

Six Facts on Induced Earthquakes

Fact 1: In the United States, fracking is not causing most of the induced earthquakes. Wastewater disposal is the primary cause of the recent increase in earthquakes in the central United States.

Wastewater disposal wells typically operate for longer durations and inject much more fluid than hydraulic fracturing, making them more likely to induce earthquakes. Enhanced oil recovery injects fluid into rock layers where oil and gas have already been extracted, while wastewater injection often occurs in never-before-touched rocks. Therefore, wastewater injection can raise pressure levels more than enhanced oil recovery, and thus increases the likelihood of induced earthquakes.

Fact 2: Not all wastewater injection wells induce felt earthquakes.

Most injection wells do not trigger felt earthquakes. A combination of many factors is necessary for injection to induce felt earthquakes. These include the injection rate and total volume injected; the presence of faults that are large enough to produce felt earthquakes; stresses that are large enough to produce earthquakes; and the presence of pathways for the fluid pressure to travel from the injection point to faults.

Fact 3: Wastewater is produced at nearly every oil and gas well, not just hydraulic fracturing sites.

Most wastewater currently disposed of across the nation is saltwater that is a byproduct of the oil and gas extraction process. Saltwater is found in nearly every oil and gas production well, regardless of whether the well has been hydraulically fractured.

Fact 4: The content of the wastewater injected in disposal wells is highly variable.

In many locations, wastewater has little or nothing to do with hydraulic fracturing. In Oklahoma, less than 10 percent of the water injected into wastewater disposal wells is used hydraulic fracturing fluid. In some parts of Oklahoma where very high volumes of wastewater are injected, no hydraulic fracturing is occurring at all, so the wastewater is purely saltwater that comes up with oil in the extraction process.

In contrast, the fluid disposed of near earthquake sequences that occurred in Youngstown, Ohio and Guy, Arkansas, consisted largely of spent hydraulic fracturing fluid.

Fact 5: Induced seismicity can occur at significant distances from injection wells and at different depths.

Earthquakes can be induced at distances of 10 miles or more away from the injection point and at significantly greater depths than the injection point.

Fact 6: Wells not requiring surface pressure to inject wastewater can still induce earthquakes.

In operations where engineers pour fluid down the well without added pressure at the wellhead still increase the fluid pressure within the formation and thus can induce earthquakes.

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Why is This Happening Now?

Wastewater disposal from oil and gas operations has increased greatly in the United States in the past decade, especially in Oklahoma where wastewater disposal volume doubled in recent years. This is due to changes in oil and gas production practices that ultimately result in more wastewater production.

Next Steps: Start with Science

Moving into the future, scientists continue to develop new strategies to help reduce the chances of damaging induced earthquakes. The USGS is dedicated to research leading to a detailed understanding of the physical processes involved in inducing earthquakes.

The USGS has developed preliminary methods to estimate the earthquake hazard associated with induced events. The models calculate how often induced earthquakes are expected to occur in the short term and how hard the ground will likely shake as a result. USGS scientists are working to further refine these models.

Close cooperation between scientists, regulators and industry will be necessary to mitigate the hazard from induced earthquakes. Of particular importance is making data on injection pressures and flow-rates freely available. With these data, scientists will be better able to assess the changing hazard from injections, potentially allowing operators to prevent damaging earthquakes from occurring.