Academic journal article Science Scope

The Human Impact on Earthquakes: Natural Fracturing versus Fracking

Academic journal article Science Scope

The Human Impact on Earthquakes: Natural Fracturing versus Fracking

Article excerpt

When we present the unifying theory of plate tectonics to our students, we can provide multiple lines of evidence that support it. The fossil and rock record demonstrate that the continents were once joined in a supercontinent, Pangaea, while paleomagnetic data reveal that lithosphere is created at oceanic spreading ridges. Additionally, plate tectonics explains the locations of mountains, natural resources, volcanoes, and earthquakes (Figure 1).

Undoubtedly, plate tectonics influence the hazards humans will face in their geographic locations: People near plate boundaries--whether these are divergent, convergent, or transform margins--will encounter earthquakes as the plates shift and move. Because California is prone to earthquake activity, the state's building codes are more stringent and are designed to protect people from frequent seismic events.

Do earthquakes only affect people? Can people influence earthquake activity?

One of the more popular geosciences topics in the media today is whether hydraulic fracturing, or "tracking," can pollute groundwater and trigger earthquakes. This activity allows students to investigate potential connections between tracking and earthquakes--or the human impact on seismic events.

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Hydraulic fracturing

What does hydraulic fracturing involve? Although this method of fossil fuel recovery has been around for more than a half century, it has recently become a popular option for methane ([CH.sub.4]) recovery in subsurface shales. Sandstones tend to be porous and permeable, and "traditional" petroleum recovery is used. This involves the drilling of a well--and the hydrocarbons, which are under pressure in the subsurface, rise through the well or are pumped to the surface. Alternatively, shales consist of clay-sized particles and are typically impermeable and nonporous. While gas may be trapped within shales, it isn't easy to directly retrieve the hydrocarbons from traditional well-recovery methods.

Hydraulic fracturing involves drilling a vertical well and then horizontally drilling (or directionally drilling) many kilometers in the subsurface (Figure 2). Water, along with chemical additives and proppants (e.g., sand, ceramic pellets), is forced at high pressure to produce fractures in the shale. The trapped hydrocarbons then flow through the newly created fractures and can be recovered at the surface. The injected fluids, also under pressure, are forced back to the surface. (Proppants remain in the fractures and keep them open.) These used injection fluids can be recycled off site but are often injected in the subsurface in a deep-injection well for disposal.

Environmental concerns arise if the hydrocarbons or chemicals in the injected fluids breach the well's casing system and enter the groundwater, either during the tracking process, during recovery of hydrocarbons, or after hydrocarbon recovery is complete. The injection of used fluids has also been associated with low-magnitude earthquakes. (See Palliser 2012 for more details about the basic hydraulic fracturing process, along with descriptions of benefits and consequences.) The number of earthquakes near hydraulic fracturing sites has continued to rise with increased hydraulic fracturing activity and waste injection, especially in Oklahoma. There have been concerns in other U.S. states, as well as in Europe, with the increased seismic activity associated with hydraulic fracturing waste-fluid disposal.

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Fluids that are injected into the subsurface can lubricate existing fault zones, facilitating movement along the fault that results in an earthquake. The classic example cited in Earth-science textbooks is the 1960s increased seismic activity associated with the Rocky Mountain Arsenal deep-injection well that disposed of fluids from Arsenal activities (Evans 1966). Scientists correlated Oklahoma's increased earthquake activity with wastewater injection from tracking processes (Hand 2014). …

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