Shake Rattle & Roll: The Rise of Man-Made Earthquakes

On September 3rd, 2016, the earth beneath Pawnee, Oklahoma, shook. A large earthquake hit the town, damaging several buildings and injuring a few residents. Seismologists from the United State Geological Survey scaled it as a magnitude 5.8 quake, the strongest seismic event recorded in the state’s history. To out of state observers, this quake may seem surprising. But to Oklahomans, earthquakes like this one have become the new norm.

Over the past decade, several central states like Oklahoma have become the focus of seismological research. Geophysicists have also flocked to small towns in Texas, Arkansas, Colorado, and Ohio, intrigued by an increase in earthquake activity. Geologists have recently come to a consensus that this tremor spike is linked to the rise of two unconventional oil and gas extraction practices – fracking and horizontal drilling. They agree, though, that it isn’t the drilling per se that causes the tremors, but the wastewater that’s injected into the earth as part of the process. From this growth in geophysical research, scientists are uncovering just how the fuel industry has unleashed these earthquakes and how scientifically backed policy can quell them.

The Midwest historically has never been known to be a seismic mover and shaker. From 1973 to 2008, central and eastern states recorded only 855 earthquakes with a magnitude greater than 3.0. But just in the past seven years, the region has racked up over 2,300 quakes with the same magnitude range. In 2014, Oklahoma dethroned heavyweight champion California as the state with the most seismic activity.

Most of this activity would hardly cause damage in areas accustomed to quakes. If the Pawnee earthquake occurred California, “it would probably startle a lot of people, it might knock some things off shelves, but it’s unlikely to do any damage” says William Ellsworth, a geophysicist at Stanford University. That’s because California’s infrastructure is built to withstand tremors. Midwestern communities have never had to reinforce their buildings to muscle through powerful earthquakes. Yet reality has changed: according to the Oklahoma Insurance Department, within the first month following the 5.8 M earthquake, Pawnee residents filed 274 insurance claims for earthquake-related damages. Additionally, the nearby Pawnee Nation Tribal Complex reported that more than half its buildings suffered extensive damage.

Seismologists see these earthquakes as an indirect consequence of two recently developed extraction techniques – fracking and horizontal drilling. Before the 1990s, oil and gas companies obtained resources by drilling straight down until they struck a reservoir. Then after sucking the site dry, they would pick up and find another place to dig. However, over the past fifteen years, these businesses have developed new ways to extract more fuel within a single site. Fracking involves shooting water into the ground, creating a high pressure that cracks open neighboring rocks and releases gas. Horizontal drilling involves digging into the earth then pivoting laterally to collect nearby fuel deposits. The advent of these two methods allowed the oil and gas industry to tap into reservoirs previously out of reach.

These new techniques unearth an abundance of oil and gas, but also produce a lot wastewater, fluid that is too rich with chemicals or salty brine to be cleaned. Instead companies inject the wastewater deep into the earth. These injection wells are the core of middle America’s seismic situation.

“There is an increasing realization of a potential linkage between deep-well injection of oil and gas wastewaters and earthquakes,” reported environmental policy specialists Peter Folger and Mary Tieman in the Congressional Research Service’s 2016 overview on human-induced earthquakes. Starting in the 2000s, many states began reporting instances of earthquakes near deep-well injection sites. In 2012, one study from the Stanford Center for Induced and Triggered Seismicity observed that tremor activity in central Arkansas had increased following the onset of 8 deep-well injection operations in 2009. The number of earthquakes greater than magnitude 2.5 ballooned from 3 in total between 2007 and 2009 to 211 between 2010 and 2012. They also found that 98 percent of earthquakes during the 2010-2012 period occurred within six kilometers of one deep-well injection site.

The idea of a human-induced earthquake is not completely novel. Since the 1920s, scientists have seen that human activities like reservoir filling and mining can create quakes. Injection well-induced earthquakes first came into notoriety in 1967 when a series of tremors, including a magnitude 4.8 quake, rumbled just outside of Denver. Many geologists then and now suspect that the seismicity was a consequence of wastewater disposal at the 12,000-meter deep Rocky Mountain Arsenal Injection Site.

Scientists are now learning how these human-powered earthquakes occur. They know that injection wells can increase pore pressure, which is the pressure water exerts in porous rock formations. When this pore pressure reaches a certain threshold, it can disrupt the equilibrium of forces acting on a fault in the earth’s crust. This in turn can cause faults to slip, releasing seismic energy in the form of an earthquake. Manoochehr Shirzaei, a geophysicist at Arizona State University, equated this fault failure to rubbing lotion on your hands. If close enough to a fault line, increased pore pressure from wastewater lubricates the earth’s plates, allowing them to slide more easily past each other.

Scientists have made other discoveries relating to this connection between injection wells and earthquakes. Recently, they’ve discovered a few parameters that dictate injection-induced seismic activity. In 2015, Stanford researchers Rall Walsh and Mark Zoback observed in Oklahoma’s Arbuckle formation that pore pressure generated by injection wells could spread throughout the expanse of a rock layer. Their study helped explain why some earthquakes occur at a distance from injection well sites or even after peak injection rates have passed. “You can easily imagine that if a fault wasn’t located directly beneath a well, but several miles away, it would take time for the fluid pressure to propagate,” stated Walsh.

In 2016, a team of scientists, including Shirzaei and Ellsworth, found from satellite images that wastewater injection could cause the earth’s surface to rise by several centimeters. This small but measurable observation may help researchers pinpoint areas of earthquake risk.

They also discovered that wastewater wells that drill through thick rock layers in the crust are more likely to access fault lines than shallower wells Deeper wells increase the pore pressure around faults, stimulating seismicity, while shallow injection wells, bounded below by impermeable rock layers, cause little earthquake activity. This finding challenges the current injection well regulations based from the 1974 Safe Drinking Water Act. To minimize the risk of drinking water contamination, policy officials have historically encouraged industries to inject wastewater deep into the earth’s crust. Now scientists caution that these regulations, first enacted to protect clean water, may in fact increase the potential for damaging earthquakes.

In many cases, the public has responded to this new research. In November 2016, The Pawnee Nation filed a class action lawsuit against 27 oil and gas companies occupying Oklahoma, stating, “the 5.8 M earthquake near Pawnee on September 3, 2016 was not a naturally occurring earthquake, or an act of God. Instead, the Defendants’ pollution of the environment caused it.”



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