Oklahoma Earthquakes

(scroll down past updates for original post)
Fracking well

Update, October 11, 2014: Recently Kansas had an uptick in earthquakes, too. One of them, a magnitude 4.4, was the strongest quake in Kansas for over a century.

Meanwhile, since August, a USGS seismologist published a study (PDF) that mentioned “earthquakes in the central and eastern United States that are thought or
suspected to be induced by fluid injection.” This inspired a legal blogger to comment on “the need to rely on sound science and evidence before making conclusions about what may or may not have caused a seismic event in the first place” (a free Lexology subscription may be needed to view the entire blog post).

In mid-September, the USGS put up an online feature report on induced earthquakes. It’s very informative.


Original post:

On October 22, 2013, the United States Geological Survey and the Oklahoma Geological Survey released a joint statement warning of a “significantly increas[ed] … chance for a damaging magnitude 5.5 or greater quake in central Oklahoma.”

This is on top of the magnitude 5.6 quake in 2011.

How can this be? Earthquakes are caused by plate tectonics, right? (Well…see below.) Oklahoma sits in the middle of the North American plate, so why is it shaking? (Good question.)

What is an earthquake?

Earthquakes happen when two blocks of rock suddenly slip past one another on a fault line, releasing a lot of stored-up energy.

Ansate/Sam Hocevar
Ansate/Sam Hocevar

The spot where this happens is called the hypocenter, and it’s usually deep underground. The surface point directly above it is called the epicenter. What we call the earthquake is actually seismic waves that have been caused by the energy released into the ground.

Those waves can either travel along the surface (Love and Rayleigh waves) or through the ground as body waves (P and S waves). The P waves arrive first and are fairly small. The other three kinds of waves are the ones that cause the most damage.

Seismometers record the waves as a function of time.

A low-frequency 3-direction ocean-bottom seismometer (cover removed).  Hannes Grobe
A low-frequency 3-direction ocean-bottom seismometer (cover removed). Hannes Grobe

The Richter magnitude was based on the maximum seismograph signal amplitude and the instrument’s distance from the epicenter.

It worked well enough with smaller earthquakes but tended to underestimate larger ones. As technology got better scientists developed a new scale – the moment magnitude (MW) scale.

For lower amplitudes, MW matches the Richter number. For big quakes, MW is more accurate than the Richter scale because it takes into account the volume of material that has moved and therefore how much energy has been released.

Here is a video by the IRIS Consortium that explains moment magnitude far better than I could:

The magnitude 5.5 quake mentioned in the USGS/OGS statement would be 1 strand of pasta in that example.

Japan’s magnitude 9 Tohoku earthquake would be a million strands.

Such a seismic apocalypse probably isn’t hanging over Oklahoman heads, but that 2011 MW 5.6 was bad enough – it injured two people and destroyed 14 homes. If the next such quake happens in a heavily populated area, it could be devastating.

What causes an earthquake?

Plate tectonics certainly plays a role.

Earth’s crust is broken up into pieces called tectonic plates. These plates move around, jostling each other. Sometimes one edge will sink down underneath another plate’s edge. All this movement really strains the ground along the plate edge, causing many earthquakes, including major ones.

San Andreas Fault. Leohotens.
San Andreas Fault, with the Pacific plate on the left and the North American plate on the right. Leohotens.


However, there have been too few earthquakes this century to account for all plate edge movement. In fact, only about 30% of plate edge slip has been linked to earthquakes.

The plate edges might also be moving without strain (fault creep), as well as through slow-slip earthquakes – large earthquakes that happen in slow motion over days, weeks or months and that have been detected in Japan, New Zealand, the United States and Canada.

Things should be quiet far from plate edges and generally this is true. Intraplate earthquakes happen, but their energy release is usually very small when compared to earthquakes along plate edges.

From Intraplate Earthquakes. Matt Fillingim.
Intraplate Earthquakes Look how few quakes there were in Oklahoma back then. The concentration to the right (east) of Oklahoma is the New Madrid Fault Zone.

The key word there is “usually.”

In 1811-1812, New Madrid, Missouri, was hit with a series of three major earthquakes, the largest one an estimated MW 8.0. Seventy-five years later, Charleston, South Carolina – far from the eastern edge of the North American plate, which sits in the middle of the Atlantic Ocean – was hit by a MW 7.6 quake. On January 26, 2001, Gujarat, India, had a MW 7.6-7.7 earthquake that killed some 20,000 people.

Scientists aren’t sure what causes intraplate quakes, though suggestions have included glacial rebound, convection in Earth’s mantle, and a failed rift.

However, in Oklahoma’s case, USGS seismologists analyzed the increased frequency of earthquakes in Oklahoma over the past couple of years (one to three MW 3 or larger earthquakes from 1975 to 2008, then around 40 earthquakes per year from 2009 to mid-2013, with a 50% increase in frequency after October 2013).

Here’s an animation from January 2005 through April 19, 2014, with a “beat” (make sure your speakers are on):

Yeah, that’s concerning.

The USGS decided that “a likely contributing factor to the increase in earthquakes is triggering by wastewater injected into deep geologic formations. This phenomenon is known as injection-induced seismicity, which has been documented for nearly half a century, with new cases identified recently in Arkansas, Ohio, Texas and Colorado.”

Induced seismicity

Hydraulic fracturing is big business these days. The process involves injecting fluids deep into shale rock, fracturing the rock and extracting its oil and gas. After the well is played out, the used fluid is re-injected and the well is then sealed.

Oddly enough, the fracturing process itself only very rarely causes a felt earthquake. The main problem, as far as scientists know right now, is waste water storage.

USGS scientists say in a Web post dated January 17, 2014 (link added):

Wastewater injection increases the underground pore pressure, which may, in effect, lubricate nearby faults thereby weakening them. If the pore pressure increases enough, the weakened fault will slip, releasing stored tectonic stress in the form of an earthquake. Even faults that have not moved in millions of years can be made to slip and cause an earthquake if conditions underground are appropriate.

Although the disposal process has the potential to trigger earthquakes, not every wastewater disposal well produces earthquakes. In fact, very few of the more than 30,000 wells designed for this purpose appear to cause earthquakes.

What can we do?

William Ellsworth, one of the USGS scientists quoted in the last section, had this say in Science Magazine, July 12, 2013:

Injection-induced earthquakes, such as those that struck in 2011, clearly contribute to the seismic hazard. Quantifying their contribution presents difficult challenges that will require new research into the physics of induced earthquakes and the potential for inducing large-magnitude events. The petroleum industry needs clear requirements for operation, regulators must have a solid scientific basis for those requirements, and the public needs assurance that the regulations are sufficient and are being followed. The current regulatory frameworks for wastewater disposal wells were designed to protect potable water sources from contamination and do not address seismic safety. One consequence is that both the quantity and timeliness of information on injection volumes and pressures reported to regulatory agencies are far from ideal for managing earthquake risk from injection activities. In addition, seismic monitoring capabilities in many of the areas in which wastewater injection activities have increased are not capable of detecting small earthquake activity that may presage larger seismic events.

Last night, central Oklahoma residents met with lawmakers, regulators, and geologists to discuss the situation and how to protect themselves. They left unsatisfied:

Holland said Oklahoma lacks the necessary resources to diagnose the problem because of its previous lack of seismic activity, but he is dedicated to finding answers.

Skinner said the industry has been very cooperative. Some operators voluntarily shut down injection wells tied to seismic activity, while others shared expensive 3D seismic data that will help researchers identify faults where earthquakes may occur.

Earl Hatley, the Grand Riverkeeper, said he supports a one-year moratorium on injection wells, a notion that drew a sturdy round of applause.

Tim Baker, who oversees the corporation commission’s injection well program, said that likely won’t solve the problem because many of those deep wells predate Oklahoma’s recent earthquake swarms.

“There’s something going on that we don’t understand,” he said.

Regulators have enacted new rules that will go into effect in September to bring more scrutiny to injection well applications near where earthquakes have been recorded, officials said. They also created a six-mile buffer zone around the epicenter of earthquakes that register above a 4.

Skinner said officials from several oil and gas-producing states are working together on the issue, but the process will take time.

That didn’t sit well with many at Thursday’s meeting.

“You want to study us like animals,” one man complained. “Do you want a 7.0 (earthquake) that leveled Haiti to occur in the middle of Edmond?”

Oklahoma is facing a problem that won’t go away overnight. Social, economic and political factors must be considered along with the scientific data.

This is an ongoing story, one that will probably have many updates.


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