That something strange is going on beneath the central and eastern US is not in doubt. Since 2008, there has been a remarkable rise in seismicity, a good deal of it attributed to anthropogenic causes. States like Ohio, Oklahoma, Arkansas, and Texas have all seen significant increases in seismicity.
A new study published in Science suggests that 20% of all central and eastern US seismicity since 2008---one-fifth---can possibly be laid deep at the feet of only four wells. 45% of of all of that seismicity comes from Oklahoma, a state that in 2014 has had more earthquakes than California, which as we've learned, has its own possible problems.
In the
last diary I wrote on this, we discussed the large November 2011 central Oklahoma earthquake (see above). This latest study discusses a seperate earthquake swarm near Jones, which is closer to Oklahoma City.
The study, titled "Sharp increase in Central Oklahoma seismicity since 2008 induced by massive wastewater injection" is just the latest in a series of really great scientific papers that have come out of the sharp increase that's happening right now in Oklahoma. The abstract reads:
Unconventional oil and gas production provides a rapidly growing energy source; however, high-production states in the United States, such as Oklahoma, face sharply rising numbers of earthquakes. Subsurface pressure data required to unequivocally link earthquakes to injection are rarely accessible. Here we use seismicity and hydrogeological models to show that fluid migration from high-rate disposal wells in Oklahoma is potentially responsible for the largest swarm. Earthquake hypocenters occur within disposal formations and upper-basement, between 2-5 km depth. The modeled fluid pressure perturbation propagates throughout the same depth range and tracks earthquakes to distances of 35 km, with a triggering threshold of ~0.07 MPa. Although thousands of disposal wells operate aseismically, four of the highest-rate wells are capable of inducing 20% of 2008-2013 central US seismicity.
(Paywall, but I believe you can find the study
here. I found this link
here.)
The Jones swarm has produced hundreds of earthquakes, some that exceed M4, even within the last month. What the researchers have found, and this is actually observed elsewhere, is the fluids, once injected, can migrate a significant distance. In the past, in order to conclusively link seismicity to an anthropogenic cause, the seismicity had to be very, very close. The four wells in question are some 20 kilometers away in southeast Oklahoma City.
It isn't that the fluids are lubricating the faults. That's not what is happening. Instead the fluid injection and its pressures change the stress conditions (pore pressure) that are acting on any possible subsurface faults. Because all of the continents have an overall stress and strain field due to their motions relative to each other, a fault that is oriented in a favorable fashion may find its clock advanced significantly forward if the local stress and strain field is changed.
Remember our clock? It still applies here:
The clock will represent the stress that builds up in the upper crust of the Earth, everywhere. This stress acts on all faults that cross through the crust and along the Earth's surface. Every fault has a clock. Sometimes, several clocks, as many faults act in segments. One segment's clock may "tick" at a different rate than others. The fault-clocks of the earth all tick at different time-scales compared to others. When the clocks reach midnight, that means the shear stress has overcome the fault's resistance to slip. An earthquake occurs.
Please keep in mind that I'm greatly simplifying some very complex physics, and I'm probably not doing it the proper justice.
The researchers built a 3-D model using actual data from 89 wells in the region, starting their data set in 1995. The pore pressure rose over time and migrated away from the wells. As the pore pressure increased, it wound the clocks on several small faults closer and closer and closer to failure until it did, which then triggered the ongoing swarm that's rattled nerves around Oklahoma City.
The researchers found that these wells pump approximately 4 million barrels a month into a formation about 2.2 to 3.5 kilometers deep. This is part of a process known as dewatering; it is used to recover gas and oil from watery resevoirs that otherwise would be abandoned. A pump is used to force water, gas, and oil up the wells. The gas and oil is seperated, and the waste fluid is disposed of in a seperate area. The wastewater is produced by the oil and gas extraction boom ongoing in Oklahoma.
The paper notes the Jones earthquake of August 2, 2012 ruptured part of a known 7 kilometer fault. There are scaling laws for earthquakes and fault lengths; they are fairly complex but in short the paper speculates the maximum magnitude for that particular fault could have been M6. I know our Californian readers may scoff, but an M6, especially in the nation's midsection, is nothing to sneeze at.
There are also two major fault systems in this part of Oklahoma. The Wilzetta, in the east of this area, we discussed in a previous diary and parts of it have been confirmed as likely active, the oil and gas fluid injection likely having induced one earthquake that then triggered several others. The Nemaha Ridge (technical PDF), which actually trends roughly north-south across three states, is the fault system closest to this particular swarm. The Nemaha is actually a broad uplift--the buried remains of the ancestors to the Rockies, and associated with it are several fault systems in Nebraska, Kansas, and Oklahoma. Note here that "fault system" does not necessarily mean "active fault system". Its folds and thrusts trap a great deal of oil and gas. It runs directly beneath Oklahoma City. This 50km-long segment near and beneath Oklahoma City, using the scaling laws for earthquakes and fault lengths could have the potential to rupture in an M7 earthquake. Such a quake would ring all of the midwest and much of the east like a bell because the rigid nature of the continental crust transmits seismic energy across a far larger distance compared to the West.
Now let me be very clear here; I'm not saying it is going to happen, and neither are the authors of the paper. There's little evidence that the Nemaha is active in Oklahoma, although for its strands beneath Kansas there's some scant evidence. The paper authors note the following:
Earthquakes in 2009 primarily occurred, within location un-certainty, near injection wells or on the nearest known faults to the northeast of the wells (fig. S9). Focal mechanisms near the swarm onset indicate fault planes at orientations favorable to failure (19, Fig. 2, inset B). Faults subparallel to the NNW-SSE-trending Nemaha fault would not be well-oriented for failure in the regional ~N70E stress regime (25) and would require substantially larger pressure increase to fail. Recent earthquakes near the fault may be evidence for continued pressure in-crease. This 50-km-long segment of the Nemaha fault is capable of host-ing a M7 earthquake based on earthquake scaling laws (20) and the fault zone continues for hundreds of kilometers. The increasing proximity of the earthquake swarm to the Nemaha fault presents a potential hazard for the Oklahoma City metropolitan area.
To translate this into plain English: The Nemaha and its associated faults isn't oriented in the right way, however the swarm continues to migrate closer to it. If the pore pressures increase, the Nemaha or some other unmapped portion of the trend could be induced to fail--in short winding a clock, assuming this segment of the Nemaha even has one ticking, remember the Nemaha is very, very old--all the way to midnight.
Oklahoma has played host to very large earthquakes in the past and the state is indeed "Earthquake Country" regardless of the activities of the oil and gas industry. Southwest Oklahoma's Meers Fault is the only known surface faulting known in the central and eastern US. Its last large earthquake was perhaps a thousand years ago. This earthquake was likely in the neighborhood of M6.5 to M7. Faults in this part of the country have clocks that tick for tens of thousands of years before they rupture. The great "experiment" being done in Oklahoma could be winding some other fault's clock to midnight.
The oil and gas industry continues to remain highly skeptical, not of the fact that their activities can induce and/or trigger seismicity but that it could be happening on this large a scale. It's pretty much impossible to deny that human beings can cause earthquakes via a surprisingly large variety of means. We have almost 70 years worth of data. But at the very least, these four wells should be shut down for use as disposal wells, and plugged. I'm willing to bet that the hypothesis that the swarm will die off over time, as observed in Ohio, Colorado, Arkansas, and other locales is correct.
The US is in the process of finalizing its seismic hazard maps. These maps display the probability of any location experiencing damaging ground motions. They're updated every six years and should be ready for public use this fall. Draft maps and their documentation indicate Oklahoma's going to have its hazard, which was already not minimal, increased, however I'm not sure by how much, as in the past hazard maps are built using a combination of techniques including historical data. It's why there's considerable controversy about New Madrid (is it shutting down? is it winding up? do large earthquakes in intraplate regions just move around at random?), and why the Japanese utterly failed to appreciate what they had lurking off of their east coast until it was too late. Oklahoma's new hazard is very likely anthropogenic and recent. It's tough to display probabilities for that on a map, but Oklahomans are flocking to purchase earthquake insurance. I can't say I blame them.