Earthquakes are becoming much more common in parts of the United States. Just within the past few years, Oklahoma has been hit worst, going from roughly one moderate earthquake per decade from 1882 through 2009 (on average) to 24 earthquakes in the year 2014 alone. While all of these are relatively small seismic events so far, they still do damage. Understanding what is causing them and possibly preventing them is a good goal.
Two studies, published today in Science Advances and Science, pin the blame for the rise in earthquakes clearly on saltwater disposal related to oil drilling. The story is clear: wherever high-concentration salt water produced in oil extraction is injected into the ground, earthquakes follow. The increase in this disposal practice is directly linked to the increase in earthquakes in Oklahoma and across the United States.
Earthquakes since 1975. Bars in red are associated in time with saltwater injection while those in... [+]
The power of the news studies lies in statistics: in the last few years, there is now enough data on earthquakes and saltwater disposal activity in both place and time. The practice of pumping saltwater back into underground reservoirs isn't new, but the practice has increased dramatically in the last few years. Some wells pump more than 300,000 barrels of water each month now, an increase over previous decades. That change in practice correlates perfectly with the rise in earthquakes, which means we should reexamine disposal methods. Notably, since Kansas called for a moratorium on disposal in March of this year, the rate of earthquakes dropped, though it's too soon to see if that trend will continue.
Most of the increased seismic activity consists of relatively small earthquakes, but since geologists adopted modern seismic recording practices in 1974, they have been able to track everything at least of magnitude 3. During the period from 1974 to 2008, Oklahoma had about about one magnitude 3 or greater earthquake per year, then saw a sharp increase, with more than 100 at least this powerful in both 2013 and 2014.
The rise in seismic activity is even more dramatic when magnitude 4 or higher earthquakes are considered, since older methods of recording data can track those. Oklahoma had roughly one magnitude 4 or higher earthquake every decade from 1882 through 2009, when the rate jumped nearly 200 times higher: the state had 24 magnitude 4 or greater earthquakes in 2014. (The magnitude scale is "logarithmic": a magnitude 4 earthquake releases about 32 times, or 101.5 times more energy than a magnitude 3 earthquake. The modern magnitude scale replaces the old Richter scale.) Other states have seen similar increases, albeit less dramatic in terms of sheer numbers.
Earthquakes associated with saltwater-injection in 1975 and 2015. (Credit: Matt... [+]
To see what could be responsible, the authors of the new studies looked at several possibilities. While it's probable some earthquakes are due to fracking — water pumped into the ground to crack rock and ease extraction of fossil fuels — this research shows the more likely culprit is saltwater disposal. Water is a byproduct of both oil and natural gas extraction, and when it is pumped up, it contains toxic concentrations of salt. The typical means of disposal involves pumping it back underground under high pressure.
And that's where the trouble lies: if fluid is added to a place where faults are already a little unstable, the additional pressure is enough to cause seismic shocks. Oklahoma is full of small faults, cracks in the deep rock that are under some stress. The extra fluid from saltwater disposal is sufficient to let those faults release their pent-up energy. When saltwater injection was relatively small scale, earthquake numbers were hard to link to human activity; when the rate of injection increased in the last few years, the seismic activity shot up to the point where the connection was statistically significant.
Much of the work was led by University of Colorado graduate student Matthew Weingarten and done very quickly, so there's a good chance policy and practice can respond. The notion that humans could cause earthquakes has been around for decades, but now that we know the connection is real, it's time to do something about it.
(Thanks to my Forbes colleagues Hillary Brueck and Eric Mack for help with background material for this story. Obviously all errors and misconceptions are my own.)
