The National Research Council published a report today, reviewing and analyzing peer-reviewed literature, federal and state documents, data requested from private companies, and more ... all in an effort to better understand the link between earthquakes and natural gas fracking techniques.
Because this is the National Research Council, you can read the whole thing online for free. But here are the three key takeaways:
First: The actual process of hydraulic fracturing—injecting fluid into the ground to break rocks and release trapped natural gas—doesn't seem to come with a serious seismic risk. This process has been definitively linked to small earthquakes—no greater than 2.8 magnitude—at one location.
Second: Injecting wastewater from fracking back into the ground has a much more noticeable seismic effect. What's more, this effect goes far beyond fracking. Injecting liquids into the ground is part of advanced recovery for oil, conventional drilling for oil and gas, carbon capture and storage, and geothermal electricity generation. This should not be a surprise. We've known that human can induce small earthquakes since the 1920s and injecting large amounts of liquids into a space that previously didn't hold much liquid—what the NRC calls a fluid imbalance—is part of that.
The strongest induced earthquakes are related to hydrocarbon withdrawl—basically, oil drilling—in California, Illinois, Oklahoma, Texas, and Nebraska. Some of those events have reached magnitudes of 6.5.
But the highest frequency of events seems to be associated with a California geothermal site called The Geysers, which has experienced 300-400 events a year since 2005, some of them reaching a magnitude of 4.6.
Basically, whether we should be really worried about this effect or not is up for debate. None of these human-induced earthquakes have led to significant property damage or loss of life and, historically, human-induced earthquakes have been on the small side, magnitude wise. Also, we've been injecting liquid into the Earth for decades and the overall number of induced earthquakes remains small. But, if we are going to be concerned, we have to understand that this is not just a fracking problem. It's going to come into play any time you're extracting or injecting a large quantity of liquid from the earth without counterbalancing that liquid loss or gain.
Finally: The thing to keep your eye on is carbon capture and storage. There's only one commercial scale project in the whole world right now. So we really don't have enough data to know what's going on here. But, the paper points out, CCS involves injecting very large quantities of liquid. Larger than the quantities involved in, say, storage of wastewater from fracking. So while we don't know what will happen with CCS, there's definitely a potential for those sites to cause some earthquakes at the larger end of the human-induced-earthquake scale.
Read the full report (Executive Summary and Summary are particularly useful if you want a quick overview.)