Damage caused by a 2011 earthquake in Oklahoma, likely caused by waste-water injection by humans. USGS. Earthquakes are common across much of North America. The most famous might be the area along the San Andreas fault in California, where the Pacific and North American plates are sliding side-by-side, generating earthquakes in the process. Even the interior of North America has some major earthquake zones, like the New Madrid Seismic Zone that runs along the Mississippi River from Illinois to Arkansas. However, some parts of North America are suddenly seeing big jumps in earthquakes and it's in places that you might not expect, like the middle of Texas. The question has become: is this a naturally-occurring process that is the consequence of the shaping of the continent or are these new earthquakes being triggered by human activity, namely the pumping of waste-water from hydrofracture (a.k.a. "fracking") operations or excess water from oil/gas wells. A new study by Maria Magnani and others in Science Advances supports the idea that at least for one major area, humans must be the culprits and they offer some techniques to possibly identify whether spikes in earthquakes are human-induced or the product of geologic events. It all boils down to being able to image the interior of the Earth. Magnani and others examined how seismic waves travel through the Earth to make a portrait of the ground beneath their study areas (the Fort Worth Basin in Texas and the northern Mississippi embayment). Seismic waves generated by earthquakes (or human sources like air guns) travel at different speeds depending on the rocks they are passing through and certain geologic features like faults can reflect seismic waves differently as well. So, much like a CAT scan, you create an image of what the structure of the Earth might be down many kilometers depth. What they found was that for the Fort Worth Basin, there is no evidence that the faults they imagined have moved in millions of years or more. This is because the sediment and rocks that have accumulated since the last big tectonic event hundreds of millions of years ago (thus, on top of the old rocks with faults) show no signs that they have moved. So, there can't have been a long history of earthquakes in the Fort Worth Basin area. Instead, they postulate that given the lack of any structural evidence of a long-term history of earthquakes in the geologic record and the very recent occurrence (since 2008) of new earthquakes in the Fort Worth Basin, there is really only one reasonable conclusion: humans. As waste-water from fracking operations is pumped into the ground, it reactivates those ancient faults that haven't moved in hundreds of millions of years, creating the new spike in earthquakes. Wastewater injection is common in the Fort Worth Basin, so the simplest solution is that humans are creating their own earthquake hazard zone. Cool thing is that before this study, most of the evidence that humans are triggering earthquakes was related to the notion that now there are earthquakes where there haven't been any recently. Although that is good evidence most of the time, it isn't conclusive as earthquake swarms can happen even in places with little seismicity, all as a part of the release of stress over millions of years on old faults. Magnani and others show that you can actually look for geologic evidence that shows whether faults have moved or not over millions of years and if they haven't, then that new seismicity is likely being brought on by something humans are doing.
Map showing the potential for earthquakes across the United States in the next 50 years (mapped as ground acceleration - shaking). Areas in black boxes are places where human-induced earthquakes have occurred and were not included in the hazard assessment. USGS. Now, the question becomes: how much do we care that we are creating earthquakes? Most are fairly small, less than magnitude 3 ... but we don't know what the long-term impact might be. As we've seen in Oklahoma where wastewater injection is causing seismicity as well, some of the earthquakes are getting larger and beginning to pose real dangers. We have the science to show the cause and effect for these new places of earthquake hazard, but it is up to the people to decide if we want to accept these changes in the name of extracting resources from the ground.