In some places, notably Ohio and Oklahoma, the injection of used fracking fluid in deep disposal wells appears to have produced a significant uptick in earthquake activity. The earthquakes are mostly much too small to be felt at the surface, but a magnitude 5.6 quake in Oklahoma was large enough to cause some damage in 2011.

This has made lots of news because of its scale, but it’s not our first experience with injection-triggered earthquakes. It’s a concern for geothermal power designs that inject water to depths where it can turn to turbine-driving steam, for example. And in the future, it could be a concern for efforts to store carbon dioxide in underground reservoirs.

Earthquakes occur where two blocks of rock suddenly slip past each other along a fault, releasing energy that causes the shaking that bothers us up at the surface. The blocks are generally stuck in place by friction, but the strain of being pushed (or pulled) in different directions slowly builds. Eventually, that strain overcomes the friction keeping it in place and the rocks slip some distance along a portion of the fault, relieving strain.

It doesn’t always look quite like that, though. In many instances, sliding friction increases strongly as the rocks increase their slipping velocity from zero, keeping the strain bottled up. Instead of a sudden jump releasing seismic energy, the rocks “creep” imperceptibly along the fault.

The standard explanation for injection-triggered earthquakes is that they occur because the pressurized water counteracts some of the downward force clamping faults shut; as a result, the friction preventing an earthquake weakens. (We can’t cause an earthquake where there isn’t built-up strain waiting to be released.) But there have been indications that quake-free creep might have occurred during injections—faults have moved more than we can account for based on the resulting earthquakes.

To investigate what's going on and see how it might relate to earthquakes, a team led by University of Aix-Marseille geologist Yves Guglielmi picked a fault and pumped it full of water. The (inactive) fault ran through some carbonate rocks in southeastern France at a site used for seismological research. After drilling a borehole through the fault—a steeply inclined plane—they placed a special instrument they developed into the borehole, straddling the fault. The instrument was secured into the rock on either side of the fault, and it contains sensors that can detect incredibly minute movements of the rock in any direction. Seismometers in nearby boreholes monitored for even the tiniest earthquakes.

The researchers then injected water at pressure into the fault, near the borehole with their motion-detecting instrument (about 280 meters below ground). No earthquakes were detected in the first 18 minutes, even as the pressure increased, but the fault creeped about 0.3 millimeters. Shortly after, a little earthquake registered. More followed over the next five minutes, and the creep rate picked up as well. Before the first earthquake, the fault actually opened up a couple tenths of a millimeter, but that opening slowed as the earthquakes continued.

By the end of the experiment, the rocks had moved a total of about 1.2 millimeters along the fault, and the vast majority of that was due to creep rather than earthquake movement. (Even added together, the 80 earthquakes wouldn’t equal a zero on the magnitude scale—which can go negative since it’s logarithmic.)

Even more interesting is the fact that the earthquakes appear to have taken place outside the portion of the fault where the water pressure was significantly raised. So it seems that in this case, it wasn’t the water pressure that directly triggered the earthquakes but the slow creep of the fault that caused them to occur nearby as the creep transferred localized strain there. This could be because the fault isn’t a uniform surface, and friction can vary over very short distances.

Generally, the fault was limited to creeping because friction grew with motion faster than the motion accelerated, according to the researchers’ analysis.

The experiment showed that creep is definitely a possible consequence of injections, and it can even play a leading role in the way the fault responds. We clearly still have a lot to learn about what exactly goes on in these situations where injections trigger quakes.

In an article accompanying the paper in Science, Francois Cornet of the Institut de Physique du Globe de Strasbourg notes that the motion-detecting instrument these researchers have developed could potentially be used to help monitor injection sites for signs that an earthquake could be on the way, allowing us to reduce the pressure before it does. That’s the goal many are aiming for.

Science, 2015. DOI: 10.1126/science.aab0476 (About DOIs).