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Rock Type Affects Reach of Injection-Induced Earthquakes

Injection sites in Central Europe, the U.S. and Australia may show similar earthquake patterns, due to injection into similarly soft rock. | Thomas Goebel

Earthquakes caused by injecting fluids into softer rock travel farther and shake more strongly than those resulting from injections into harder basement rock, a new study in the August 31 issue of Science shows.

As previous policies designed to mitigate induced quakes have encouraged injecting into soft, sedimentary geology, the findings of this new report may prompt a reevaluation of hazard assessments and regulatory approaches, the authors say.

Most natural earthquakes occur along fault lines and in areas where powerful tectonic forces conspire to push, warp and crack the Earth's crust; however, they can also be triggered by human activity and in regions far from those prone to natural seismic activity.

The number of earthquakes caused by unconventional energy exploration activities or wastewater disposal using high-pressure injection wells has greatly increased in parts of North America and Central Europe — in the state of Oklahoma alone, seismic events increased 50% between 2013 and 2014 — developing into a substantial source of seismic hazard that puts infrastructure and the public at risk.

These types of induced earthquakes have been difficult to manage because in many cases, such quakes occur far away — in some cases nearly 20 miles (30 kilometers) distant — from the original fluid-injection activities. As such, little is known about the seismic potential of a given injection activity.

Understanding the mechanisms underlying injection-induced seismicity is critical to improving hazard assessment and mitigation. To address this need, the research team examined the seismic reach of induced earthquakes worldwide. The authors focused on well-documented quake activity surrounding isolated injection sites in areas predominantly located in regions with low natural seismic activity throughout the U.S., Central Europe and Australia.

"Our study is the first to compile a large number of earthquake data-sets from single injection wells," said Thomas Goebel, the study's lead author from University of California, Santa Cruz.

"We wanted to do something comparative," said UCSC co-author Emily Brodsky. "We have seen a lot of studies focusing on particular sites and we were wondering if there were some more general patterns. It turns out there were."

By analyzing the spatial decay of each induced quake — a measure of how far and quickly its energy travels before dissipating — the authors revealed two distinct patterns of quake activity. Some tremblors traveled longer distances from injection sites and grew steadily weaker, while others abruptly ended closer to the injection site. They also were able to determine variations in the maximum magnitudes between the two types of quakes — fluid injection into soft rock produced the largest seismic events in their analysis with magnitudes of up to 4.7.

The propagation of injected earthquakes differs depending on the hardness of the injected layer of rock. | Thomas Goebel

The authors found that quakes induced by injection activity into shallow, softer underlying geology, like sedimentary rock, traveled farther and were generally stronger, while those resulting from deep injection into harder basement rock stayed closer to home.

"Our empirical observations suggest that injection into sedimentary rocks has a larger zone of influence. The extended spatial footprint may promote more felt events during injection," said Goebel.

These findings run counter to the basis of previous induced seismicity mitigation strategies, which have encouraged injecting into sedimentary units and as far away from basement rock as possible.

By forcing large volumes of fluid deep into the earth, injection wells create added pressure inside and around underlying geological formations. The added stress can fracture or deform subsurface rock or cause existing nearby faults to slip, thereby triggering seismic events.

"The additional pressures are often quite small, however, the Earth almost always has some strain energy built up, so even very small changes in pressure can trigger earthquakes," said Brodsky, who added that even small seismic events can greatly impact natural faults, which can result in large events. The farther each quake reverberates from its point of origin, the more faults it could potentially encounter.

"So, the problem turns into a competition between the number of faults available, which increases with distance, and the size of the perturbation from the well, which decreases with distance," said Brodsky. "It is not clear which process will win the competition."

Furthermore, these risks are not specific to soft-rock injections. According to the authors, injection into basement rock also poses a source of seismic hazard and has led to damaging earthquakes when faults are disturbed, like the magnitude 5.3 Rocky Mountain Arsenal, Colorado earthquake in 1967.

While the number of fluid injection projects that induce earthquakes is small compared to the more than 30,000 U.S. wells in operation, according to a report by the Congressional Research Service, the seismic risk is now elevated in some parts of the country with injection activities. In the central and eastern U.S., the number of earthquakes with a magnitude greater than 3.0 has increased dramatically, from approximately 20 a year between 1970 and 2000, to over 100 per year for the period of 2010-2013. Furthermore, fluid-injection projects are growing worldwide.

[Credit for associated image: Jason Bondy/ Flickr]