Giant, distant earthquakes may help scientists identify places where humans are liable to set off smaller tremors when they inject fluid deep into geologic deposits.
Scientists have known for decades that injecting huge volumes of liquid underground — such as waste from hydraulic fracturing, or fracking — can set off quakes. But in most cases it doesn’t, and scientists can’t predict when or where such human-induced earthquakes will happen.
In the July 12 Science, seismologists report that massive earthquakes unleash seismic waves that can trigger tremors near wastewater disposal wells half a world away. The tiny quakes may be a warning sign that a fault is close to rupture.
“When we do see remote triggering, it seems to foreshadow larger induced earthquakes,” says coauthor Nicholas van der Elst of Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y. “It shows the faults are reaching a tipping point.”
Concerns over human-caused quakes have grown in recent years as earthquake activity has shot up in unexpected places. From 1967 to 2000, the central and eastern United States experienced an average of 21 earthquakes per year of magnitude 3 or greater. From 2010 to 2012, the region saw more than 300 such quakes, reports William Ellsworth, a seismologist at the U.S. Geological Survey in Menlo Park, Calif., in the same issue of Science.
A rise in earthquakes in Arkansas, Colorado, Ohio, Texas and elsewhere seems to coincide with an increase in extracting natural gas and oil from shale formations in those states. Oil and gas don’t easily flow through impermeable shale, so petroleum companies drill horizontal wells and pump in pressurized fluid to fracture a small section of rock (SN: 9/8/12, p.20). This fracking process itself doesn’t cause earthquakes, Ellsworth says. But the fracking fluid comes to the surface along with the oil or gas. Contaminated by a mix of chemicals, the fluid is disposed of when companies inject it deep underground, where it puts pressure on faults.
“Any time we change the fluid regime underground, we could bring a fault closer to failure,” Ellsworth says.
The United States is home to tens of thousands of wastewater disposal wells. “The vast majority,” Ellsworth says, “don’t appear to induce earthquakes.” Scientists don’t understand why only some pose a risk.
The discovery by van der Elst and colleagues may help scientists identify spots where quakes will pop up. In reviewing seismic activity in the Midwest from 2003 to 2013, the researchers discovered that distant earthquakes appeared to initiate small tremors near some wastewater disposal wells, which in turn presaged larger quakes. At each of three sites, in Oklahoma, Texas and Colorado, the team found spikes in seismic activity in the days following at least one massive earthquake, a 2010 magnitude 8.8 quake in Chile, a 2011 magnitude 9.1 event in Japan and a magnitude 8.6 earthquake in Sumatra in 2012. Six to 20 months after the initial tremors, each of the three sites experienced quakes ranging from magnitude 4.3 to 5.7.
The seismic waves from the giant temblors probably perturbed fluids in the faults, increasing the pressure, van der Elst says.
If operators can identify when a fault is about to rupture, they can adjust how much fluid they are injecting into a well or stop the injection altogether. However, the usefulness of this foreshadowing as a monitoring tool is limited because big earthquakes that can remotely trigger tremors occur only rarely, says Cliff Frohlich, a seismologist at the University of Texas at Austin. They happen roughly once a year.
Another limitation is that not all wastewater sites that produced earthquakes during the study responded to remote triggering. Van der Elst suggests that when a well is very close to a fault, just a few months — rather than years or decades — of fluid injection can build pressure and cause a fault to slip. In these cases, there’s little chance of a big earthquake happening before the fault reaches its tipping point.
Even with caveats, the work helps scientists better understand the nature of induced quakes, as does a separate study in Science, published July 11. It may offer a way to evaluate the seismic risk of pumping water in and out of the ground. Examining a geothermal field in California, researchers find that the net volume of water withdrawn mirrored the field’s pattern of seismic activity over the last 30 years. The results could help guide decisions about how much water to extract and inject at such facilities and shed light on the forces that generate human-related earthquakes.
In the work, Emily Brodsky and Lia Lajoie, of the University of California, Santa Cruz, looked at seismic activity from 1981 to 2012 at the Salton Sea geothermal field, which sits near the southern end of the San Andreas fault in California. At the geothermal plant, hot water is extracted from the ground to run turbines and then some of the water is recaptured and put back in the earth. Over the 30 years, thousands of quakes up to about magnitude 5 took place at the site.
After statistically accounting for aftershocks, the researchers found that the pattern of induced quakes over time was related to the volume of water extracted minus the water injected. Because more water ends up withdrawn than replaced at a geothermal field, pressure should decline in affected faults. So Brodsky and Lajoie’s work suggests that rising pressure isn’t the only culprit behind induced quakes. Other factors, such as subsidence, may place stress on a fault, Brodsky says. “The balance of emphasis had been on the effect of injected fluid,” she says. “It turns out there are cases where extraction matters.”
The next step is to study other geothermal fields and wastewater disposal sites to see if similar relationships turn up. Knowing how the amount of fluids affects seismic activity is important for l planning, Frohlich says.
For the most part, earthquakes related to wastewater disposal have been small to moderate and not caused much damage. But such fluid injection has only occurred for a few decades. It’s too soon to know how risky these wells really are and whether they are worth the risk, Brodsky says. When such wells were limited to remote places like West Texas, small rumbles in the ground didn’t bother anyone, Frohlich adds. Now that they are being built near densely populated areas like Dallas, people are rethinking the hazards.
