Two new studies of earthquakes near injection wells have seismologists using words rarely heard these days in earthquake science: prediction and warning. The research has also renewed calls for better seismic monitoring and reporting in regions experiencing man-made earthquakes.

"Shale gas operations have completely changed our energy policy and people are injecting in places they've never injected before. If we're going to do this safely, we need to address the environmental issues, including protecting water supplies and earthquake risk," said Cliff Frohlich, a seismologist at the University of Texas Institute for Geophysics who was not involved in the new studies.The two reports appear in today's (July 11) issue of the journal Science.
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Links between injection and earthquakes

In the Midwest, researchers discovered a warning signal that moderate-sized earthquakes may strike near injection wells, where mining companies dispose of waste fluids. At three sites in Oklahoma, Colorado and Texas, passing seismic waves from faraway earthquakes - the recent massive temblors in Japan, Sumatra and Chile - triggered swarms of small earthquakes. The seismic activity continued until magnitude-4 and magnitude-5 earthquakes struck, such as the large earthquakes near Prague, Okla., in November 2011.

"We've been telling our operators for some time that this is one of the warning signs to look for," said Austin Holland, a seismologist with the Oklahoma Geological Survey who was not involved in the study. "If you see remote triggering of your wells, it's a clear indication that your faults are right at the failure point. It just took a little tickle, if you will, to trigger the earthquakes."

In a separate study, researchers documented a clear link between earthquakes and production, or removing and injecting underground fluid, at Southern California's Salton Sea Geothermal Field power plant. Every 500 million gallons of water pumped out of the ground caused one detectable earthquake per 11 days, according to the report.

"The thing that best predicted the earthquake rate was the net amount of water extracted from the ground," said Emily Brodsky, lead study author and a seismologist at the University of California, Santa Cruz.

Scientists have long known that geothermal projects can trigger earthquakes, but some communities are wary of the seismic risk. Construction on a geothermal energy plant in Basel, Switzerland, was shut down in 2009 after fluid injection triggered earthquakes up to magnitude 3.4. Brodsky's study offers a new statistical model for predicting how many earthquakes to expect at a geothermal plant, based on the amount of fluid going in and out of the ground.

"This paper has made a very direct and compelling correlation between the net of fluid out and fluid in and the rate at which these small earthquakes are happening," said William Ellsworth, a seismologist at the U.S. Geological Survey's Earthquake Science Center, who was not involved in the study. "This seems like a very promising way of applying this particular statistical model."