The middle of Oklahoma has become an earthquake hotspot because of the oil and gas industry—and also from powerful temblors around the world. In the area near Prague, Okla., where wastewater from oil and gas production has been injected down disposal wells for decades, a series of earthquakes broke out following the massive magnitude 8.8 earthquake off the coast of the Maule region of Chile in 2010. For months the grounds in Oklahoma periodically shook, culminating in a destructive 5.7 magnitude quake in November 2011.

According to a new paper published in Science on July 12, that makes the Prague earthquake not only the largest earthquake associated with wastewater injection but also the largest linked to another seemingly natural quake as an initiating trigger, despite the distances involved. And that suggests that such relatively small, remotely triggered earthquakes might serve as a warning sign of bigger shocks to come, according to geologist Nicholas van der Elst of the Lamont–Doherty Earth Observatory at Columbia University, who led the research.

Oklahoma is not alone. The Cogdell oil field near Snyder, Texas, had a similar reaction to the March 2011 massive magnitude 9.0 Tohoku earthquake in Japan, which caused a tsunami that resulted in more than 15,000 deaths and multiple nuclear meltdowns at the Fukushima Daiichi power plant. And the same Maule earthquake also set off a similar swarm of small quakes near the border of Colorado and New Mexico, where water was also being reinjected. "We were somewhat surprised, though, to find that the triggering actually foreshadowed the occurrence of moderate to large earthquakes at these sites, showing the buildup of underground fluid pressure," van der Elst notes.

The key is all that wastewater pumped back down under pressure as part of oil and gas production, which is on the rise in the U.S. again. As van der Elst describes it, slow-moving seismic waves from the distant earthquakes squeeze that fluid in and out of the deep rock, setting off local rumbling. The same thing happens in natural underground water-filled formations, too, like the magnitude 7.9 earthquake near Denali, Alaska in 2002, which set off temblors in the Yellowstone Caldera and changed the timing of geyser eruptions.

Such man-made earthquakes are not rare at all, and some come quickly after injection of wastewater, as in quiescent Youngstown, Ohio, which endured a magnitude 4.0 quake after wastewater injection started at a nearby disposal well. The shaking has not recurred since pumping stopped.

That such wastewater injection triggers quakes comes as no surprise. Evidence for them began in the 1960s, when the weapon–making Rocky Mountain Arsenal near Denver attempted to dispose of hazardous chemicals by pumping the liquid underground. During four years of such disposal, the pumping triggered 16 earthquakes, even after the injection stopped. And the U.S. Geological Survey (USGS) deliberately triggered quakes in the Rangely oil field in northwestern Colorado to determine what types of underground pressures might set off temblors.

Of course, it's not just injecting wastewater or deliberately cracking deep rock with high-pressure water—the technique known as fracking—that can set off quakes, notes a review of such injection-induced earthquakes by William Ellsworth of the USGS, also published in Science on July 12. Intensive water use ranging from creating large-dam reservoirs to excessive pumping of groundwater has set off earthquakes. Perhaps the best-studied example is the attempt to harvest heat deep underground to generate electricity, or geothermal power. At the Salton Sea in California, which boasts 10 geothermal power plants, earthquakes can actually be predicted based on the amount of brine extracted or injected, according to a paper published online by Science on July 11. "As operations expanded, so did the seismicity," the geophysicists from the University of California, Santa Cruz wrote.

But wells that pump large volumes of fluids underground seem to pose the most risk, and they may be responsible for the rapid rise in the number of temblors in the middle of the U.S. in recent years. There are at least 140,000 such injection wells in the U.S., the bulk of them used for pumping water or carbon dioxide underground to flush out additional oil from old fields. But 30,000 or so are used for permanent disposal of the nine billion liters of contaminated water that flows back up wells along with oil or natural gas in the U.S. each day. Given the lack of alternatives for disposing of such wastewater, the increase in small earthquakes from a few dozen to several hundred a year in the last few decades may prove to be a necessary risk.

Even if such wastewater injection ceases, the risk of earthquakes from the wells will remain, van der Elst says. "There's no guarantee that stopping injection would prevent diffusion of the fluids that are already present into susceptible faults." All that water underground may just be waiting for the next big earthquake—anywhere in the world—to set the ground in motion.