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.