ROBERT SIEGEL, HOST:
From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.
AUDIE CORNISH, HOST:
And I'm Audie Cornish. In the continental U.S., there are small earthquakes every day and their numbers are increasing. Geoscientists have concluded that there's a new epidemic of quakes. They say it's related to industrial wastewater being pumped into underground disposal wells. And new research reveals two trigger mechanisms that may be setting off these wastewater quakes. Here's NPR's Christopher Joyce.
CHRISTOPHER JOYCE, BYLINE: Most of these quakes are too small to feel. They tend to happen in swarms. Over the past year, geoscientists tracked down some of these swarms to underground faults near deep wastewater wells. Many of these wells are filled with waste fluid from the oil and gas drilling boom. Nicholas van der Elst is a geophysicist with Columbia University's Lamont Doherty Earth Observatory. He says there are lots of small faults all over the country.
NICHOLAS VAN DER ELST: The injection of fluids goes inside the fault itself and kind of pushes outward on the fault walls and makes it easier for the fault to slip.
JOYCE: The wastewater loads up these faults with tension until, at some point, they slip and the earth moves. So, what van der Elst wanted to know is what triggers that slip. Sometimes it's just all that water building up. But van der Elst discovered that in three cases over the past decade - in Oklahoma, Colorado and Texas - the trigger was another earthquake, a really big one thousands of miles away.
ELST: They set up these big waves that travel around the surface of the earth kind of like ripples. You can even see them on seismometers going around the world multiple times.
JOYCE: Those three big quakes rang the planet like a bell. And when their seismic waves reached underground faults near waste wells in those three states, they nudged them over the brink. Soon, they swarmed with mini-quakes. And some of those swarms eventually culminated with pretty big quakes, in the magnitude four and five range, big enough to do some damage locally. Van der Elst's findings appear in the journal Science. In the same issue, geoscientist Emily Brodsky at the University of California Santa Cruz reports yet another trigger mechanism for mini-quakes: geothermal energy. The power plant in question, near the Salton Sea in California, extracts hot water from underground. It turns it to steam to make electricity and then returns most of that underground.
EMILY BRODSKY: What we found is that the earthquake rate correlates quite strongly with the extraction of water from the field.
JOYCE: On average, extracting half a billion gallons of water per month resulted in one detectable earthquake every 11 days. Now, scientists already knew that geothermal power plants can cause small quakes when they cycle water from underground. But Brodsky's research actually matches the amount of water moved to the frequency of quakes. Brodsky notes that these quakes are very small but adds that the geothermal plant is near the southern tip of California's San Andreas Fault, the source of many of the state's big quakes.
BRODSKY: We ought to know what's happening on the southernmost terminus of the San Andreas Fault. Of various places in the world to induce earthquakes, this is a particularly sensitive one.
JOYCE: Brodsky says it's unlikely that the geothermal plant could induce a major quake on the San Andreas Fault, but it's theoretically possible. The company that owns the plant, Mid-American Energy Holdings, declined to comment on the research. The U.S. Geological Survey's William Ellsworth has been in the thick of all this research.
WILLIAM ELLSWORTH: One of the major questions we're concerned about is how large might an induced earthquake be. We don't have the answer to that and that's really one of the keys to being able to better forecast the seismic hazard going forward.
JOYCE: Ellsworth says to do that, geologists need more seismic monitoring stations and more data from waste wells and geothermal plants. Christopher Joyce, NPR News.