Small earthquakes in Ohio and Arkansas associated with hydraulic fracturing for natural gas have taken many people by surprise. Gas industry executives say there's no hard evidence that their activities are causing these quakes. But some scientists say it's certainly possible; in fact, people have been causing quakes for years.
In the 1960s, geologists realized that gold mines in South Africa had created small earthquakes. Caverns dug into the earth thousands of feet below the surface collapsed. The "pancake" effect caused quakes — in one case a magnitude-5.2 temblor.
Since then, scientists have found that even pumping water away from underground mines (to keep them from flooding) changes the dynamics of stress in rock formations enough to trigger a quake.
Some rock is saturated with water — the water occupies pores between rock particles. This creates what's called "pore pressure" and keeps the formation in a sort of equilibrium. If you suck the water out, particles tend to collapse in on themselves: the rock compresses. Add water, and you push particles apart. So moving water around underground can affect the stresses on those formations.
Now let's say there's a fault in the earth. If the water content around the fault is changed, the fault might slip. If the water gets into the fault itself, it can lubricate the fault and trigger a quake.
Hydraulic fracturing pumps a lot of water underground, where it's used to crack the rock and liberate gas. This may cause tiny quakes, but fracking goes on for a day or two, and the quakes are small.
Recent quakes reported in Ohio and Arkansas are associated with wastewater wells, not fracking wells. The water first used in fracturing rock is retrieved and pumped into these waste wells, which take in lots of water. And at more than 9,000 feet deep, the water is under high pressure that can build up over months or years. It's this pressure that can actually create earthquakes.
In the 1960s, a wastewater well in Colorado at the Rocky Mountain Arsenal is believed to have been the trigger for a magnitude-4.8 quake.
A few geologists are familiar with these induced or triggered quakes. They're rare and usually small, but now fracking is creating thousands of wastewater wells, often in heavily populated areas that historically have not been seismically active. That means even small quakes get noticed.
Shipping Wastewater Out Of State
It can be hard, even then, to definitively nail down the source of a quake. And there are several ways big infrastructure projects can create them: People have created quakes when they excavate quarries — removing all that rock, or "overburden," changes the vertical stress on rock and the faults below. Likewise, dams increase the stress below when a lake is created. The Aswan Dam in Egypt and another in India are believed to have triggered quakes.
One way to avoid creating earthquakes is not to inject fracking wastewater into waste wells, but to recycle it instead. The state of Pennsylvania tried that, but they found that wastewater treatment plants couldn't get all of the toxic material out of fracking water, and the "cleaned up" water returned to rivers wasn't clean enough. So well operators in the state decided to ship wastewater to Ohio, where it has been going down into wells.
The U.S. Geological Survey is working on ways to head off quakes from wastewater wells. That would include performing seismic surveys before drilling the wells. Permeable rock like sandstone is better than hard, brittle basement rock that is riddled with faults. Operators might also limit the amount of water going into wells: USGS geologists have learned that the more water injected, the bigger an ensuing quake.
LINDA WERTHEIMER, HOST:
The idea that pumping water into wells could be causing small earthquakes has taken many people by surprise. Gas industry executives say there is no hard evidence that their activities are causing these quakes. But some scientists say it's certainly possible. In fact, people have causing earthquakes for years. We're joined by NPR science correspondent Christopher Joyce.
CHRISTOPHER JOYCE, BYLINE: Glad to be here, Linda.
WERTHEIMER: So what is the evidence that human activity can cause earthquakes? What sort of thing could do it?
JOYCE: Mining, for one thing. This is one of the first places they got this notion. It was in South Africa, actually, in the '60s, and it was gold mines. And what happened, you know, you create a void underground, and then sometimes it collapses and you create an earthquake.
But there's another way that mines alter what goes on underground. You pump water out of mines, because you can't work in a mine when it floods. And when you move water around underground, you know, you're messing with a very complicated array of stresses along fault lines underground.
WERTHEIMER: Now, hydro-fracking operators do pump a lot of water underground.
JOYCE: They do. Well, when they frack, as it's called, and they break the rock with the water, they use a lot of water. But that's not really what these earthquakes are all about. These quakes in Ohio and Arkansas are associated with wastewater wells. What happens is you use the water to frack for a day or two and then you retrieve it, and then you haul it off and you pump it into these wastewater wells. And this is a lot of water. And 'it's quite deep.
And so, the deeper you go with the more water, the more pressure you create underground and that builds up. And if you get it near a fault line, it can trigger a quake. And actually, this happened in the 1960s. In the Rocky Flats Arsenal, in Colorado, there was a wastewater well and they pumped a lot of water down there and they got a 4.8 magnitude quake.
WERTHEIMER: So, what exactly does the water do to the fault?
JOYCE: Well, if you want try to visualize a fault, and let's say you put your hands together like praying, OK? You open up with your fingers extended, and then that line between your hands is the fault. So press your hands together hard and that's called clamping pressure. It's keeping that fault stable. You add water, like a lubricant, inside that fault line, and boom, it slips and slides and you got a quake.
WERTHEIMER: So, people have been moving water around underground. They've been doing it for a long time, from mining to get rid of wastewater. Why haven't people associated that activity with quakes before?
JOYCE: Well, geologists have, a few geologists. It's not an area that people paid that much attention to. The geological conditions had to be just right, the quakes generally are very small, and it's hard to tell what's causing them. It could be natural or it could be other human activities.
WERTHEIMER: But, Chris, now these fracking wells are proliferating in places that are densely populated - places like Pennsylvania and Ohio, places where we haven't seen this thing before. So, are we going to see more earthquakes?
JOYCE: That's definitely a possibility. What's happened is that you're doing this in places that are geologically different. It's not very active, but there are faults there. They are near heavily populated areas, so people are going to be paying attention.
WERTHEIMER: So, is it possible to avoid creating quakes?
JOYCE: Well, that's one of the things that the U.S. Geological Survey is working on right now. And they're saying, look, if you do a seismic survey before you create a wastewater well, you can avoid possibly the worst area. You can look for faults and stay away from them. You can look for the kind of rock that's more permeable, that's not so brittle. Sandstone, for example, it absorbs the water like a sponge instead of letting the pressure build up.
WERTHEIMER: Presumably, it's not cheap. Can you do these preventative things in a sensible way that will still make the fracking a reasonable thing to do?
JOYCE: I'm not in the gas business and I don't know their bottom line, but it's certainly not out of the question to do seismic surveys. People do them all the time for other underground and for mining obviously, and for doing oil exploration. It just is something that's not required at the moment and probably might well be required in the future. But either that or recycle the water and clean it up, but that's also an expensive process.
WERTHEIMER: Chris, thanks very much.
JOYCE: You're welcome.
WERTHEIMER: NPR's science correspondent Christopher Joyce.
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