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.

Chris, welcome.

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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