MELISSA BLOCK, HOST:
A couple of centuries ago, there was a series of big earthquakes in what we now think of as the country's midsection. Geologically speaking, it's the New Madrid fault zone. Since then there's been some small seismic activity. And scientists have gone back and forth on whether this area still poses a threat for another major earthquake. Some say it's seismically dead or at least in a deep sleep. Well, now research suggests that the New Madrid fault zone is alive and kicking. Joining me to talk about that is geologist Susan Hough. She's with the U.S. Geological Survey. She's coauthor of the study published this week in the journal Science. Ms. Hough, welcome to the program.
SUSAN HOUGH: Thank you.
BLOCK: And let's talk a bit about these big earthquakes in the Midwest over several months in 1811 and 1812. How big were they and how far away were they felt?
HOUGH: Well, we don't know the magnitudes precisely because we didn't have seismometers, but we've estimated magnitudes of at least 7.0 for the four largest earthquakes. They were felt all the way to the east coast. They rang church bells in Charleston, South Carolina. One of the documented fatalities was actually near Louisville, Kentucky, which is relatively far from the New Madrid zone. But as we know, in the east coast, the seismic waves travel quite efficiently. So, the effects of any one earthquake are felt more broadly.
BLOCK: Now, I mentioned thought that the New Madrid faults were dead, even though there were some smaller earthquakes since those big ones. So, why the thinking that the fault zone was, if not dead, at least really sleeping?
HOUGH: Right. Well, it has been assessed as a high-hazard zone, so this isn't entirely new. But there were really two arguments that it might in fact be dead. One was that you don't observe a lot of warping at the surface from GPS measurements and the other is the argument that the earthquakes we observe at the surface in present daytimes are actually aftershocks of the earthquakes 200 years ago.
BLOCK: And your research seems to be saying otherwise, right? You're saying this fault zone is alive and kicking.
HOUGH: Right. So, that was the part of the story that we focused on. If you sort of look at the activity that we know about, it kind of looks like an aftershock sequence, but we really tested that hypothesis very rigorously. And when you do that, the hypothesis just fails, because we know it's not a plate boundary, where big plates are moving past each other. Every indication is that there's active processes going on deep in the crust and we're not sure what they are and why they're happening. But our results tell us that something is alive and kicking down there to keep generating these small earthquakes, which we conclude are not aftershocks.
BLOCK: Well, for folks who hare living in the New Madrid fault zone area, they're going to be wondering, you know, what's the likelihood, what's the probabilities that a really big earthquake could strike here again. Is there any way to answer that question?
HOUGH: We can just sort of give an assessment of the overall probabilities. And what we know about New Madrid, the expectation is that a magnitude 7.0-ish earthquake might happen once every 500 years. And a magnitude 6.0-ish event might happen, on average, once every 50 years. It's been 1895 since we've seen the last event close to magnitude 6.0.
BLOCK: Susan Hough is a seismologist with the U.S. Geological Survey in Pasadena. Thanks so much for talking with us.
HOUGH: Thank you. Glad to be here.
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