JUDY WOODRUFF: Now, the earthquake that rocked the middle of the United States today. Ray Suarez has more.
RAY SUAREZ: Here to tell us about the quake and the forces that caused it is Timothy Kusky, director of the Center for Environmental Studies at St. Louis University and author of the book “Earthquakes: Plate Tectonics and Earthquake Hazards.”
And people reported across a big chunk of the Midwest feeling the tremors, Professor, but what was happening under their feet? What was going on below the ground that they were feeling?
TIMOTHY KUSKY, St. Louis University: Well, there was an earthquake with a magnitude 5.2 centered in eastern Illinois. And the energy moved out from the epicenter in all directions and was transmitted as far as Chicago, as far as Atlanta. It shook St. Louis. And people felt the ground shaking as a result of that energy being released.
RAY SUAREZ: Well, you live a couple of hours’ drive from the epicenter. Was it a substantial feeling where you were?
TIMOTHY KUSKY: It sure was. It woke me up out of bed. It woke my whole family up and the pets up. And the house was shaking back and forth for maybe 5 or 10 seconds.
RAY SUAREZ: Is 5 or 10 seconds about how long quakes last along this particular fault line that runs through Indiana and Illinois?
TIMOTHY KUSKY: That’s pretty typical, yes. People who live on softer soils can feel the shaking last for a longer time, because the very thick soils can amplify the amount of ground shaking. But the actual earthquake event that shook the ground was about 5 or 10 seconds in duration.
Midwest quakes rare but devastating
RAY SUAREZ: Do they get big earthquakes along this fault line?
TIMOTHY KUSKY: They do. This earthquake was located along the Wabash fault zone, and this fault line can give magnitude 7.0 earthquakes about once every 1,000 years.
But the real concern in this area is that the Wabash fault is an indirect way connected with the New Madrid fault zone, which has historically given magnitude 8.0 earthquakes. For instance, in 1811, 1812, there was a series of around three magnitude 7.5 to 8.0 earthquakes on that fault zone.
RAY SUAREZ: There were also aftershocks in this episode, weren't there?
TIMOTHY KUSKY: There were. There were a couple of 2.5 aftershocks. And then, around 10:15 this morning, there was an aftershock of a magnitude around 4.5 or 4.6.
RAY SUAREZ: Well, let's talk a little bit about the Richter scale. Is there a big difference between a 4.5, 4.6 aftershock and the original earthquake at 5.2?
TIMOTHY KUSKY: There is. The Richter scale is a logarithmic scale, so the way it was devised, there's an increase of tenfold in the amount of energy between a 4.0 and a 5.0. Actually, the amount of energy released is a little bit more than tenfold, maybe 20 or 30 times, because there's more seismic waves released for each increment on the Richter scale. So that's a very big difference.
RAY SUAREZ: And what causes an aftershock? Does the Earth, in effect, still have some more energy to get out of its system?
TIMOTHY KUSKY: That's right. The earthquake releases a lot of the stress that is built up for a long time along the fault, so the main earthquake releases the biggest part of the stress.
And then the rest of the fault zone around the main area that slipped during the main earthquake has to recover and respond to the main shock. And so the aftershocks are the bits around the area that slipped recovering and responding to the first earthquake shock.
RAY SUAREZ: So it's not felt as widely as the original earthquake?
TIMOTHY KUSKY: Well, a smaller amount of the fault actually moves during the aftershocks, so the amount of energy released is less, and the energy doesn't get transmitted quite as far.
And that's actually a very interesting point, because a magnitude 5.2 earthquake is very large for the central USA, because the bedrock here is very hard and solid, and the energy gets transmitted very far and very efficiently in these rocks.
If an earthquake of magnitude 5.2 happened, for instance, in California, where the rocks are much softer, the energy would be absorbed much more quickly and it would do much less damage and would travel less far.
RAY SUAREZ: And from what we know, and from what you in the science know from observing these things, is this it for a while? Or is there a pattern where there will be some repetitions?
TIMOTHY KUSKY: Well, there's likely to be small aftershocks for a number of days. And it's very hard to predict earthquakes, especially in this part of the country, because we're not along a plate boundary here.
In places like California, we have one tectonic plate that's sliding past another tectonic plate at a very regular, specific rate, and we can say that there's three or four inches of movement every year, and that after 12 inches of movement accumulates, we're going to have an earthquake, so we can assign a likelihood of that earthquake happening.
Here, for instance, along the New Madrid fault zone, there's historical evidence that there's been earthquakes about once every 400 or 500 years. By historical, I mean very large earthquakes of magnitude 7.5 or 8.0.
And the last major earthquakes we had along the New Madrid zone were in 1811, 1812, so probably we have a couple hundred years to go before there's another very major earthquake around here.
RAY SUAREZ: Professor Kusky, thanks for joining us.TIMOTHY KUSKY: Thank you very much.