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.