Residents of Frankfort felt a 5.4-magnitude earthquake centered six miles from West Salem, Ill., April 18. The 5:36 a.m. quake awoke citizens as shockwaves reached the Bluegrass. State Journal Reporter John Zambenini spoke with James Cobb, director of the Kentucky Geological Survey, about the likelihood of an earthquake in Kentucky.
What sort of geologic features make us susceptible to earthquakes here?
In the center of the continent there's kind of a weakness, a sag, you might say. The Gulf of Mexico came all the way up (to the lower Midwest) in previous geologic periods and beneath this part of the continent there are structures and faults and that is why it's prone to earthquakes. California is the most seismically active part of the U.S. On the order of once in 100 years there'll be a 6.5 or better earthquake. Now the sagging part of the interior of the continent is not as dynamic as that so once in every 1,000 years there's a significant earthquake. So it's not as active but still, we get these earthquakes.
For someone who might not know, what exactly is a fault?
A fault is a weakness in the earth's crust upon which movement has taken place. To be a real fault it's got to have some movement. There are some faults where the rocks are sliding. There are others where they are going past each other or even thrusting over the other one.
Geologically, are there certain hallmarks that indicate that we have had an earthquake?
There are features in the geology that record the occurrence of earthquakes from the past. They are known as liquefaction features and scientists have gone up and down the Wabash River and there are places in the bank of the river where earthquakes have caused a fissure to open and it filled with mud, a liquefied material from below. Those are in the Wabash River as well as along the Ohio and down the Mississippi. So we do know previously there have been big earthquakes that left behind these structures.
There's a really cool one in Missouri where a Paleo-Indian skeleton was cleaved cleanly in half by one of these Paleo-earthquakes and liquefaction of the material from beneath has risen up and actually kind of separated the skeleton. We know the age of the burial. You can date the age of the earthquake. There are other occurrences where either carbon or plant material or something has been caught up in one of these and it gives us a date. That's how we know the recurrence interval of large earthquakes on the New Madrid. They're about once in every 500 to 1,000 years and in the Wabash, the big earthquakes of a 6 or better are about once every 2,000 years. In other words it takes 1,000 years to accumulate the strain that would then be released in a big earthquake. So there are fossil earthquakes.
You mentioned every 500 to 1,000 years for a big one but some people say that the New Madrid fault or the Wabash Valley fault might be overdue. What do you make of that?
They're just uninformed. It's just kind of hype. There's no such thing as overdue. That's just Hollywood.
What sort of seismic activity is normal here? You mentioned that there are quakes that we don't feel.
Normal is, along the Mississippi River, along the New Madrid, we have a network of seismographs across Kentucky and these are seismic stations for normal seismicity. You can log onto our Web site and you'll see little blips of a magnitude of a 1 or 2 occurring every day. We had a bit of an increase in activity a couple of weeks ago. There were just dozens and so our seismologists get very curious, you know, does this mean something? We still think that the larger earthquakes are predicted to be about 1,000 years a part. We have daily occurrences of 1's and 2's. We have every few years maybe a 4 or 5.
You mentioned lots of 1 or 2 magnitude quakes. Would someone feel a quake like that?
No.
In the event of a catastrophic temblor, what should be of particular concern to Kentuckians? Will the earth split open? Will hot magma bubble up?
No, absolutely not. That's totally beyond the range of any possibility. When the big ones did occur in 1811 and 1812, there were certain structures formed. Some of them liquefied the underlying clay material, which boiled up to the surface and covered farm fields and actually spread out over many acres. Reelfoot Lake was created so the course of the Mississippi River was altered. There were some small ridges formed also changing some surface drainage a bit but that was it. I mean this is not Hollywood where Los Angeles is going into a gaping crevasse that's thousands of feet deep.
Would you tell us about the scope and the role of the Kentucky Geological Survey?
We monitor seismicity and by monitoring seismicity you get a sense of where the faults are. We have an interesting situation in the mid-continent " we don't see at the surface the expression of the fault, so you have to use the collection of epicenters as your representative of where the fault is. So that's what we do " we collect the data, we interpret the data and that gives us a clear indication of where the more seismically active parts of Kentucky are.
What engineers, bridge designers, the people who design schools and hospitals want to know is this: They want to know what the vibrations " if you've got an earthquake like we had of a 5.2, what is the amplification? It's that relationship that's everything to a bridge designer, hospital building designer, that's the key.
How much of its resources does KGS invest in things like earthquake prediction and response?
You can't predict. There is no such thing as prediction. We have recurrence intervals. We know from looking at the records what to sort of expect but expectation and prediction are different. We don't presume to predict an earthquake. To our knowledge there's only once instance and that was in China where they may have just gotten lucky.
How fast would your average seismic waves travel?
It will go across Kentucky in minutes. I don't know, maybe all the way across Kentucky in 20 minutes. So it travels at a pretty high rate.
Is there any correlation between volcanic activity and seismic earthquake activity?
Oh, yes, it's big. When you've had a big outpouring of lava in a volcanic eruption you end up with a void down there and it will collapse and generate an earthquake. While active volcanoes are going on, you always register earthquake.
But there's probably not much of a volcanic connection with the New Madrid Fault or the Wabash Valley?
No, there's none. There is no hot spot in the mid-Continent. Now, if you move to like Yellowstone, there's a big hot spot and they're active but we don't have any beneath Kentucky.
How many seismic stations are there in total?
There are 25 stations.
What do they consist of?
A well, probably plastic pipe into the ground, into bedrock, and then in the well is a seismic instrument. It's actually based on a magnet. With even the slightest, little vibrations these very sensitive magnets will pick up and make electrical signals that can be turned into seismic readings.
And they send those signals back here?
Yes. Some are hardwired into the phone system, which comes directly to the KGS. Some of them we have to go out and download the memory because they're too remote to get good phone lines. We've got a brand new very deep well called the Central U.S. Seismic Observatory which we put in a couple of years ago and this is a very deep well. It's about 2,000 feet deep.
According to the KGS mission statement, you deal with other geologic hazards. Describe those that you deal with.
It's particularly sinkholes and landslides. We have a group that goes out and maps landslides particularly in northern Kentucky and eastern Kentucky where you have higher relief and more hillsides, the soil will creep as you get real heavy rains that will saturate the soil and then they'll start sliding down the mountainside. In northern Kentucky we've got some of these real soft clays, they're subject to causing landslides so if people build a house and they don't understand where those layers are then pretty soon the house will be, you know, just a little movement of a foundation will destroy the house so we get a lot of that.
Earthquakes can precipitate things like that?
Oh, yes, they can. We haven't had any reports of that for this last quake. If it had been in Kentucky we probably would have but we didn't.
What sort of a magnitude quake would you say is necessary to record some real damage?
In the 5's. We had the Sharpsburg earthquake in about 1980 or '81 and we recorded some real damage between Lexington and Maysville. Chimneys were knocked down, brick homes were broken, things like that. Maybe some sewer lines broken, gas lines, things like that will happen.
At what point would an earthquake be categorized as catastrophic?
Six and above. Seven is pretty catastrophic; eight is absolutely devastating, and nine is what Sumatra experienced in that horrible one that caused the 2004 tsunami and that is almost unprecedented. There has only been one other more powerful earthquake ever recorded on earth and that was on the west coast of South America.
The network gives us location, depth and magnitude information. We put it online so that people in emergency services and everybody else can actually see it. It's supported by the state and we've had this since 1990. We began making the statewide seismic network. It is operated by the University of Kentucky and the Kentucky Geological Survey, and the funding and support comes from the state.