Q&A: Earthquake risk in the eastern United States

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Outside of the west coast of the United States, few Americans have much experience with earthquakes. We may have felt smaller earthquakes, of magnitude 4 or 5 in the east, but nothing that approaches the terrible power of the earthquake that occurred in Japan on March 11.

I spoke with Prof. James F. Tull, a professor of Geological Sciences at Florida State University, to find out just how likely such a catastrophic event might be and whether the earthquakes around the Pacific Rim make it any more likely that the west coast of the United States will experience a major earthquake in the near future.

The risk of earthquakes in California and along the west coast of the United States is fairly well-known, but what other areas of the country might be at risk?

One of the highest seismic risk areas outside of the west coast is called the New Madrid fault system, which is along the Mississippi River, in northeast Arkansas, southeast Missouri, Kentucky, west Tennessee. There was a major earthquake there in the early 1800s. It was probably one of the largest earthquakes ever experienced in the United States. That zone is still very active. The areas that would be at risk are from Memphis all the way north to St. Louis. Many of the government agencies in those areas are certainly aware that there's major seismic risk there, and they do have emergency plans.

What kind of an impact would a major earthquake there have on the flow of the Mississippi River?

Well, of course, you may have heard of 1811 and 1812, the New Madrid Earthquakes. The Mississippi actually flowed north. It tilted the land enough immediately after the shock for the Mississippi to flow north. It produced some very large lakes in that area. Reelfoot Lake, I think, was produced during that earthquake. There weren't too many European-Americans living in that area then, so the reports aren't very good. Nothing like that has happened in that area since then, and that's about 200 years ago.

Do the building codes in that area take into account earthquake risk?

I don't think so. They're certainly not to the Japan standards or the California standards. That's obviously something that all nations, or nations that have earthquake threats, need to work on, but it's a matter of getting the legislation passed and having the new buildings built to code. Of course, the [older buildings in the] rest of the city are usually not up to code. So there are plenty of buildings that would go down, given a large earthquake like that.

Given the state of construction in a location like that, what magnitude of earthquake would be required to see real damage?

It depends on the geology, the distance to the quake, and the magnitude of the quake. It would be hard to say, but I would say certainly [a magnitude of] 6 or 7 would cause significant damage. Anything larger than that would be pretty catastrophic. The Mississippi delta sediments are relatively weak, when it would come to transmission of these seismic waves, so you would have a lot of ground motion associated with the geology. It's not good strong bed rock until you get over west of Little Rock. The delta sediments would exacerbate the ground motion.

What about other areas that might be at risk? I'm looking at a U.S. Geological Survey (USGS) Map and they have a spot listed in South Carolina.

The Charleston Earthquake was [1886], and that's not really very well understood. It was felt as far away as New York City. Some geologists think that was related to igneous activity, to magma motion, but I don't really know enough about that. It's obviously going to show up on a seismic risk map anytime you have a historical event that's significant. One of the ways that you predict earthquakes is to look at where they've happened before. Charleston was one of the largest earthquakes on the east coast, but there hasn't been anything nearly that magnitude since [1886].

The other east coast location that shows up, and from which I've felt earthquakes, is up in Quebec.

Yes. Those are ancient fault systems that are still adjusting partly to the removal of the glacial ice, from the great glaciation 100,000 years ago. They are crustal faults that have been there since the Appalachians were formed and they're still adjusting to plate motions. You could feel those, but I don't think there is any danger of a large magnitude earthquake along those faults in the St. Lawrence valley.

When an earthquake occurs, the type of buildings that are damaged is related to the wave frequency.

Well, number one, the damage is related to the nature of the construction. A masonry building is by far the most susceptible to damage if it is not steel-reinforced. That's what you see in older buildings that have not been brought up to code. In third-world or developing countries, their construction technique is mainly masonry, and those types of buildings simply can't take that kind of ground motion. You get a surface wave coming through from a seismic wave with rapid ground motion in one direction and then back in the other direction, and that's the shaking. A masonry structure can't maintain itself under that kind of ground motion, whereas a wooden structure or a steel-reinforced structure can do that. Of course, with more advanced designs, like we have for skyscrapers in most of the Pacific countries, they are designed to take that motion, allowing them to basically twist, and turn, and sway, and still remain intact. If you were on the upper floor you'd be thrown around, and anything else would be thrown, but the integrity of the building is not affected.

There's some talk that if you look at the Ring of Fire and put four corners on it, three of them have had major events recently. The corner near California and Canada hasn't.

Christchurch, Japan, where's the third one? Chile. It's getting pretty far-fetched to link those together. If you push your hands together, and back off enough to allow your hands to slip, that's what happens along a fault. Of course, most faults are locked by that pressure. They are not slipping. They are building up that stress. At any given time, any fault system might be ready to go, but having one of these faults move and produce an earthquake and another one [occurring], miles and miles or thousands of kilometers away, is not what's happening.

You can relate these events geologically. They are all subduction zone earthquakes, but they're not associated with each other, other than that. The fact that they happened this year... geologist and seismologists have never become convinced that there's any real change in the abundance of earthquakes over time. In other words, if you look at the record of earthquakes we have over time, they are random events, temporally. The world is not coming to an end. They are not getting more and more rapid or higher intensity.

The fact there was an earthquake in Japan has no bearing on the likelihood of a major seismic event somewhere else.

No, absolutely not. That's an independent force field or stress field on those faults and they are going to adjust and move according to that. In Alaska or in California, there's a totally different stress field and it's not going to be affected by the part of the segment in Japan. People like to try to come up with world-wide scenarios, but earthquakes don't work that way.

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