Engineering Professor Answers Earthquake Questions

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Brady Cox

EDITOR'S NOTE: This question-and-answer article about recent earthquakes was prepared before the occurrence of another relatively strong earthquake in Oklahoma on Monday evening. The first paragraph has been updated to reflect the Monday earthquake; however, professor Brady Cox's answers remain on the mark.

On Saturday, Sunday and Monday, Nov. 5-7, more than 20 earthquakes shook the area of central Oklahoma. The three largest — one with a magnitude of 4.7 on Saturday morning, a second on Saturday evening with a magnitude of 5.6, and a third on Monday evening with a 4.7 magnitude — originated approximately 40 miles east of Oklahoma City.

Many people in Northwest Arkansas felt the earth shake on Saturday night at 10:53 p. m., and, combined with the earthquakes in central Arkansas earlier this year, this event has brought attention to seismic activity, something that has not been a big topic in our area for many years.

Brady Cox, assistant professor of civil engineering and an expert on the way seismic waves affect soil and structures, has answered a few questions about the Oklahoma earthquakes and the earthquake threat in our region in general.

Q: From a scientific point of view, were these earthquakes an unusual or unexpected phenomenon?

A: No, central Oklahoma is known to be an area of seismic hazard. They’ve historically had earthquakes in this part of Oklahoma. This was a large earthquake for the area, but it’s relatively small when compared to significant earthquakes expected in other seismically active areas such as the west coast of the U.S. or the New Madrid seismic zone in northeastern Arkansas.

Q: Is this activity a sign of more serious earthquakes in the future?

A: Not necessarily. Most people have heard of aftershocks, but there is also a thing called a foreshock. Foreshocks are earthquakes of smaller magnitude that happen before the main shock, and sometimes people will ask “can we use these foreshocks to predict earthquakes?” The answer is no, we’ve never really been able to do that on a consistent basis.

Most people would probably call the magnitude 4.7 earthquake a foreshock for the 5.6, but is the 5.6 a foreshock for an 8.0? The answer is no, it’s not. That area can’t produce an earthquake that big. Was the 5.6 a foreshock for a 5.7 or an 5.8? I have no idea, but more than likely, no.

Q: How does the soil in our area react to earthquake activity?

A: By and large in Northwest Arkansas, we have great soils for construction, not just for earthquake loading, but in general. Because rock is fairly close to the surface, we can found structures right on rock in many cases, and even if we don’t put the foundations right on rock, we have great, really strong soils that don’t tend to amplify earthquake shaking that much.

Q: Are buildings in Northwest Arkansas built to withstand earthquakes?

A: Any substantial structure built recently has to be designed and constructed according to building codes, and in those codes are provisions for seismic design that engineers have to follow. There are more detailed analyses that can be done for special buildings, like nuclear facilities. I get involved with those a lot.

Q: Has anyone suggested that these recent earthquakes are related to hydraulic fracturing used in natural gas extraction and commonly referred to as “fracking”?

A: People have written papers about links between seismicity in Oklahoma and other areas and production/drilling for oil and gas. However, you can’t cause an earthquake just by doing that. You have to be in an area where there’s already tectonic stress built up. Drilling or fracking might be a catalyst for low-level seismic activity that is already prepared to occur; it may just speed it up a bit.

Q: Did you feel the earthquake?

A: I recognized it instantly as an earthquake and started counting the seconds. I counted over 30 seconds, and because of that I knew it was a decent sized earthquake but I also knew that it was a long way away.

A: Should Northwest Arkansas residents be worried about earthquakes?

Q: The main threat in our state and what people need to be concerned about is the New Madrid seismic zone. That has potential to cause huge shaking. The New Madrid zone is almost the exact same distance away as Oklahoma City. So if people can imagine that they felt an earthquake near Oklahoma City that was a magnitude 5.6, what are you going to feel if a magnitude 8.0 earthquake happens the same distance away? It will be a lot stronger.

It doesn’t mean that we would have a ton of damage in Northwest Arkansas, but we could have problems with small things, like what happened in Oklahoma City with this earthquake. People would be seeing their chimneys cracked, drywall cracked, stuff like that.

Q: Is there a way to prepare for earthquakes?

A: Not really, other than the standard stuff. We have hazard from earthquakes, which are pretty minimal in Northwest Arkansas, but the hazard from tornadoes is much greater. The hazard from ice storms as well. If people are prepared for those things, they are going to be prepared for any earthquake shaking that could happen here.