Geologists try to map potential seismic hazard risks of New Madrid region

April 2015 – NEW MADRID – The most recent update of the U.S. Geologic Survey earthquake hazard maps shows a big bull’s-eye over the Bootheel. But the agency says there’s no reason to panic. “We don’t know that an earthquake is imminent — we can’t predict earthquakes. But we do expect the past behavior in the New Madrid region to be a representation of how things might happen,” said Robert Williams, Central and Eastern U.S. regional coordinator for the USGS in Golden, Colorado.

“This is a ground motion hazard map, not a map showing the expectation of where earthquakes will actually hit,” Williams said. The map shows there’s a 2 percent chance over the next 50 years the area will exceed 80 percent of the acceleration of gravity. The USGS builds its earthquake hazard maps for engineers based on quakes that have occurred, Williams said. “But it’s estimating the chance of exceeding ground motions in the next 50 years.” Williams said the 50-year period is a common engineering design time interval for the life of a building. “Obviously you have to have an earthquake to get the ground motions, but doesn’t try to say exactly where the earthquakes are going to occur and when they’ll happen, just an estimate of future ground motions.”

Cape Girardeau is in an area with a high chance of experiencing strong ground motions in the next 50 years, Williams said. The USGS assessed earthquake occurrence in the past, including the New Madrid earthquakes in 1811 and 1812 as well as earlier, large sequences of New Madrid quakes going back to 900 A.D. “The 1811-1812 events were very important, but geologists have dug into the earth and found or imaged faults and evidence of past earthquakes in prehistoric times,” or paleoseismic quakes, Williams said. Mark Hasheider, assistant Cape Girardeau Fire Department chief and the city’s emergency management coordinator, said they plan for the eventuality of an earthquake, but “that doesn’t necessarily mean in my eyes that it’s going to be as significant as the one in previous history. But any earthquake could be somewhat devastating in our area, so we do plan for that.”  

Information at earthquake.usgs.gov on eyewitness accounts credit the local 1811-1812 quakes with major geological changes: causing the Mississippi River to flow backward, raising Crowley’s Ridge and creating Tennessee’s Reelfoot Lake, for example. Williams said physical changes to the natural environment are difficult to predict.

“If we had a repeat of the 1811-1812 earthquakes, we would expect similar kinds of ground failure that happened then, like the very large sand blows, huge volumes of water flooding onto the ground surface because of the liquefaction and strong shaking.” Cape Girardeau; Memphis, Tennessee; and Paducah, Kentucky, all would experience strong shaking, he said. The local area at risk for an earthquake is large. Dick Knaup, Cape Girardeau County emergency management director, said Mississippi, New Madrid and Pemiscot counties are ranked 10, the highest degree of potential damage, by the State Emergency Management Agency. Scott, Stoddard and Dunklin counties are ranked 9, and Cape Girardeau Bollinger, Perry and Butler counties are ranked 8. “Around here, it all depends on where the epicenter is going to be,” he said. “The New Madrid fault runs all the way up to the bridge at Cape Girardeau. … And then the Wabash fault hangs a hard eastward, if you’re going north, a right turn and heads up the Wabash Valley up toward Indianapolis.”

In a rank 8 area, according to the SEMA map among other damage, drivers would have trouble steering, poorly built structures would suffer severe damage, substantial building could partly collapse, tree branches would break, houses not bolted down could shift off their foundations, tall structures could twist and fall and springs and wells could see temporary or permanent changes. Knaup said he’s been told the Bill Emerson Memorial Bridge has 250 sensors and is monitored closely by Central United States Earthquake Consortium in Memphis. “If the epicenter is on the northern edge and could conceivably be inside Cape County, that recalculates everything,” Knaup said. “That puts us in a 10 zone.” Damage in a rank 10 area, according to the SEMA map, would be worse: Most masonry and frame structures and their foundations would be destroyed, some bridges would be destroyed, large landslides would occur, railroad tracks could be bent slightly; and cracks in asphalt and cement roads would open.

Knaup said a magnitude-6 or stronger quake would trigger National Guard activation, SEMA responses and Knaup would be required to call Jefferson City, Missouri, with an evaluation. “It would be hard to specifically say for Cape Girardeau what would happen there,” Williams said. There would be variability of damage based on what type of ground buildings sit. Williams said the hazard map is based on “pretty firm ground. Parts of Cape Girardeau, I know, are situated on harder rock, older rock, and other parts of the city are on softer ground, so you could expect some variability in ground motion.”

Hasheider agreed, and added different types of construction could meet different fates. “Unreinforced masonry construction that has a history back 50, 60, 75 or more years could have more damage” than buildings being constructed today, he said. Hasheider said many area buildings are built with tornadoes in mind, which also makes them a little more resistant to earthquakes. “Mobile homes or modular homes are … tied down, restricted from moving because of tornadoes, high winds from blowing them over. … Well, that same tie-down would keep them from sustaining a lot of damage from earthquakes,” Hasheider said.

“The key thing that we preach — SEMA, us, FEMA — is preparation,” Knaup said. “Even though it’s gone out of date, having your earthquake kit — that never goes out of date when you’re living in Southeast Missouri along the New Madrid fault. You always want to check it, keep it up to date. Make sure the water in it is fresh, if your food staples aren’t fresh, rotate them out and put fresh in. Eat what you had there. Things like that.” Williams also said people still should make an effort to prepare for the possibility of an earthquake.

“There’s a lot of natural disasters for people to think about” in Southeast Missouri, such as thunderstorms and Mississippi River flooding, Williams said, “so it’s a lot for people to absorb and prepare for. But preparation for getting through an earthquake is in some respects similar to other natural disasters in terms of what you need to have for food and communication with your family and things like that.” The last substantial earthquake in the area was in 2012. The epicenter of the 4.0-magnitude quake was at East Prairie, Missouri. “The hazard, over the decades, that red bull’s-eye has not changed a great deal over the past 20 years,” Williams said. “We’ve known about the New Madrid earthquakes for quite a while now; we’ve known about the occurrence of larger earthquakes in prehistoric time … so this is not a new result, really. I’d like to emphasize that.” –Semi Missourian