State geologist: ancient rift gives NMSZ explosive earthquake potential

Thursday, March 16, 2023
Scott M. Ausbrooks, Arkansas State Geologist and Director and Arkansas Geological Survey, speaks Thursday on the science and history of the New Madrid Seismic Zone.
DAR/Donna Farley

There is a 25-40% possibility of a magnitude 6 or greater earthquake occurring in the New Madrid Seismic Zone within the next 50 years, said Andrea Spillars, Region 7 administrator for Federal Emergency Management Agency. She shared this prediction Thursday during the 2023 Earthquake Summit in Portageville.

“Having this event here is so important to making sure we are communicating and collaborating with the boots on the ground,” she said. “Our folks are so resilient in Missouri... but (if) a devastating earthquake were to happen in the (NMSZ), it’s going to take everyone in this room plus the whole of the government to make sure folks are okay.”

Sen. Jason Bean said the 4.0 earthquake that occurred in 2021 helped demonstrate some of the area’s needs, and he hoped events like Thursday’s summit would help his region better prepare.

“The more prepared we are, the better,” Bean said during a break. “I think that showed us even though the earthquake was very, very mild, it overloaded the 911 system in Poplar Bluff. Hopefully, this can help us be even better prepared.”

The event brought together key people not only from Southeast Missouri, he said, but from Kentucky, Arkansas, Illinois and other regions.

“We’re going to have to work with people across the region and this (helps) get to know the people involved,” he said.

Poplar Bluff Police Chief Mike McClain agreed.

“This is an effort to increase our level of preparedness for natural disasters and take advantage of the networking,” he said.

McClain was joined by members of the Poplar Bluff Housing Authority, Poplar Bluff city government and Butler County Health Department.

Earthquake science

The NMSZ is shaken by tremblors on an almost daily basis, said Scott M. Ausbrooks, who is the Arkansas state geologist and Director of the Arkansas Geological Survey within the state’s Department of Energy and Environment.

A 1.2 magnitude was recorded Thursday morning, he shared, but it was likely unfelt and far below the magnitude of a damaging quake.

The NMSZ has had about 20 damaging earthquakes since the catastrophic 1811-1812 series that was reported to severely damage communities in Southeast Missouri and made the Mississippi River run backwards for a short time.

The more catastrophic quakes have occurred on average about every 500 years, but it doesn’t mean that larger earthquakes can’t happen at any time.

While people normally think of earthquake hazards as occurring along tectonic plate boundaries, the NMSZ is an intraplate hazard. It occurs within a tectonic plate, making it more difficult to directly measure, Ausbrooks said. Only about 5% of earthquakes occur within plates.

Earthquakes occur in the NMSZ because of a very old geologic structure — the Reelfoot Rift, a crustal flaw. It is more than 600 million years old, and a place where the tectonic plate tried to pull apart, he said.

“Compression forces are reactivating the faults. It manifests itself in the earthquakes we’re seeing now along the New Madrid zone,” Ausbrooks said.

In places like California, the movement and strain caused in plate boundaries can be monitored, he continued.

“Intraplate (strain) is a new set of problems because it’s hard to directly measure how much strain... and how much energy is being transmitted into this zone. We don’t know,” Ausbrooks said, explaining all of this is occurring far below the surface.

One thing he hoped each attendee would take away from Thursday’s event was a better understanding of the magnitude of a potential NMSZ quake.

“A similar magnitude earthquake here is going to have a much greater impact than a similar magnitude earthquake in California,” Ausbrooks shared.

The soil types and age of the rocks in the NMSZ are part of what cause that, he shared.

While a 5.4 magnitude in California is not felt outside the state, the same quake in the NMSZ is felt across the Midwest, Ausbrooks said. The 5.4 magnitude in California in 2007 was limited to a felt radius of about 200 kilometers, while the 2008 quake of the same magnitude in Mt. Carmel, Illinois, was felt well beyond a radius of 400 kilometers.

“Our rocks are much older (than in California). They’re hard, they’re cold, they’re dense. They’re very efficient at transmitting seismic energy,” Ausbrooks said.

Each step up in magnitude means 32 times more energy is being released. A magnitude 5 quake releases 900 times more energy than a magnitude 3. A magnitude 7 would release 1 million times more energy than a magnitude 3.

“It takes a tremendous amount of small earthquakes to release the same energy as a major earthquake,” Ausbrooks said.

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