A repeat of the 1811 New Madrid quakes would shake more than 8 million people

A repeat of the 1811-1812 New Madrid earthquakes would expose more than 8 million people to potentially damaging shaking, according to a set of computer simulations led by seismologist Leonardo Ramirez-Guzman. In the same scenarios, strong shaking in the greater Memphis area could last from 30 seconds to more than 60 seconds. Both numbers describe a hypothetical repeat of a magnitude 7 to 7.7 sequence in the central United States, and neither is a forecast of when, or whether, one will happen.

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The New Madrid seismic zone spans portions of seven states: Illinois, Indiana, Missouri, Arkansas, Kentucky, Tennessee and Mississippi. The earthquake of December 16, 1811 is estimated at magnitude 7 to 7.7, and the 1811-1812 sequence ranks among the largest in U.S. history since European settlement. The simulations ask what that same ground would do to the cities, towns and river crossings built around the zone today.

Twenty hypothetical earthquakes on two active faults

The U.S. Geological Survey described the work in a national news release dated July 30, 2015, issued alongside a paper in the Bulletin of the Seismological Society of America. The research team combined scientists from the Universidad Nacional Autónoma de México, the USGS, San Diego State University, AECOM (formerly URS Corporation) and the University of Memphis. They modeled 20 hypothetical but plausible earthquakes of magnitude 7.0 to 7.7 on two faults that are active today, moving the epicenter from run to run. Ramirez-Guzman said that if the 1811-1812 earthquakes were to repeat today, more than 8 million people living and working near the zone would experience potentially damaging shaking, at Modified Mercalli intensities of VI to VIII.

The full study, Ground-motion simulations of 1811-1812 New Madrid earthquakes, central United States, runs pages 1961 to 1988 of volume 105, issue 4, and carries the digital object identifier 10.1785/0120140330. Ten authors are listed, among them Robert Graves, Kim Olsen, Oliver Boyd and Chris Cramer. Low-frequency motion, up to 1 hertz, came from a regional three-dimensional velocity model, while higher frequencies came from a one-dimensional model, and the two were joined into broadband ground motions. The result is a modeled range of shaking, not a single predicted outcome.

Memphis and a minute of strong shaking

Ramirez-Guzman, a professor at the Universidad Nacional Autónoma de México and a former USGS contract scientist, said strong shaking in the greater Memphis area “could last from 30 seconds to more than 60 seconds,” depending on the magnitude and the epicenter of the simulated earthquake.

The spread between 30 seconds and more than a minute is itself part of the finding. With 20 scenarios, the team did not get one answer for Memphis, because the length and strength of shaking changed with where the rupture began and how large it was. The release notes that it is not possible to know which direction a fault will rupture once an earthquake starts. A figure in the San Diego State University account of the study maps peak ground motion for a magnitude 7.7 earthquake, with warmer colors marking stronger motion and the stronger motions stretching northeast and southwest of the fault.

Reelfoot rift channeling and tall structures

Two effects shape that stretched pattern. The first is rupture directivity, in which seismic energy piles up in the direction a fault is breaking. The second is channeling along the Reelfoot rift, a buried northeast-southwest valley in the zone. According to the Southern California Earthquake Center record of the paper, strike-slip events on the northeast-southwest arm of seismicity send long-period energy, in the two to ten second range, down the rift, and a western epicenter amplifies shaking toward the northeast while an eastern one amplifies it toward the southwest. Some scenarios focused strong shaking 100 to 200 miles northeast, near Paducah, Kentucky, and Evansville, Indiana, and about 150 miles southwest toward Little Rock, Arkansas.

Frequency matters for buildings. The USGS release says large earthquakes on the roughly 80-mile fault produce strong shaking at frequencies that pose risks to mid-rise and high-rise buildings and tall bridges. Reverse-fault scenarios on the Reelfoot fault showed those regional effects less strongly, with very high ground velocities mostly confined to the area near the source. The authors also concluded that rift channeling and directivity add uncertainty to magnitude estimates for 1811 and 1812 that rest only on historical reports of how hard the ground shook.

Uncertainty in the simulations and the preparation advice

Kim Olsen, a San Diego State University professor of geophysics who contributed a set of the simulations, said the possible range of shaking in a specific area is still large. His advice was blunt: “the best advice is to be prepared for the stronger shaking.” The preparation steps listed in the university account are modest ones, namely securing bookshelves and heavy furniture to walls, keeping drinking water and food within reach, and getting under a solid piece of furniture if shaking starts indoors.

The 8 million figure is a count of people exposed to potentially damaging shaking at intensities VI to VIII, not a casualty estimate, and the release gives no building-damage totals. It also comes from a 2015 study, so a count made on today’s population could differ. What the work establishes is the shape of the problem: a long fault, a buried valley that steers energy toward Paducah, Evansville and Little Rock, and a Memphis area that, in these simulations, could be shaken for as long as a minute or more.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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