A U.S. Geological Survey fact sheet puts the odds of a magnitude 6 or larger earthquake in the New Madrid seismic zone at 25% to 40% over a 50-year period. The estimate appears in USGS Fact Sheet 131-02, “Earthquake Hazard in the Heart of the Homeland,” published in 2002 and written by USGS seismologists Joan Gomberg and Eugene Schweig of the agency’s Memphis office.
The same fact sheet gives a separate, lower figure for a repeat of the 1811-1812 earthquakes: 7% to 10% over the same 50 years. Both numbers are more than two decades old, but the fact sheet remains one of the clearest official statements of how the central Mississippi Valley is assessed.
Two probabilities over the same 50 years
The probabilities are laid out on the fact sheet’s fourth page. The probability of a magnitude 6.0 or larger earthquake is listed as 25 to 40 percent for a 50-year period, and the probability of a repeat of the 1811-1812 earthquakes, magnitude 7.5 to 8.0, is 7 to 10 percent. The estimates were made by the USGS and the University of Memphis Center for Earthquake Research and Information. The 50-year window is short against the region’s geologic clock, which helps explain why the odds for the larger event are far lower than the odds for a magnitude 6.
The fact sheet explains why the two sizes behave differently on the ground. Magnitude 7.5 to 8.0 earthquakes can cause widespread damage over a large region, while magnitude 6.0 earthquakes can cause serious damage near the epicenter. In other words, the higher probability attaches to the smaller but still dangerous event, and the lower probability attaches to the regional catastrophe.
Soft sediments and slow-fading waves in the Mississippi Valley
Shaking, not magnitude alone, drives damage, according to the fact sheet. Seismic waves lose energy more slowly in the central and eastern United States than in the West, so an earthquake of a given size shakes a larger area there. The thick sediments beneath the New Madrid seismic zone can amplify the motion at the surface, and the page describes ground motions that can be many times larger than those in the underlying rock. The national hazard maps do not include those amplification effects, the authors note, and USGS work was under way to produce complementary maps.
The second page adds that the national maps show a zone of high hazard in the New Madrid region, with hazard as high as in California despite lower modern earthquake activity. It also warns that the usual rule of thumb, in which frequent small and moderate earthquakes indicate the likelihood of larger ones, does not appear to apply there.
Sand blows and a 500-year clock
The odds rest on geology as much as on instruments. The third page says the New Madrid zone is far from any tectonic plate boundary, and that large earthquakes there occur more often than satellite measurements of ground deformation would predict, because the crust is strained extremely slowly, if at all. Scientists therefore dig into the record. In the central United States they examine sand blows, which form when seismic waves liquefy wet, loose sand and force it to the surface. Material buried by a sand blow dates the earthquake to after that date, and a sand blow buried by dated material places it before.
That work indicates that earthquakes similar to those of 1811-1812 have occurred twice in the past 1,200 years, between A.D. 800 and 1000 and between A.D. 1300 and 1600, roughly one every 500 years. The second page cites deposits dated to about A.D. 900 and A.D. 1450 as evidence of magnitude 7 or larger earthquakes recurring on a similar schedule, with the most recent sequence in 1811-1812.
What 1811-1812 looked like
Otto W. Nuttli’s account in the USGS overview of the 1811-1812 earthquakes describes three principal shocks, around 2 a.m. on Dec. 16, 1811, and on Jan. 23 and Feb. 7, 1812, with the February shock judged the largest from newspaper accounts of building damage. Reelfoot Lake in northwest Tennessee formed from the ground sinking during the quakes, and stumps of trees killed by the submergence are still visible. Effects of intensity V or greater covered about 2.5 million square kilometers, against about 150,000 for the 1906 San Francisco earthquake, and chimneys fell in Louisville, Kentucky, about 400 kilometers away.
The felt-area contrast ties back to the 2002 fact sheet. Readers who want the full document, including the maps and contact details for the Memphis office, can find it on the fact sheet’s main page, which lists HTML and PDF versions. Because the probabilities date from 2002 and the page shows no update, they should be read as the agency’s published estimate in that document and not as a current forecast.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
More from Morning Overview
- The NSA says three phone features should be off whenever you aren’t using them
- Hurricane Hunter radar shows four warning signs that a tropical cyclone is about to strengthen, a University of Miami study found
- Tropical Storm Rachel is dumping up to 12 inches on four Mexican states on its way to major hurricane strength
- The FTC says Lens.com doubled the price shoppers saw in Google ads
