The size of the sand blows indicates especially powerful earthquakes in 1811-12, Tuttle says. “I have seen the geological record of these events, and I can attest that the ground was basically ripped apart.”
Further evidence for the earthquakes’ ferocity, Tuttle notes, is that sand blows can be found up and down the Mississippi River Valley from Illinois to Arkansas. Sand blows across a similarly vast area appeared during the 2001 earthquake in Bhuj, India, which killed 20,000 and was recorded at magnitude 7.7. Like New Madrid, this temblor happened mid-continent in a locale not severely shaken since 1819.
More to come?
As she works with archaeologists in probing sand blows, Tuttle finds arrowheads and even whole Indian mounds on top of some of them. Radiocarbon dating of buried leaves and twigs confirms that these blows are much older than 200 years, suggesting that New Madrid-size earthquakes have happened in this region repeatedly. In fact, Tuttle and her associates have documented that the valley rocked to monster quakes around the years 300, 900 and 1450 as well as 2350 B.C. The 1811-12 event was one in a series, which could point ominously to the future.
One of Tuttle’s colleagues, Haydar Al-Shukri at the University of Arkansas at Little Rock, had a hunch that similar, prehistoric earthquakes might have struck 50 miles south of the New Madrid seismic zone, in an area long thought to be geologically quiet. In the cotton fields of the Arkansas Delta he has recently found huge patches of gleaming white sand, cursed by farmers but invaluable as scientific evidence.
The largest of these covers 29 acres, so sprawling that Al-Shukri nicknamed it Daytona Beach.
Using a new dating technique called optically stimulated luminescence, Al-Shukri can tell how long it has been since samples of soil have been touched by sunlight. Daytona Beach formed about 3500 B.C., presumably the result of a magnitude 7.0 quake (about the strength of Haiti’s 2010 disaster) or stronger. His findings prove that such events can happen in places well outside the known seismic zones and that periods of violent activity may be followed by thousands of years of torpor.
This poses a conundrum for emergency management experts: How to plan for earthquakes if seismic activity wanders around the map?
To add to the confusion, Northwestern University geophysicist Seth Stein argues that there won’t be any future earthquakes at New Madrid. He has gathered GPS data that show no pressure developing in the Earth’s surface there; hence, he argues, no energy is building for a convulsion. He believes mid-continent seismicity moves around, so that as the New Madrid zone “turns off,” future quakes may happen elsewhere.
Others, though, say Stein puts too much weight on GPS data. Strain may be building deep inside the Earth, where GPS cannot peer. And GPS data have been gathered for only a few years. The U.S. Geological Survey continues to warn of the potential for “a major destructive earthquake” from Memphis north to southern Illinois, relying in large measure on the sand-blow evidence.
“Based on this history of past earthquakes, the USGS estimates the chance of having an earthquake similar to one of the 1811–12 sequence in the next 50 years is about 7 to 10 percent, and the chance of having a magnitude 6 or larger earthquake in 50 years is 25 to 40 percent,” a fact sheet says.
Most scientists seem to agree. “My goal is not to scare people,” Al-Shukri says. “I’m not saying it will come tomorrow. But it might come.”
Maynard has taught environmental history at Johns Hopkins and Princeton universities and written five books on American history and culture.
Loading...
Comments