An anniversary worth sleeping through – Dec. 16, 2011
(This article is a taste of the print Traveler… or Jo’s part, anyway. In addition to my other duties, since 2006, I’ve come up with a monthly article on some aspect of the geology/hydrology or landscape related southeast or south central Missouri topics called Rock Talk. This month we were so crammed with stuff it couldn’t make print. Because the topic is timely, I decided to publish it here. If you like what you see, well, I do things like this every month of the year, almost always about publicly accessible places. I’m putting it out early because the town of New Madrid and the New Madrid Museum is holding a special commemoration this weekend, with a first person theatrical production, then a talk by Dr. David Stewart, a man who has studied the quakes for the better part of his life, for a while (before its demise) at the Southeast Missouri Earthquake Center at SEMO in Cape.)
Visit New Madrid this weekend or next week. If you dare. *|:-)
It’s an anniversary worth sleeping through.
By Jo Schaper
On December 16, at just past two in the morning 200 years ago the first of the famous New Madrid quakes of 1811-1812 shook the Bootheel awake. The quake is estimated to have been between 7.2 and 8.2 on the moment magnitude scale (with the most severe shaking being 10).
The earth rolled in swells of ten to twelve feet. Water and sand spurted from the ground. Island No. 94 in the Mississippi disappeared, along with people (river pirates?) camped there overnight. Fissures appeared in flat countryside, including one deep beneath the river, causing it to appear to flow backward above the vanished settlement of Little Prairie.
Twenty-seven temblors struck between two and eight a.m., the final one being as intense as the first. Over the next three months, until mid-March, 1784 separate aftershocks were recorded, especially intense ones on January 23 and February 11, 1812. The quake in January opened Reelfoot Lake in Tennessee. Farmland in the area was ruined, and people fled.
Because the quake happened in a sparsely settled area before the invention of modern seismology, precise knowledge is difficult. Can it happen again? Is it likely to?
It all depends on the geology in a 400-mile wide area now centered on the town of New Madrid. The New Madrid Seismic Zone (NMSZ) extends from Marked Tree near Blytheville, Ark., through the Bootheel, crossing Tennessee, Kentucky and extending into the related Wabash Valley fault zone, which roughly parallels the Wabash River between Indiana and Illinois as far north as Vincennes, Indiana.
Along this southwest-northeast trending geological line (called a lineament) lies a failed rift zone. Five hundred million years ago, the earth’s crust tried to pull apart here, to form an ocean basin. It failed to separate, but left a down dropped section of crust as a scar, called a graben.
Earth’s tectonic plates move similarly to solving a 15-square in a frame puzzle. Add the complexity that the earth can buckle up squares on edge (mountain building), or dive below them (subduction.)
Unlike California earthquakes where squares slide sideways, the New Madrid’s squares move vertically at skewed angles. The NMSZ is like a slit in the middle of a piece of paper. Push the edges of the paper, and it’s the first place to buckle. Pull the paper apart, and it gaps at the slit. When the paper buckles or gaps, it folds or tears. When the same thing happens in rock, it’s called an earthquake.
The amount of force, both electrical and kinetic (motion force) released by an earthquake is tremendous, and likewise, large amounts of stress are needed to jolt the earth to move. Earlier this year, when the Army Corps of Engineers set off a series of explosions to blow the Birds Point Levee in Mississippi County, and open two drains below New Madrid, some feared the explosion might set off a sleeping giant below. Their fears were unfounded, and we reported on the reason back then.
There is a certain amount of “give” in the ground. Another way to imagine New Madrid geology is as a turkey wishbone. As two people turn and twist on the bone, a certain amount of force will cause the bone to give. More force will break it. Just exactly where it breaks depends on the forces applied. Instead of two people, tectonic forces push and pull along the NMSZ.
The New Madrid is most closely monitored by the Center for Earthquake Research and Information at Memphis State University, http://www.ceri.memphis.edu/, which lies a scant 25 miles from the Reelfoot Rift section of the lineament. If things kick on the New Madrid, they rarely need instrumentation to feel it, though the instrumentation is vital for analysis.
Although most earthquake data is now obtained digitally from disturbances in electronic detection equipment called seismometers, old habits die hard. Paper traces of earthquakes are still generated off these helicorders, the grandchildren of the old seismograph. CERI posts helicorder traces for professionals and the public online at http://www.ceri.memphis.edu/seismic/heli/index.html By comparing traces and waveforms recorded at different stations, seismologists can infer something about the actual ground motion at various locations.
In 2008 and 2009, Dr. Seth Stein and colleagues of Northwestern University teamed with Purdue University under a USGS grant. They concluded NMSZ seismicity is declining, and the Wabash Valley Fault section, which had five 5+ magnitude quakes in the 20th century, is becoming more active. Other seismologists feel the New Madrid activity of the last 10,000 years is related to something called “isostatic rebound” — the movement of the earth reshaping itself vertically as the compression of the crust, once mired down under literally miles of ice during the last Ice Age, expands. Neither explanation takes into account the deep structure of the rift valley.
People in eastern Missouri and western Tennessee aren’t so sure. Approximately 7 million people, over half of those in St. Louis and Memphis metro areas, and all of Traveler Country, now live where they would be seriously affected by a New Madrid earthquake series similar to that 200 years ago.
The New Madrid Museum will present Eliza Bryan, New Madrid Survivor, (as portrayed by Marian McDonald) and Dr. David Stewart, seismic consultant, at the Dixie Theater in New Madrid, Dec. 9 and 10, in commemoration of the quakes.
As for what happens on December 16? We hope it will be an anniversary worth sleeping through.