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July 18, 2014

USGS: Tennessee moves up on the earthquake-threat map

Shake, rattle and roll. A new earthquake map rachets up the earthquake threat for Tennessee.

The United States Geological  Survey has released a new earthquake map that raises the threats for just over half of the United States and lowers the threat for a quarter of the USA.

Called the “Seismic Hazard Map,” the update is the first update since 2008, and reflects data accumulated from the 2011 earthquake/tsunami that hit Japan and the surprise earthquake that rattled nerves in Virginia in 2011. Of course, the highest risk remains on the west coast and Alaska — the “ring of fire” that is often the target of science fiction “earth in peril” films that people love to watch and laugh at. But another potential hotspot of note is in the States of Tennessee, Arkansas, Kentucky and Missouri, the region affected by the massive quakes of 1811-12 on the New Madrid fault line.

In all, Washington, Oregon, Utah, Oklahoma, Colorado, Wyoming and Tennessee were upgraded to the top two hazard zones.

The sixteen states with the highest risk for seismic shaking are Alaska, Hawaii, California, Oregon, Washington, Nevada, Utah, Idaho, Montana, Wyoming, Missouri, Arkansas, Tennessee, Illinois, Kentucky and South Carolina.

So what about this New Madrid fault line that runs through the middle of the Mid-Mississippi valley?

USGS 1The New Madrid Seismic Zone (NMSZ) is the most active seismic area in the United States east of the Rocky Mountains. The NMSZ is located in southeastern Missouri, northeastern Arkansas, western Tennessee, western Kentucky and southern Illinois. Southwestern Indiana and northwestern Mississippi are also close enough to receive significant shaking from large earthquakes occurring in the NMSZ.

The active faults in the NMSZ are poorly understood because they are not expressed at the ground surface where they can be easily studied. The faults are hidden beneath 100- to 200-foot thick layers of soft river deposited soils called alluvium. Fault scarps and traces in the soft alluvium erode in a very short time or may be rapidly covered by new deposits, thereby quickly hiding evidence of earthquake fault lines. Faults in places like California, where rocks are at or near the ground surface, are much easier to study because the faults are readily found, seen, measured and analyzed.

Researchers have long debated just how much of a hazard New Madrid  poses. The zone stretches 150 miles, crossing parts of Arkansas, Illinois, Indiana, Kentucky, Mississippi, Missouri and Tennessee.

In 1811 and 1812, it unleashed a trio of powerful jolts, one measuring a massive 8.8  and the others measuring magnitudes 7.5 to 8.1 — that rattled the central Mississippi River valley. Chimneys fell and boats capsized. Farmland sank and turned into swamps.  The Mississippi River ran backwards. The death toll is unknown, but experts don’t believe there were mass casualties because, unlike today, the region was sparsely populated then.

The force of the land upheaval 15 miles south of New Madrid created Reelfoot Lake, drowned the inhabitants of an Indian village; turned the river against itself to flow backwards; devastated thousands of acres of virgin forest; and created two temporary waterfalls in the Mississippi. Boatmen on flatboats actually survived this experience and lived to tell the tale. Because of the geology along the New Madrid, earthquakes such as the ones that rattled the region in 1811-12 were felt as far away as Boston, MA, and the Province of Quebec. 

The world’s largest sand boil was created by the New Madrid earthquake. It is 1.4 miles long and 136 acres in extent, located in the Bootheel of Missouri, about eight miles west of Hayti, Missouri. Locals call it “The Beach.” Other, much smaller, sand boils are found throughout the area. 

Though previous analysis suggested the New Madrid was shutting down, scientists have now reversed their thinking.

“Our new results tell us that something is going on there, and therefore a repeat of the 1811-1812 sequence is possible,” said  U.S. Geological Survey seismologist Susan Hough.

The maps are key to the National Earthquake Hazards Reduction Program, which includes four agencies: USGS, the Federal Emergency Management Agency (FEMA), the National Institute of Standards and Technology, and the National Science Foundation.


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