The sunken lands are part of the New Madrid Seismic Zone -- shaky ground.

The U.S. Geological Survey warns that 655,000-population Memphis lies "near faults capable of producing major earthquakes." The Center for Earthquake Research and Information (CERI) keeps track of the tremors at the University of Memphis.

CERI director Charles Langston explains the ups and downs of what happened in this region more than 200 years ago:

"Any time that a fault breaks the surface, as happened in 1811-1812, there are places where the land will go up and other areas where the land gets lower," Langston says.

"During the 1811-12 events, several faults broke and produced areas of uplift and down drop. Because these areas occurred in the river valley, the low lands filled with water only to be drained after the [U.S.] Army Corps of Engineers constructed the ditch and levee system.

"There is a relatively high area on both sides of the river in the New Madrid/Tiptonville area called the 'Tiptonville dome' where the land has gone up about 30 feet because of repeated earthquakes on the Reelfoot thrust fault.

"Both the areas of uplift and sinking are unusual for the central and eastern [United States], since large earthquakes here are unusual. But these effects from surface faulting occur anywhere there is significant tectonic activity."

He continues, "In the west, such as in the Los Angeles area, mountains are there because of repeated earthquakes along faults at the base of the mountains. Directly in front of the mountains are deep basins that drop down a little with each earthquake and then fill up a little more with sediment afterward."

The sunken lands sometimes appear to be still sinking -- for example, causing road damage. But steady the nerves. Langston points to other forces at work.

"The fact that road repairs do not last very long probably means that the water table is close to the ground surface," he says. "Sediments and artificial fills under the roads are relatively weak and will flow or move as the road experiences the stress of traffic."

Style on 05/20/2014