Mississippi Valley At Risk Of Big Quake - Report

By Maggie Fox, Health and Science Correspondent

WASHINGTON (Reuters) - Mississippi
River Valley cities such as St. Louis and Memphis are in grave danger of being hit by a severe earthquake, geologists said Thursday.

They found faults, similar to those responsible for the 1994 Northridge quake that badly damaged the Los Angeles area, all along the New Madrid fault region that extends through Arkansas, Missouri, Kentucky, Illinois and Tennessee.

That region was hit by three big quakes with magnitudes greater than 7.0 on the open-ended Richter scale in 1811 and 1812 -- quakes so strong they reportedly caused church bells to
ring all the way to Boston.

Writing in the journal Science, a team at the University of Colorado at Boulder and the University of Arkansas said they had found evidence that explains just why the quake threat there is so serious.

``Our evidence shows the New Madrid seismic zone is indeed a threat, which contradicts a recent study of the seismic hazard of the region taken with satellite data,'' Karl Mueller, who led
the study, said in a statement.

``For the first time we can see how fast the earthquake engine is running and how long it takes to build up energy for a quake.'

Not only would any quake be strong, but because the valley is made up of layers of silt, the shaking would turn much of the land to jelly in a process known as liquefaction.

"`New Madrid is the world's most spectacular example of liquefaction,'' Mueller said in a telephone interview. ''When you take a fine-grained, saturated sediment like mud or
silt and you shake it during an earthquake, it turns to the consistency of jello,'' Mueller said.

``If you have a building that is sitting on top of jello, the building falls down. The scary part about New Madrid is that ... we see liquefaction all over the place there.''

And people in Memphis, for example, would be caught unprepared. ``They really don't think that (quakes) are a threat because they haven't happened in a lifetime,'' Mueller said

Mueller said he thinks his team's data is reliable because they did so many different measurements of the faults.

They hired backhoe operators to dig long, deep, trenches along the Mississippi floodplain in the Reelfoot Lake region of Tennessee to expose recent deformations of the sediments.

Geologists have recently been studying ``blind-thrust'' faults that cannot be easily seen or mapped. Such a hidden thrust-fault was responsible for the 1994 Northridge quake,
which measured 6.7 on the Richter scale, killed 57 people and caused more than $40 billion in damage.

One signature of blind-thrust faults is the folding of rocks or sediments beneath the surface as they absorb the upward motion of the energy released by the faults.

They found a pattern of faults linked into a forked, lightning-bolt shape. ``If you know the shape of the fault, then you know where the seismic gun is pointed,'' Mueller said.

They also used an imaging technique that helped enhance three-dimensional topographic maps of the region, and data from other researchers using ``seismic reflection profiles.'"

"`They are like CAT scans through the upper couple kilometers (mile and a half) of earth and they give us a pretty good picture of what the shape of these big bends are, these big
wrinkles,'' Mueller said.

He said aftershocks from the 1812 quake, which continue to this day, help scientists see where the faults are. ``The aftershocks illuminate the faults and it's like shining a lot of little flashlights on these faults,'' he said.

Mueller said there is no telling just when a quake will hit, but it was likely a quake of magnitude 7.2, enough to cause serious damage to buildings, would strike within 500 years of the 1812 quake.

He said right now, about three feet (one meter) of ``elastic energy'' is stored in the fault system. ``If it popped off now, that would be a Northridge-sized event,'' he said. ``That's something in the mid-to-high 6's.''