A day of disaster in Mid-South

New studies focus on worst-case scenarios for 11 million at quake risk

By Tom Charlier
Commercial Appeal
December 11, 2005

THE BIG QUAKE How ready are we?

Today: A magnitude 7.7 would kill, injure thousands
Monday: Our children are at risk in vulnerable schools
Tuesday: Too little EMA clout could hurt preparedness

Miles beneath the deceptively serene patchwork of rice fields and farm towns covering East Arkansas, the grinding of rocks that has been building up stress along a fracture in the Earth's crust finally brings forth a momentous, shattering rupture.

From Marked Tree, Ark., to near Caruthersville, Mo., a fault peels open as if it were a 75-mile-long zipper. The half-mile-thick sediments underlying the region start shaking like a Jell-O salad.

Some 35 miles to the south, in Shelby County, the Earth is at war with itself, heaving back and forth so violently it's almost impossible to stand. All across the county, the shaking produces G-forces much greater than those felt by airline passengers flying through severe turbulence.

The convulsion lasts perhaps three minutes and makes a shambles of a region that's home to 10.9 million people and stretches from central Missouri to northwest Alabama, from southern Indiana to south-central Arkansas.

As many as 4,300 people are dead and at least another 65,000 injured. More than 179,000 homes and 500 bridges are destroyed. Nearly 1 million households are without water service.

The debris choking the landscape could fill 1.24 million trucks. Economic losses exceed $77 billion.

That, according to a pair of new studies, is a sketch of what geologists and emergency-planning officials now consider the most likely "Big One" scenario for Memphis and the Mid-South: a magnitude-7.7 earthquake along the southwestern edge of the New Madrid seismic zone.

The studies indicate the Memphis area would be subjected to greater ground motion than those in earlier projections. And they underscore the regional scale of the expected devastation.

"There are 11 million people at risk in the Central U.S.," said Jim Wilkinson, executive director of the Central United States Earthquake Consortium, a Memphis-based information agency.

"That's my biggest fear about New Madrid. It's not going to be a community that gets hit, it's going to be the whole region."

A 7.7 quake occurs every 200 to 1,100 or so years in the New Madrid zone, with an average interval of 400-500 years. There's a 7-10 percent chance of it happening within any given 50-year period, according to the most recent projections.

"That's pretty high," said Joan Gomberg, a geophysicist with the U.S. Geological Survey. "If you thought you had that much of a chance of getting a serious disease, you'd be concerned."

The studies -- involving a hazard-mapping initiative by the USGS and a computer-modeled "earthquake event" by the Federal Emergency Management Agency -- are based on some of the most detailed soil and building inventory data gathered.

They coincide with the heightened attention focused on disaster-planning nationwide in the aftermath of Hurricane Katrina.

In the Memphis area, where earthquakes are considered the gravest threat, officials this year have scrambled to get federal approval for a plan showing how they'll try to minimize damage and casualties from a disaster.

Local officials also have slightly stiffened local building standards to deal with earthquake hazards. Slated to take effect Jan. 1, they'll require "essential facilities," such as fire and police stations, to be constructed to the updated seismic specifications contained in the 2003 International Building Code. Other structures will remain subject to seismic standards in a 1999 code.

With the new studies, officials say, planners will be better able to prepare for large quakes and the critical recovery period.

"Science and emergency managers need to come together and look at worst-case scenarios, even if there are a lot of uncertainties," said Gary Patterson, director of information services with the Center for Earthquake Research and Information at the University of Memphis.

The New Madrid zone, a network of three main fault segments zigzagging southward from near Cairo, Ill., to Marked Tree, is the most active seismic region east of the Rockies.

Though covered by sediments that have been eroded, plowed and built upon, the faults remain partially visible at the surface as "uplifts," ridges that were pushed up several feet on opposite sides of the fault.

Between Dec. 16, 1811, and Feb. 7, 1812, the New Madrid zone produced three of the most powerful quakes known to have hit the continental United States. They created Reelfoot Lake, caused the Mississippi River to flow backward and rang church bells on the East Coast. Casualties were relatively light because the area was largely wilderness.

A magnitude of 7.7 was selected for the hazard scenarios because it's in the middle of the range of estimates of the magnitude of the 1811-12 quakes. The southwestern fault segment -- extending to near Marked Tree -- is the one closest to Memphis.

One of the problems in forecasting that size of a temblor, researchers say, is that in the past they've tended to occur in bunches. As a result of a process called "stress transfer," a huge quake on one fault segment can help trigger a temblor on another, said Arch Johnston, director of the UofM earthquake center.

Researchers also have been hampered by the sketchy record of quakes in the New Madrid region. Compared to, say, the San Andreas in California, the New Madrid faults produce sizeable quakes very infrequently.

And unlike most other seismic regions, New Madrid isn't along a continental plate boundary, where quake forces are better understood.

"As to why we have the earthquakes, we really don't have the answer," said Eugene Schweig, geologist and regional coordinator with the USGS earthquake hazards program.

The USGS hazard maps show that a 7.7 quake on the Marked Tree fault segment would generate an even greater amount of shaking in the Memphis area than shown in earlier studies.

The seismic motion would be greatest in northwestern parts of Shelby County, which is nearest to the fault. There, the peak ground acceleration -- a key measurement of movement -- could reach 0.55 unit of gravity, or more than half the "G" force felt by a free-falling sky-diver before the parachute opens.

Throughout the Memphis area, even in southeastern parts of the city, the peak ground acceleration would be at least 0.3 unit of gravity. That's about the force with which a sports car driver is pushed back in a seat after mashing the accelerator following a dead stop. Passengers in a jetliner in heavy turbulence experience about 0.2 G.

"People will have trouble standing," said Chris H. Cramer, another geophysicist in the USGS earthquake hazards program.

Older masonry buildings in the city "would be in a world of hurt," he added.

Cramer said the shaking will begin after an initial sound wave -- a locomotive-like rumble -- reaches Memphis at a speed of about two miles per second. The ground probably will rock back and forth, followed by waves -- like those in the ocean -- rolling across the surface.

"Earthquakes don't always do the same thing," Cramer cautioned.

Based on the Modified Mercalli Intensity Scale, which is used to depict the severity of shaking, the effects of the peak ground accelerations in Memphis would be "violent."

"Damage considerable in specially designed structures and great in substantial buildings, with partial collapse," the scale index states. Well-designed frame structures would be thrown "out of plumb" and buildings would be shaken loose from their foundations.

The USGS maps don't focus on liquefaction, the process by which intense shaking turns soil into virtual quicksand. Previous studies have found liquefaction would be a problem mainly in floodplains.

CUSEC director Wilkinson said the scenario studies, however ominous they might be, will be useful in disaster planning.

"The challenge is to take the visions and fold them into a mitigation plan for the city and county," he said.

-- Tom Charlier: 529-2572

Worst-case scenario

Two new studies offer these estimates of the consequences in the Mid-South and Midwest of a magnitude 7.7 New Madrid earthquake. Casualty estimates vary depending on which time of day a quake occured. They're lightest for an event at 2 a.m., when most people would be asleep in their homes, which tend to be smaller and more resilient than multistory buildings.

Deaths: Between 3,212 and 4,289.

Injuries: 65,840 to 71,951.

Homes: More than 200,000 destroyed or extensively damaged.

Hospitals: More than one-third of all beds unavailable for use the day after the quake.

Schools: 19% sustain at least moderate damage.

Police stations: 23% sustain at least moderate damage.

Damage to buildings and contents: $51.7 billion.

Loss of wages and income: $6 billion.

Damage to transportation facilities: $7.3 billion.

Damage to utilities: $12 billion.

Quake sitemap