The crew of the steamboat New Orleans awoke the morning of Dec. 16, 1811 to a surprising sight - or rather a lack of one.

While on the boat's maiden voyage, the crew had moored the boat on an island in the Mississippi River the previous night. When they awoke the next morning, the island had sunk beneath the river's surface.

Miles away, the 400 residents in the Missouri city of New Madrid were abruptly awakened by violent shaking and a tremendous roar. Survivors reported cracks in the earth's surface, seeing the ground roll in visible waves and witnessing large areas of land sink or rise.

Damage was reported as far away as Charleston, S.C., and Washington, D.C. Vibrations were strong enough to ring church bells in Boston. It was the first of three major earthquakes and thousands of aftershocks to hit the central United States that winter, according to the U.S. Geological Survey.

No one can verify the exact intensity of the earthquakes because of the technological measuring seismic activity back then. But most experts have the magnitudes fluctuating between 7.5 and 8.0, making it the strongest earthquake in the recorded history of the contiguous United States.

Looking back at the 9.2 magnitude quake and following tsunami that claimed more than 100 lives in Anchorage and other parts of Alaska in 1964 and the 6.9 quake that interrupted the 1989 World Series and killed nearly 70 people can provide more recent perspective. The issue became very real again for people last month when Southern Illinoisans, along with people as far away as Chicago and Atlanta, felt the shock of a 5.2 magnitude quake.

While most experts agree there's no reason to doubt a repeat of the high-magnitude quake in the central United States, scientists, engineers and emergency personnel can only wonder what will happen when it strikes.

The region has become a different place in the nearly 200 years since the disaster hit an area anchored by St. Louis, which had a population around 5,000 at the time. Not only were people scarce to the central United States, but so was infrastructure. Nuclear power plants, interstates and skyscrapers weren't even thoughts on peoples' minds.

A modern repeat of the historic quake along either the New Madrid or Wabash Valley faults would take a toll on the entire country, not just the region near its epicenter, said Robert Bauer, engineering geologist for the Illinois State Geological Survey at the University of Illinois at Urbana-Champaign.

Strong seismic activity could knock out automotive and rail bridges across the Mississippi and Ohio rivers, damage interstates and limit travel and cause pipelines crossing state lines to burst.

While it's difficult to imagine the aftermath of a large earthquake, Southern Illinois University Carbondale geology professor Harvey Henson believes it's important to recognize the reality of the possibility.

The U.S. Geological Survey has determined there to be a 7 to 10 percent chance of a repeat of the 19th century quake in the next 50 years, and a 25 to 40 percent chance of a 6.0 or higher magnitude occurring on the fault in the same time frame.

"The possibilities are there, and each day that goes by just gets us that much closer to it happening," said James Wilkinson, executive director of the Central United States Earthquake Consortium based in Memphis. "There's no reason to expect they're not going to happen again."

Experts stress the fact it's impossible to ever predict when or where an earthquake will strike, but geological differences between seismic zones in central Illinois and the west coast make them even more unpredictable for those in this area, Bauer said.

Two tectonic plates, the building material of Earth's surface, exist under west coast locations like California. These two plates, the Pacific and North American, slide past each other at a known rate of one inch per year, he said. This movement created the San Andreas Fault.

In comparison, the New Madrid and Wabash Valley faults in the central part of the country were formed when one plate cracked but never fully separated. Energy becomes stored in different ways under each fault system, and when this energy releases, the vibration felt on the surface becomes an earthquake.

"In California with the San Andreas Fault, you could look at all the little earthquake epicenters and see areas where there is no seismic activity in a long period," Bauer said, adding this helps identify pods of stored energy that, when released, can become seismic activity. "We don't have that luxury in the central United States."

Last month's early morning 5.2 magnitude quake, epicentered near West Salem, in the Wabash Valley Seismic Zone served as a wake-up call for people in the region, many experts said.

"These small earthquakes are an opportunity to remind Southern Illinoisans that we are at risk," Henson said. "We don't need to be afraid, but we need to be aware."

While experts say earthquakes with magnitudes in the 4's and 5's are not uncommon for the two local seismic zones, Jale Tezcan, an assistant professor of civil and environmental engineering at SIUC, explained how magnitude scales are based on exponential numbers, meaning what may seem like a slight variation in magnitude number may actually be a tremendous difference.

As an example, she said an earthquake with a magnitude of 8.0 releases 30 times more energy than one with a 7.0 magnitude and 27,000 times more energy than a 5.0.

Local, state and federal officials living within the New Madrid Seismic Zone will be learning about the threat of earthquakes and how they can prepare at a conference in Metropolis Tuesday and Wednesday.

The New Madrid Fault Region Earthquake Preparedness Conference will feature speakers like Bauer and Wilkinson discussing issues of preparation, reaction and action plans in the event of a major earthquake, said James Becker, community economic development specialist for the U of I Extension office in Mount Vernon. The two-day conference will help bring together people from the central United States and help them prepare for a disastrous earthquake.

"There's always room for improvement, and we need to stay on top of that," he said.

adam.testa@thesouthern.com / 351-5031

What to do

Jale Tezcan, assistant professor of civil and environmental engineering at Southern Illinois University Carbondale, offers these tips for residents to remember during an earthquake:

• Stay inside; the outside can be more dangerous because of falling debris and other harmful materials

• Try to stay under a heavy desk and avoid other furniture than can be blown or moved naturally

• Don't believe anyone who tells you when and where an earthquake will occur