It's hard to find anything positive in the wake of the devastation wrought by Hurricane Katrina on the once-romantic and glorious city of New Orleans. But for what it's worth, a lot of geoscientists were expecting something far, far worse to happen in the immediate future, destroying New Orleans and other cities in the area more completely, with less warning and with casualties in the millions throughout the Mississippi valley.
In the winter of 1811-1812 a fault line located in the Mississippi valley released a series of four powerful earthquakes - the strongest estimated at over 8.0 on the Richter Scale - which were reported in the small town of New Madrid in what is now Missouri. At that time very few people lived in the Mississippi valley, but there were multiple reports of giant tidal waves in the river, of the river reversing course in the upper Mississippi and its tributaries, of massive landslides along the banks, of entire islands disappearing, houses being swallowed whole in the town of New Madrid, entire lakes being drained and 150,000 acres of forrest being swallowed whole. The last and strongest of the earthquakes was felt as much as 1000 miles away where it rang church bells in Boston. Because so few people lived in the region the loss of life and property was minimal - and very hard to estimate, because those impacted likely disappeared in the flood waters or were swallowed whole, with no one really aware that they had ever been there. A much weaker magnitude 6.8 earthquake in 1895 caused substantial physical damage in New Madrid and surrounding towns, and again was felt up to 1000 miles away.
The New Madrid fault line is still there and still active. There are several small tremors registered there every week. Because it is an intraplatal fault burried under layers of sediment its behavior is unpredictable and the impact of earthquakes there is very different from what we are used to seeing in California. A quake of magnitude 6.0 in California is significant, but a quake of the same strength on the New Madrid fault line would be substantially more damaging. A full 8.0 earthquake like the 1812 event would produce almost unimaginable destruction all along the river.
Today there are large cities and millions of people along the Mississippi. A major New Madrid earthquake would cause damage in at least 20 states, with a massive tidal wave in the river, immediately destroy enormous amounts of river traffic and all the bridges. It would shut down power throughout the region, wipe out every small riverbank town, destroy every levee protecting New Orleans, virtually wiping the city off the map, it would likely cause a large portion of Memphis to be destroyed in massive landslides, and there would be major building damage and loss of life in St. Louis as well as short-term flooding. Because of the large cities now in the region, the likely result of such an earthquake would probably be millions of lives lost and hundreds of billions of dollars in damage.