And here's something that no one thinks about. Quakes in the middle of a plate.
http://www.cnn.com/2006/TECH/science/04/18/new.madrid/index.html?section=cnn_latest
Scientists: Quake could devastate America's heartland
New Madrid a mysterious and potentially dangerous fault zone
By Marsha Walton
CNN
MEMPHIS, Tennessee (CNN) -- Scientists and residents alike are focusing on the devastating San Francisco earthquake 100 years ago. But some researchers also want to bring public attention to seismic events in a region where strong earthquakes just don't make a lot of scientific sense.
During the winter of 1811-1812, at least three powerful earthquakes (believed to be magnitude 8 or above) and thousands of aftershocks were felt in America's heartland, in what's known as the New Madrid Seismic Zone. The region impacts parts of eight states, Illinois, Indiana, Kentucky, Tennessee, Alabama, Missouri, Arkansas and Mississippi.
"They were felt into Canada, up in the Montreal region and that would be almost 1,200 miles away," said Arch Johnston, director of the Earthquake Research and Information Center at the University of Memphis.
Fortunately, the area was sparsely populated at the time. There were only about 400 residents in the frontier town of New Madrid, Missouri. They were awakened in the middle of the night when the first big quake hit December 16, 1811. (Watch what could happen if a quake hit the U.S. heartland -- 2:45)
"The comparable size 1906 San Francisco earthquake was barely felt outside the state of California. That's not because the New Madrid quakes were so much larger than the San Francisco quake. But our crustal rock transmits seismic waves a lot more efficiently," Johnston said.
That seismic efficiency could prove disastrous with today's population.
"We're looking at about 11 million people at risk, of that 11 million about 2.5 million respectively from Memphis and St. Louis," said Jim Wilkinson, executive director of the Central United States Earthquake Consortium, based in Memphis.
Scientists have a pretty good understanding of more than 90 percent of earthquake activity, such as the active area known as the "Ring of Fire" that runs along the west coasts of South and North America, up and across the Pacific Ocean to Asia.
"Most earthquakes occur along what we call plate boundaries," said Joan Gomberg, research seismologist with the U.S. Geological Survey in Memphis.
"The Earth's surface down to about 100 kilometers (62 miles) is broken up into these big pieces, like a big jigsaw puzzle. Those pieces move around, at the edges of those pieces, where they are moving in different directions, is where we have earthquakes, and volcanoes, and where most of the action is," Gomberg said.
She said it is challenging work studying a region that does not fit that pattern.
"Here in the New Madrid seismic zone, we are in the middle of a plate. And it's really a big mystery as to why we have big earthquakes here," Gomberg said.
One mystery has been solved. The events of 1811-1812 were not a fluke. Geologic records show that similar strong earthquakes took place in the same region about 900 AD, and about 1450 AD, roughly at 500 year intervals.
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