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6.24.2005

Strong Earthquake Risk to Central U.S



From Live Science

A colossal earthquake that caused damage from South Carolina to Washington D.C. and temporarily reversed the course of the Mississippi River nearly two centuries ago could be repeated within the next 50 years, scientists said.

Strain is building on a fault near Memphis, Tennessee that was the site of a magnitude 8.1 earthquake in 1812, according to new observations that settle a debate on the risk of another huge quake.

The odds of another 8.0 event within 50 years are between 7 and 10 percent, geologists said today. The assessment, based on new data from a recently installed array of sensors, puts to rest a 1990s claim that strain was not increasing.

Such a strong earthquake would rock the entire eastern half of the country and prove devastating to the local region. A lesser but still damaging quake of magnitude 6 or greater has a 90 percent chance of striking in the next five decades.

The new study, detailed in the June 23 issue of the journal Nature, reveals a vexing characteristic of the fault that traverses the region. The ground moves more near the fault, creeping a few millimeters every year, than it does farther from it.

"I can't explain how the movement is driven," said study team member Michael Ellis, a geologist at the University of Memphis.

That lack of understanding makes the task of pinpointing when the next quake might hit even more challenging.

Repeating history

In a three-month period in 1811-12, three major earthquakes rattled a broad expanse of the United States, causing damage as far away as Charleston, South Carolina and even rattling nerves in Boston. The quakes triggered landslides into the Mississippi River and, according to some boaters who were not drowned, sent part of the river running the other direction for a time.

The earthquakes were centered around New Madrid, Missouri. They measured 8.1, 8.0 and 7.8 and represent three of the four strongest earthquakes ever recorded in the lower 48 states.

Over the past 12 years, geologists have found evidence for other prehistoric calamities along the New Madrid fault. Sandy soil in some areas became liquefied in past events, leaving telltale "sandblows" when the material was squished to the surface. This tendency for soil east of the Rockies to liquefy, along with other differences in geology, means earthquakes there pack more potential for damage and are felt over a much wider region than western temblors. The sandblows indicate that three or possibly four earthquakes of magnitude 7.6 or better struck the region in the past 2,000 years, in addition to the incredible series of three in the early 1800s.


An earthquake East of the rockies is felt across a wider area. SOURCE: USGS