Some predict big quakes for central U.S. again

In the early hours of Dec. 16, 1811, an earthquake struck the area where Missouri, Kentucky, Arkansas and Tennessee come together today. Named for the small town of New Madrid, Mo., the quake was the first of four major quakes that were the largest to hit the United States in modern history.

The biggest occurred on Feb. 7, 1812, and resulted in huge sections of land sinking and rising. Large fissures opened up and trees snapped as the ground rolled as if it were the surface of the ocean. Hundreds of “sand volcanoes” sprayed water and sand high into the air.

The quakes were strongly felt across 50,000 square miles and moderately felt across 1 million square miles.

At magnitude 7.5, the New Madrid quakes were the strongest ever felt by non-Indians in North America.

They occurred when that part of the country was sparsely settled. If similar quakes occurred today, millions of people could die.

Scientists want to know whether these quakes were a once-and-done event or whether earth movements are building pressures that can be released only by more great quakes.

A recent paper published in the journal Science by Morgan Page and Susan Hough of the United States Geological Survey took a two-pronged approach to this question, looking at both historical records and current seismicity.

Since those big quakes occurred, low-level seismic activity has continued in the New Madrid area. Some scientists believe that these small quakes are mere aftershocks of the big events of 1811-12.

True aftershocks generally follow a pattern called Omori decay, named after the Japanese seismologist Fusakichi Omori who proposed the idea in 1894. In Omori decay, aftershock frequency decreases by roughly the reciprocal of time after the main shock.

The classic example was the Nobi quake (also sometimes called the Mino-Owari quake) of 1891, the largest to hit mainland Japan in recorded history. More than 7,000 people were killed. Following the quake, Omori decay was seen for 100 years.

Data on small quakes following the New Madrid quakes are sparse and do not fit the pattern of Omori decay, which predicts that some events equal or larger than magnitude 6 should have occurred since 1812. None has.

Furthermore, measurements of ground movement indicate an offset of 4 mm per year, which might be building stresses in the crust. Such stress might reactivate ancient faults that formed during the breakup of Rodinia 750 million years ago.

Rodinia was the supercontinent that existed beore the assembly of the most-recent supercontinent, Pangea, which began assembling around 450 million years ago and began breaking apart to form our present continents around 250 million years ago.

Both historical records and current seismic activity suggest that current quakes are not mere aftershocks. Another great quake might occur near New Madrid.  Dispatch