Midwestern Earthquakes? --Blame Old Glaciers.

• Position: Brief Article - Statistical Data Included

The ghost of past glaciers may still rattle the American Midwest, Until 20,000 years ago, a gigantic ice sheet covered North America, weighing down the hard upper crust of the continent for millions of years. Eventually, the glaciers melted. Freed from the heavy pressure of the ice sheet, North America slowly rose. This glacial rebound continues today and triggers quakes in the New Madrid fault zone in Missouri, Kentucky, Arkansas, and Tennessee, according to Stanford (Calif.) University geophysicist Mark Zoback. "It's the only hypothesis we have that I think satisfies all the diverse observations, especially that big earthquakes in this region appear to have been turned on during the past few thousands of years."

His theory explains earthquakes that occur in the middle of the North American continent, far from the usual zones of high earthquake activity along tectonic plate boundaries, such as California's San Andreas fault. Zoback's theory is intriguing because of its possible application in explaining similar earthquakes along faults in plate interiors, including the Jan. 26, 2001, earthquake in Gujarat, India, that claimed more than 20,000 lives and left up to 1,000,000 homeless.

The New Madrid fault is famous among seismologists. In 1811 and 1812, three of the largest earthquakes ever reported in the U.S. struck the fault and changed the course of the Mississippi River. Large aftershocks continued for many years. The tremors lasted so long and were so large that many of the frontier folk felt it was the end of the world. From 1811 to 1812, membership in the Methodist church increased by 50% in the earthquake zone, compared to one percent for the rest of the nation.

The cause of Midwestern earthquakes has long perplexed geologists. An answer may help scientists better predict whether earthquakes are likely to recur in the near future. Most earthquakes occur at the edges of rigid crustal plates that float atop the Earth's more fluid interior, like crackers crowded on top of thick soup. These plates cause earthquakes when they slide under, over, into, or past one another, For example, most California earthquakes happen as the Pacific plate slides underneath the North American plate along the San Andreas fault system.

New Madrid, however, has evaded explanation because it is right in the middle of the North American continental plate, far from the edges where earthquakes usually occur. "Why earthquakes occur in the middle of plates and...