Stanford University geophysicist Mark Zoback, has a theory explaining earthquakes that occur in the middle of the North American continent far from the usual zones of high activity along tectonic plate boundaries. Zoback’s theory is interesting because of its possible application in explaining similar earthquakes along faults in plate interiors. A comparable quake occurred Jan. 26, 2001, in Gujarat, India. The earthquake killed around 20,000 people, injured another 167,000 and destroyed nearly 400,000 homes.
In 1811 and 1812, three of the largest earthquakes ever reported in the U.S. struck the fault and temporarily changed the course of the Mississippi River. The cause of these Midwestern earthquakes has long perplexed geologists.
New Madrid has evaded explanation because it is right in the middle of the North American continental plate, far from the edges where earthquakes usually occur. The New Madrid area sits squarely on top of what geologists call a “failed rift,” where hot, molten rock underneath the Earth’s crust once threatened to rise and push apart the North American continent. For some reason, the upwelling stopped and never successfully split the plate, but the process left its signature, creating a unique tension in the New Madrid portion of the North American plate.
Twenty thousand years ago, the glacial ice sheet crept as far south as the middle of Illinois. The glacier did not extend all the way down to the New Madrid region of southeastern Missouri, but it was big enough and thick enough to strain the Earth several hundred miles to the south. Essentially, at the edge of the glacier, the Earth bent, like a mattress will slope under a body’s weight. When the climate grew warmer and the ice melted, the ground rose from the released pressure and began to straighten out. It is still rising, as the hot mantle underneath the crust is this very viscous goo. When the load is removed, it takes a long time to respond.
The rising of the Earth following glaciation is a well-documented process. Crust continues lifting in Norway, Sweden, and Canada’s Hudson Bay. If Zoback is right about New Madrid, he predicts that frequent, big earthquakes will continue in New Madrid for the next 10,000 years, long enough for the Earth to bounce back fully from the glaciers. Right now, earthquakes tend to fall every 200 to 900 years, and it has been [almost] 200 already.
© Ilex Farrell ~ Midwestern Plant Girl