14 DAY TRIAL // Scientists have for a long time wondered as to why certain mid-continental fault lines rupture even when they are not sited directly atop tectonic plate boundaries. Such is the case for example with the New Madrid fault line, which lies southwest from New Madrid, Missouri. Oftentimes, it produces what are known as intraplate earthquakes, which are basically tremors that occur within a tectonic plate, which affect the southern and midwestern United States. Experts at the Purdue University believe that they may have come up with an explanation as to why tremors occur at this location.
In 1811 and 1812, for example, the eastern parts of the US were shook by significant earthquakes, which apparently originated in the New Madrid fault line. The tremor had magnitudes ranging from 7 to 7.5 on the Richter scale, and scientists say they were the manifestation of pressure accumulations that began thousands of years ago. The new proposal from the Purdue team argues that the reason why the intraplate fault line did not slip and cause earthquakes before is that the Mississippi River had not yet eroded the areas that kept the New Madrid feature from “acting up.”
The team believes that the river began its erosion action thousands of years ago, most likely at the end of the last Ice Age, some 12 millennia ago. “We understand why earthquakes happen at the contact between tectonic plates, like in California, but it has always been a puzzle as to why earthquakes occur in the middle of the continent as well, and with no visible surface deformation. Our theory links an external climate-driven process, the melting of the ice sheet, and earthquakes,” says the lead researcher of the new study, Eric Calais. The expert is a Purdue professor of earth and atmospheric sciences.
“The only way to reconcile the fact that this part of the continent is not deforming but is producing earthquakes is for the stresses to have built up long ago. Old geologic processes, such as the opening of the Atlantic and the uplift of the Rocky Mountains, may have squeezed the Midwest. The resulting stress remained stored for millions of years until uplift associated with the Mississippi erosion event led to the unclamping of old faults lying beneath,” says Purdue associate professor of earth and atmospheric sciences Andrew Freed, who was a coauthor of the new investigation.
“Unfortunately, this stored stress is invisible to us, and the usual methods of measuring strain and deformation to evaluate a spot's potential for an earthquake may not apply to this region. Under these conditions, once an earthquake occurs on a given fault, it’s done; but this also means that other faults in the region that appear quiet today may still be triggered,” Calais adds, The work was supported with a grant awarded by the US Geological Survey (USGS). Full details were published in the latest issue of the esteemed scientific journal Nature.