All clear for heavy earthquakes in the heart of the USA

Model sees little danger for plate collisions in the New Madrid Seismic Zone

Earthquake in the New Madrid Seismic Zone © USGS
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200 years ago, the New Madrid Fault north of the city of Memphis was the site of the worst quakes in American history. Since then, there are fears that this strong plate collapse could be repeated. But a study now published in "Nature" gives the all-clear. The reconstruction of the geological evolution reveals the fault as basically active but extremely slow.

Almost two hundred years ago, in the winter of 1811-1812, near the village of New Madrid in the center of the US, three major earthquakes that had dramatic consequences for the entire center of the continent had changed the course of Mississippi and Ohio, leaving new lakes The landscape north of present-day Memphis was created and transformed sustainably. But these earthquakes had a peculiarity: their epicenter was not on an active plate boundary, but apparently in the midst of the continental crust of North America.

What is the risk of repetition?

The fault responsible for the earthquake, the so-called New Madrid Seismic Zone (NMSZ), is similar to the Upper Rhine Graben in Germany, an old fault that arose millions of years ago as the beginning Riftzone, but then did not move on. Seismologists - based on the relatively constant movement of the plates - now assume that the distances between the strong earthquakes remain at least approximately the same. However, this does not look the same for plate slabs. Here it is still unclear whether there are comparable repetition rates at all. However, some researchers believe that there could be a quake of at least magnitude 8 before 2050.

Geological structure in the underground of the NMSZ © USGS

Model reconstructs development

Researchers led by Eric Calais of Purdue University have now not only found a geological explanation for the historic New Madrid quakes, they also give the all-clear for the future of today's densely populated region. They developed a model that depicts the conditions in the subsoil as well as the sedimentary sediments and reconstructs their development over the last millennia. The goal was to find out how the tensions along the fault changed and what factors affected it.

Sediment erosion activated fault

The calculations reveal that sedimentation plays a crucial role in the activation of the fault. The melting processes and associated flash floods after the last ice age spilled large amounts of sediment from the earth's surface into the sea 16, 000 to 10, 000 years ago. This sudden erosion could have resulted in the previously inactive, blocked rejection loosening again and starting to move. display

Repetitions unlikely

The model also shows that this solution process continues to this day, but at a very low rate. The earthquake of that time had released the energy accumulated by the post-glacial processes and thus discharged the tensions in the subsoil. Reloading due to further movements of the underground has since been significantly slower and weaker than at the plate edges. In addition, the areas of the fault that have already broken in the last major earthquake would not continue to give way in the near future.

According to the researchers, it is therefore unlikely that strong earthquakes in the New Madrid Seismic Zone will recur with similar regularity as at the plate boundaries of, for example, California. However, they do not rule out that the slow movements of the New Madrid Seismic Zone could well trigger earthquakes in adjacent segments and failures.

(Nature, 29.07.2010 - NPO)