Mysterious earthquakes

2001/03/27 Carton Virto, Eider - Elhuyar Zientzia

• Geologia

In 1812 and 1813 lived the three most violent earthquakes in the history of the United States in the city of New Madrid of Missouri. Although the consensus among geologists is not total, it seems that the three exceeded the magnitude of 8.0 on the Richter scale and modified the course of the Mississippi River. Since then, thousands of earthquakes have occurred in the area of New Madrid, but they have never been able to be explained in detail.

The researcher at the University of Standford, Mark Zoback, has been looking for an explanation of what happens in New Madrid for years and has just launched a theory related to the ancient glaciers. And it is that, in his opinion, the fault system of New Madrid cannot explain so much earthquake alone. Zoback takes into account two factors to explain the phenomenon of New Madrid. On the one hand, the geological characteristics of the area and, on the other, the pressure of the glaciers that formerly covered these regions.

As for its geological characteristics, Zoback emphasizes that the New Madrid is located on an area that did not become a rift. The rift are long and narrow depths originated in the continental crust and frequent seismic and volcanic episodes along the depression. The evolution of the Rift can lead to the breakdown of the continent, as in the case of Rift Valley, East Africa. The New Madrid rift did not reach the surface and did not break the continent twice, but left there a very suitable tension point for earthquakes. However, for Zoback, the existence of a tension point is not enough to cause earthquakes, but the process is led by another agent: glaciers.

Ancient glaciers

North America was covered by a huge ice sheet until 20,000 years ago and its weight was so large that the continent descended, it sank in the mantle. After the end of the glaciation and the decline of the ice, the continent rose again and remains. The phenomenon is known to geologists, as the same occurs in Norway, Sweden and the Canadian Hudson Bay. The mantle is very viscous and takes a lot to respond to a change.

Zoback considers that this upward movement of the terrestrial surface can be the factor that causes earthquakes in New Madrid. The upward movement plus tension in the area are the cause of such violent earthquakes. Zoback has built a model that represents the process and considers that it can be useful to explain the earthquake that occurred last January in India. Therefore, he has shown his intention to investigate the glaciers of India, since if the model also served, we would be faced with a solid theory.

The right part of the image was the area of influence of the earthquake in 1994 in California. To the left, what happened in 1895 in New Madrid. Both had the same strength.

The search for an adequate explanation for the earthquake that occurs in the center of continental plates can be of great help, among other things because it affects a much wider area than the earthquakes that occur in the limits of the plates. Due to the geology of the center of the plates, seismic waves can extend very far. For example, those of Nueva Madrid are noticed up to a thousand kilometers to the East.