The Science Behind Mysterious, Giant, Midwest Earthquakes

Kristin Osika

About two centuries ago, a number of giant earthquakes shook the town of New Madrid, Missouri, destroying houses and collapsing buildings more than 1000 kilometers away. Seismologists today have estimated this series of earthquakes to be registered above a magnitude 7.0—which matches some of the strongest earthquakes—on the Richter Scale. This number is determined from the amplitude of waves recorded by seismographs. Even today, the New Madrid Seismic Zone (NMSZ) generates over 200 tremors, or small earthquakes, annually. With that said, geologists have been unable to gather the most accurate information, due to the lack of recent giant earthquakes.

 

The NMSZ has been and will continue to be difficult for scientists to study, as there is no obvious mechanism that drives the seismic activity at this earthquake hotspot. The NMSZ is located toward the center of the North American tectonic plate, which logically should produce little to no seismic activity. But even though the largest earthquakes tend to occur near the earth’s active faults, where tectonic plates can collide, slide, and pull away from each other, the NMSZ still consistently produces earthquakes. New studies have shown that blobs of dense rocks from the lower crust of the earth at the NMSZ rise up underneath the ground. This causes the dense rock to gravitationally tug on everything in its vicinity. This process changes the stress of the underground region, which in turn could cause the faults at the NMSZ to slip, thereby creating small earthquakes.

 

Even with the tomographic evidence that shows the dense rocks rupturing, a fascinating question arises—why do these dense rocks rise up to cause this disturbance in the earth’s lower crust? Geologists offer that hundreds of millions of years ago, the current North American Plate attempted to split and pull apart to form two plates, but the process failed, leaving repercussions below the NMSZ: a buried rift zone, with weak rocks above plutons, under an unusually dense lower crust. The plutons then rise because the lower crust is unable to stop the rocks from moving around.

 

The research has shown direct correlation between dense rocks rising up and the initiation of earthquakes. Although there could be other components that could indirectly affect the seismic activity at NMSZ, the main source comes from movement in the earth’s lower crust.

 

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This figure shows the buried rift zone along with the plutons, and the stress caused by gravity.

 

Why does this study matter for science?

 

Earthquakes are a huge problem to everyone in the world. Although many people understand they live in a seismic zone, some people do not even realize that an earthquake could occur in their area. This study proves to be a big step towards determining where and when earthquakes can occur. Geologists could study the lower crust and determine if an area is an earthquake hotspot, and check if the dense rocks located in the lower crust could cause regional stress. Will Levandowski, a geophysicist from Colorado notes, “We’re interested in creating an honest-to-goodness map of where we think earthquakes will occur in the future.” He believes that with this research, geologists will soon be able to locate other vulnerable areas, so the people will be prepared for and educated about earthquakes. Therefore, the repercussions of the earthquakes will not be as severe. With this new study, people are able to take action prior to an earthquake, instead of acting after the damage has been done.

 

Jonathan Chen ’19

 

References:

https://earthquake.usgs.gov/learn/topics/measure.php

http://www.sciencemag.org/news/2016/09/scientists-may-have-solved-mystery-giant-midwest-earthquakes

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