USA: Can rivers cause earthquakes?
By Charlie Shobe
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Most large earthquakes occur at the boundaries of tectonic plates—the slowly moving, continent-sized slabs of rock that make up Earth’s crust—in places like Nepal and California. But many destructive earthquakes have happened in the eastern United States, far from any plate boundary. What triggers these so-called intraplate earthquakes is something of a mystery. New research by geologists at Colorado State University and the University of Kentucky suggests a surprising culprit: rivers. By eroding trillions of tons of rock from Earth’s surface over millions of years, rivers might remove enough weight to change the balance of forces in Earth’s crust and cause intraplate earthquakes.
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Intraplate earthquakes require three ingredients. First, there must be faults, or weaknesses in the Earth’s crust, far from a plate boundary. Second, stresses must build up at those faults. Finally, an increase in stress above what the crust can sustain triggers an earthquake.
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But what about the third ingredient? What triggers intraplate earthquakes? If they occurred simply from continuous stress buildup along faults, they would occur all up and down the Eastern Seaboard. Yet East Coast earthquakes are concentrated into several “seismic zones,” areas with frequent quakes. The earthquake I felt in Virginia, for example, hit smack in the middle of the Central Virginia Seismic Zone. Something must be triggering earthquakes specifically within observed seismic zones and not elsewhere. The trigger, at least in one seismic zone in eastern Tennessee, might be a river.
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A new study by [Sean Gallen, a Colorado State University geologist] and geologist Ryan Thigpen at the University of Kentucky suggests that rapidly removing so much weight from the crust causes earthquakes. Like squeezing a tube of toothpaste while unscrewing the cap, removing the confining pressure of overlying rock may allow parts of the crust to move quickly, resulting in large earthquakes far from plate boundaries. The researchers use a computer model to calculate how stress in the crust would respond to the removal of 500 vertical feet of rock. Their results predict a pattern of stress change that corresponds with the earthquakes observed in [a seismic zone in eastern Tennessee] between 1901 and 2016.
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