A Fresh Look at Old Data Sheds Light on Central U.S. Quake Origins and Potential
Small variations in the density of the earth’s crust—undetectable to humans without sensitive instruments—influence where earthquakes may occur in the central United States. These new findings from the U.S. Geological Survey, published today in Nature Communications, may allow scientists to map where future seismicity in the center of the country is most likely.
Using a new technique that combines data about the earth’s gravitational field with sonogram-like images, scientists were able to map out variations in pressure in the deep crust, finding that these pressure variations appear to be responsible for earthquake activity in the area.
Earthquakes far from tectonic plate boundaries—such as in the central U.S.—are rare. Nevertheless, potentially damaging events can occur. Geological evidence from the Cheraw Fault in sparsely populated southeastern Colorado suggests that three earthquakes of approximately magnitude 7 occurred in the last 25,000 years. To understand whether similar events are likely to occur elsewhere in the central U.S., researchers attempted to identify the forces responsible for such unexpected earthquakes.
Scientists got a clue when they noticed that the pull of Earth’s gravitational field is surprisingly low near the Cheraw Fault, the result of buoyant rock units deep within the crust. These variations in rocks’ densities create differences in pressure in the subsurface.
“For the same reason that the wind blows or a balloon pops, pressure gradients in the earth are inherently unstable and can cause deformation resulting in earthquakes,” said Dr. Will Levandowski, a USGS postdoctoral fellow and the study’s lead author.
Levandowski and colleagues used gravity data together with seismic images of the earth’s interior to estimate the density of the crust and upper mantle across the central U.S. from the surface to 100 miles deep, and then calculated the pressures and stresses associated with this 3-D model.
Their calculations show that the majority of natural earthquakes happen in areas with unusually deep crustal pressures due to variations in rock density, and that these pressure variations appear to be the controlling factor in where natural seismicity occurs on the Great Plains.
“These density variations have existed for many millions of years, causing imperceptibly slow deformation and intermittent earthquakes,” said Levandowski.
This new technique may make it possible to map these ancient but persistent forces and to estimate long-term earthquake rates even in areas in which events are rare. Such estimates may help clarify the probability of large earthquakes near major population centers or critical infrastructure in the central and eastern U.S. where these events are uncommon.