On the surface, Antarctica’s vast ice sheet appears still and unchanging. But deep below, vibrations ripple through the frozen plain, transmitting the movements of Earth’s tectonic plates—and scientists now have a formidable new set of tools to listen in with. The U.S. Geological Survey (USGS), collaborating with the IceCube Neutrino Observatory at the South Pole, has installed the deepest seismometers ever deployed. At 8,000 feet under the ice, the two instruments will record earthquakes of magnitude 5 or greater anywhere on the planet with unprecedented accuracy and help to reveal new details of Earth’s deep interior in the process.
The South Pole is one of Earth’s quietest places because there is very little humanmade infrastructure and no background “noise” from the planet’s rotation, which can distort seismometer data. At their depth, the new seismometers may also be shielded from disruptive changes in atmospheric pressure, says USGS research geophysicist Robert Anthony, the Deep Ice Seismometer project manager.
Engineers “drilled” holes by shooting pressurized hot water into the ice to melt it. “The drill is producing as much energy as the most powerful steam locomotive ever made and channeling it all through an orifice the size of a penny,” Anthony says. At a rate of roughly three feet per minute, the drill can complete a hole in about 50 hours, after which the team has another 50 hours to lower strings of instrumentation before the ice refreezes. To contend with the pressure at a depth of 8,000 feet, the seismometers are encased in a stainless steel vessel built to withstand 10,000 pounds per square inch.
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Each seismometer contains a small pendulum suspended within a magnetic field; when a vibration reaches the sensor, a resistor measures the magnetic-field strength required to keep the pendulum stationary. “This lets us record lower-frequency ground motions all the way down to the solid Earth tides driven by the gravitational attraction of the sun, Earth and moon,” Anthony says. Scientists can use these data to quickly characterize how a fault moved to generate an earthquake, which can help them determine whether it also triggered a tsunami. The South Pole station fills a significant gap in global seismic coverage, researchers say, because of its distance from other stations and the lack of interference from Earth’s rotation.
“Earthquake waves don’t just shake the surface; they go in all directions, including down,” says David Wilson, director of the Global Seismographic Network. The deep seismometers will be particularly good at recording long-period seismic waves created by large earthquakes (about magnitude 7 or greater). “These waves can go on for months after a large earthquake because there’s nowhere for the energy to go to dissipate,” Wilson says. “Imagine hitting a bell. It’s going to sit there and ring until the energy completely dies down.”