Listening to the Earth at the South Pole

Science Center Objects

The darkest place on Earth may be deep within a cave, but the quietest place on Earth is deep within the Antarctic ice. If you want to listen to the softest whispers of the Earth, South Pole, Antarctica is where you want to be. Seismic station, QSPA (Quiet South Pole, Antarctica) has been allowing us to eavesdrop on the Earth for over six decades, and it may soon gain the equivalent of hearing aids.

circle with map of Antarctica at South Pole, and US map with circle on it

Map of permanent broadband seismic stations since 2010. Global Seismographic Network (GSN) and affiliate stations are shown in green stars, with labels indicating station code and parenthetical rankings (out of 150 GSN stations) used by the National Earthquake Information Center for preliminary earthquake locations during a 1-year period (14 July 2019 to 14 July 2020). The GSN station at Quiet South Pole, Antarctica (QSPA) is the only permanent broadband station within 12°, roughly the size of the combined Mountain West and Great Plains regions of the United States (inset). (from Anthony et al., 2021) (Public domain.)

QSPA is the 2nd most used Global Seismographic Network (GSN) station by the National Earthquake Information Center (NEIC) to detect and locate worldwide earthquakes. It has been the only continuously operating seismic station in the Antarctic interior since it was installed in 1957, and it is the only station on the planet at a Pole. The nearest station to QSPA is a distant 12° away on the Antarctic coast (see image). If QSPA were in Denver, Colorado, the next nearest permanent seismometer (Scott Base, Antarctica; SBA) would be in Houston, Texas. Its location fills a large gap on the planet that helps accurately locate earthquakes across the world.

Noise takes on different meanings when you talk to a seismologist, and even Antarctica has noise – both from human and environmental sources. During the field season at South Pole (November to February), the sun never sets, and tractors and other heavy equipment move around the clock to support science operations and groom the snow runway. The high-frequency vibrations from these activities became so pervasive that they were drowning out the earthquake signals, so in 1975 the seismometer vault was moved 8 kilometers (5 miles) away from Amundson-Scott station to a designated “Quiet Sector” and placed over 250 meters (800 feet) below the ice surface (and renamed from SPA to QSPA). Even so, noise from the dome was ubiquitous enough that the four daily quieter mealtimes could be seen in seismometer recordings during a study in the 2008-2009 field season.

At mid-frequencies, QSPA records unique information such as Southern Ocean wave activity as it interacts with sea ice and the Antarctic coastline. The station also records some of the movements of large ice streams in Antarctica. Therefore, seismic records from SPA/QSPA have the potential to provide information regarding how ocean wave activity, sea ice, and the movements of ice sheets have changed over the last six decades. These signals are not considered noise.

 

disc with different colors

Seismic signal recorded on QSPA as function of hour (azimuth) and day (radius) over the 2008–2009 field season. The times of the four meals in the day during the field season (including midnight rations, “Midrats”) at Amundson–Scott base are roughly plotted, as well as the start and end of the field season (red and blue circles). (from Anthony et al., 2021) (Public domain.)

High- and mid- frequency signals don’t get in the way of recording Earth whispers; low-frequency noise is the main concern, and here is where QSPA’s location at the South Pole has the advantage. Broadband seismometers, like QSPA, can record the low-frequency regular Earth tides caused by the gravitational pull on the Earth from the Moon, and the oscillations of the whole earth after a very large earthquake. These signals are the Earth whispers that provide unique and difficult-to-obtain data about the interior of the Earth. Low-frequency signals on all of the other broadband seismometers in the GSN are distorted by the rotation of the Earth, but at the South Pole, the axis of the Earth’s rotation, this doesn’t happen. This is why we call it the quietest seismic station on the planet. However, low-frequency noise caused by an outdated seismometer, and even seemingly innocuous fluctuations in the barometric pressure and magnetic field, still inhibit crucial recordings of Earth oscillations at QSPA.

Now there is a new opportunity for QSPA to eliminate more of the noise that drowns out the quiet Earth sounds. IceCube is an array of 86 boreholes drilled 2500 meters deep into the ice to house more than 5000 photodetectors; it was built between 2004 and 2010 for studying high-energy neutrino-induced events. A planned upgrade to IceCube may allow the U.S. Geological Survey (USGS) to put a low-noise, very broadband seismometer in a borehole >2 km (1.2 mi) below the Antarctica surface. This would be one of the deepest borehole seismometers in the world, far away from barometric pressure variations at the surface that are a source of noise for the QSPA site.

The South Pole presents additional unique challenges for seismic stations. Seismometers are not built for ice and extreme cold. Antarctica is covered by ice that is on average 2,160 meters (1.3 miles) thick, and the average year-round temperature is -50 degrees C (-58 degrees F), with winter temperatures as low as -70 degrees C (-94 degrees F). The USGS ASL is currently working with two manufacturers of seismic instruments, Streckheisen and Nanometrics, to develop a seismometer that will function at -50 degrees C with no heating elements required (which are another source of noise). ASL scientists are performing investigations that include how to securely install the instrument, how best to shield it from magnetic fluctuations (another source of noise), and how to communicate with it. The USGS hopes to install this new seismometer in the IceCube borehole sometime during 2023 – 2024.

So be assured, Earth, we’re listening, and with our new “hearing aids” we’re about to get better at it.

two photos of seismic instruments with two timelines above

(a) Timeline of network and station name and (b) availability and format of seismic data from South Pole, Antarctica (SPA) since 1957. (c) Charles Hutt (U.S. Geological Survey) adjusts a long-period galvanometer to reflect light onto a photosensitive recording drum during the late fall of 1969 (photo courtesy of Charles Hutt). (d) Collapse of the seismic vault in 1996 forced the primary STS-1 sensors to be relocated to the hallway in the tunnel leading to the vault. The three components shown here are covered in vacuum-sealed glass bell jars to mitigate recording pressure variations, and the vertical component is additionally covered in a mu-metal shield to reduce magnetic field fluctuations observed by the seismometer (photo courtesy of Dave Krukar). U.S. C&GS, U.S. Coast and Geodetic Survey; WWSSN, World-Wide Standardized Seismographic Network. (from Anthony et al., 2021) (Public domain.)

 

diagram with white at top, brown at bottom, and lines in an array extending from top to bottom

(a) Diagram of the IceCube neutrino detector including the area where an upgrade is scheduled for the 2023–2024 field season (red area) and where (b) a seismometer package could be emplaced as a special device. Unlike traditional borehole seismic stations, the data would need to be digitized downhole and directly interface with the IceCube Mini-Mainboard. Power routing is shown in red, digital signals in black, and analog signals in yellow. (from Anthony et al., 2021) (Public domain.)

- written by Lisa Wald, USGS, August 30, 2021

For More Information

Anthony, R. E., A. T. Ringler, M. DuVernois, K. R. Anderson, and D. C. Wilson (2021). Six Decades of Seismology at South Pole, Antarctica: Current Limitations and Future Opportunities to Facilitate New Geophysical Observations, Seismol. Res. Lett. XX, 1–18, doi: 10.1785/ 0220200448.

The Global Seismographic Network (FS-2011-3021)

The Scientist Behind the Science

person standing on rocky outcrop with water and gray clouds in the background

Robert Anthony. (Public domain.)

Robert Anthony is a geophysicist at the USGS Albuquerque Seismological Laboratory. His research is focused on understanding sources of seismic noise and developing techniques to improve seismic data quality. Rob also works with instrumentation manufacturers to test and improve seismic equipment. When not testing seismometers and looking at seismic data, Rob enjoys traveling with his wife and dog, getting to the tops of mountains, and driving/fixing old BMW cars.