The Global Seismographic Network in Antarctica

Rhett Butler, The IRIS Consortium
Kent Anderson and Charles R. Hutt, USGS Albuquerque Seismological Laboratory

The GSN operates four sites in Antarctica: SPA, South Pole; PMSA, Palmer Station; CASY, 
Casey; and SBA, Scott Base, McMurdo. The current SPA site is being replaced by a new station, 
QSPA, in the Quiet Sector at the South Pole at the South Pole Remote Earth Science and 
Seismological Observatory (SPRESSO). QSPA is located 8 km from the Pole to reduce the 
influence of man-made noise from the Amundsen-Scott Station. Boreholes have been drilled 300 
m into the ice to deploy KS54000 and CMG-3TB seismometers below the low-velocity firn layer 
to minimize near-surface and wind-generated noise. A shallow vault at QSPA will house both an 
STS-1 vertical and STS-2 sensors, for excellent ultra-long-period performance and system back-
up, respectively. SPRESSO is linked to Amundsen-Scott Station via electro-optical cable for 
power and real-time connectivity.

With the completion of QSPA, new opportunities for global seismology are presented by the 
available power, telemetry, and the expressly quiet conditions at the SPRESSO site, the 
development of reliable Autonomous Geophysical Observatories (AGOs), and the proposed fiber 
cable to run from South Pole to Dome C. All AGOs deployed in Antarctica should include an 
STS-2 (or equivalent) seismometer as a part of its basic complement of instrumentation for 
implementing a reference continental-scale Antarctic Array. Similar instrumentation installed at 
junction boxes along the fiber cable from the South Pole to Dome C will provide a linear array of 
over1700 km length. The dimensions of the quiet sector (a wedge from the Pole to 88° S) permit 
the installation of an array as large as LASA. The outer rings of this array would utilize AGOs 
for remote power and telemetry (and possibly the cable to Dome C which would pass through the 
quiet sector), and inner rings can be cabled directly to SPRESSO. By utilizing the technologies 
developed for AMANDA and IceCube, a three-dimensional array installed in the 2.85 km thick 
ice is the centerpiece of this large aperture array. A preliminary concept for a Cryo-Seismic 
Three-dimensional Array Lattice (CRYSTAL) includes a 3-D hexagonal prism of seven sensors 
strings (at each vertex and the center), each with 26, 3-component broadband seismometers at 
100m spacing from 250 to 2750m depth. The horizontal aperture of this prism would be twice 
the 2.5 km vertical extent. All sensors are initially emplaced in water at depth, and are 
comparable to Ocean Bottom Seismometers, except that they must work in the extreme cold of 
the ice, -50°C. 

CRYSTAL is as earthward-looking telescope (or telluscope, from tellus, latin for earth). The 
Southern Hemisphere has only two arrays, Warramunga and Alice Springs, which are both in 
Australia about 300 km apart. All other arrays lie in the Northern Hemisphere. Seismology has 
recognized that study of the structure and rotation of the Inner Core requires long-term 
observation near the Earth’s axis of rotation. Since the rotational axis may also be a symmetry 
axis for long-term outer core convection, the resolution of seismic structures within the Outer 
Core, and at the Inner-Outer Core and Core-Mantle boundaries, also requires an observation site 
at the polar axis. The microseismic noise is a fundamental limitation in body-wave seismology, 
and the increased resolution of body-waves with this noise background can only be 
accomplished with an array. A three-dimensional array offers extraordinary opportunities for 
seismology. The wave-field may be resolved in 3-D (wave surfaces, rather than wave fronts), 
separating vertically propagating body-waves from the dominantly horizontal propagation of the 
microseism noise.