JOHN S. DERR, Albuquerque Seismological Laboratory, U.S. Geological Survey, Albuquerque, New Mexico 87115
The new Global Seismograph Network, consisting of some 150 very-broadband, digital stations worldwide, presently includes five stations in Antarctica. Those at South Pole (SPA), Palmer (PMSA), and Casey (CASY) are sponsored by the Incorporated Research Institutions for Seismology (IRIS). Those at Vanda (VNDA) and Scott Base (SBA) are sponsored by the U.S. Air Force and include provision for real-time telemetry. Except for Casey, a cooperative project with the Australian Geological Survey, all are installed and maintained by the U.S. Geological Survey, Albuquerque Seismological Laboratory (USGS/ASL) through grants from the National Science Foundation (NSF). Instrumentation, data-acquisition systems, and data availability are described in Peterson and Hutt (1989). [The Federation of Digital Seismograph Networks (FDSN), consisting of some 250 stations worldwide, includes these five stations, plus five others in Antarctica: Dumont d'Urville, Mawson, Sanae, Syowa, and Terra Nova Bay.]
Some of these sites have been recording seismic data for over 30 years as a part of the Worldwide Standard Seismograph Network (WWSSN). This instrumentation, and its several upgrades, has now been replaced by modular, state-of-the-art sensors, digitizers, data processors, and recording systems. This new technology permits remote diagnostics (where phone lines or Internet connections are available), facilitates repairs by modular replacement, and permits evolutionary upgrades as new components become available. What has not changed is the severe antarctic environment, which continues to pose challenges to instrumentation and personnel support.
All data-processing and recording units are in heated facilities at manned bases. To minimize seismic noise, seismometers are generally placed in remote vaults requiring only minimal heat or in boreholes requiring no heat. Each year, new technicians are trained in system maintenance and seismogram interpretation and supported with extensive advice by electronic mail from Albuquerque. This arrangement minimizes special trips by experienced technicians.
The Dry Valleys Seismograph Project was established in cooperation with the New Zealand Antarctic Program to record broadband, high-dynamic-range digital seismic data at a remote site far removed from the environmental noise on Ross Island. The McMurdo Dry Valleys offer some of the few locations on the continent where bedrock can be accessed directly. The sensor systems in the Wright Valley consist of a triaxial broadband borehole seismometer located at a depth of 100 meters and a vertical short-period instrument located at a depth of 30 meters. The seismic data are digitized at this location and, along with environmental data, are telemetered via a radio-frequency link to a repeater at Mount Newall and on to the Hatherton Laboratory at Scott Base. The data processor at Scott Base processes and formats the data to be recorded on magnetic tape as well as transmitting the data to a McMurdo computer system to be put on the Internet. The data are then received from the Internet at ASL to be distributed to the seismological community. To preserve data flow from this area, we have installed a backup seismometer at Scott Base (in the old WWSSN vault), so that if the telemetry link between Scott Base and Vanda fails, the local seismometer can be substituted and can use the same data-processing system and link to Internet for near-real-time data access.
Activity this past year has been a maintenance effort on the system to improve the reliability of the thermoelectric power generators at the Mount Newall repeater site and the Wright Valley site. The maintenance effort on the existing system will continue until new power equipment and shelters are installed in the 1998-1999 austral summer season.
At South Pole, ASL now operates the University of California at Los Angeles gravimeter with a modern, 24-bit data system, collocated with the very broadband seismic sensors. These sensors have been moved from their old location in a building to their current location in a tunnel, in an attempt to avoid drift of the horizontal components from building pressures. The current location, at the edge of the tunnel, is not fully successful in avoiding the microtilt of the floor caused by the pressure of the walls, so a new site will be chosen where they can be in the center of the tunnel. Ultralong-period data from these instruments show the fundamental modes of the free oscillations of the Earth, at periods of up to 53 minutes. Everywhere else on Earth, these modes are split by the Earth's rotation into multiple peaks: only at South Pole is it theoretically possible to record unsplit modes and measure the precise center frequency.
Experience with the Global Seismograph Network is leading seismologists to place greater value on near-real-time data from selected stations around the world. One of the most useful advances in this area would be to have access to a full-time, high-data-rate channel from the South Pole. Such data could be instrumental in rapid determination of focal mechanisms for large Southern Hemisphere earthquakes, and could help in identifying suspicious events for the comprehensive test ban treaty.
The major thrust in coming field seasons will be to relocate the South Pole seismic station 5 to 10 kilometers from the present habitation. Tests have shown that the primary source of noise on the current seismometer is tractors, and that it would be attenuated into the background noise at a distance of about 8 kilometers and a depth of about 200 meters. Constructing the new South Pole Station will add significantly to this noise problem, so we are planning to relocate the seismometers to a borehole in a seismically quiet zone, for maximum noise reduction.
The lack of land and the remoteness of islands in the Southern Hemisphere have meant many fewer seismograph stations, resulting in poorer coverage of earthquakes and less knowledge of the seismicity of oceanic ridges. With the planned coverage of this hemisphere shown in the figure, IRIS hopes to improve this situation. Stations in Antarctica are especially important because they provide important azimuthal coverage for earthquakes below the equator, thus significantly improving the accuracy of epicentral locations. These data are essential to the scientific community for providing accurate locations and source parameters for Southern Hemisphere events, leading to improved understanding of the tectonics of the southern lithospheric plates and the Earth's interior.
The following people contributed significantly to supporting various aspects of these stations: Rhett Butler, IRIS; Don Albert, Cold Regions Research and Engineering Laboratory; and Bob Hutt, Bob Reynolds, Gary Holcomb, ASL. Significant financial support was received from the National Science Foundation (grant EAR 95-29992), IRIS, and the Air Force Technical Applications Center.
Peterson, J., and C.R. Hutt. 1989. IRIS/USGS plans for upgrading the Global Seismograph Network . USGS Open-File Report 89-471 (revised September 1996). [Available from Albuquerque Seismological Laboratory, Building 10002, Albuquerque, New Mexico 87115, phone: (505) 846-5646, fax: (505) 846-6973, e-mail: gsnmaint@asl.cr.usgs.gov .]