A New Quiet GSN Site at the South Pole: Comparison of Seismic Data Between SPA and QSPA.

Due to increasing noise from Amundsen-Scott South Pole Station (ASSPS), a new Global Seismic Network (GSN) station, QSPA, was constructed in the 2002-2003 field season to supplant the previous GSN station, SPA, in operation since c. 1985. QSPA is the first experiment established at the South Pole Remote Earth Science and Seismological Observatory (SPRESSO). The new instruments reside in the newly-designated seismic Quiet Sector, 8 km southeast of ASSPS. SPRESSO provides a much lower noise environment, yet is close enough to the south pole to be effectively co-sited with Earth's rotational axis for long-period seismological purposes. The SPRESSO site was selected after examining tractor surface noise propagation, balanced against power and communication considerations. Numerical modeling showed that ASSPS noise should be fairly well trapped in near-surface, lower-velocity firn zone and indicated that a burial depth of 300 m would very substantially reduce noise. QSPA consists of a Geotech Instruments KS-54000 at 275 m and Guralp CMG-3Tb at 255 m depth, separated horizontally by 10 m. These depths are approximately 160 m below the local firn/ice transition. QSPA also includes a near-surface vault ( ∼5 m depth) housing Streckeisen STS-1V and STS-2 sensors. SPA has been left in operation for a period of ∼1 yr to allow for a thorough comparison with QSPA. Although 8 km from the South Pole station activities, the QSPA site borehole instruments still sense South Pole noise. However, these noise levels are significantly diminished at frequencies >1 Hz and at periods >20 s. From 1 to 15 Hz, the improved noise environment is especially dramatic. For example, background noise at 2 Hz is 15 db below SPA, ∼20-25 dB at 3 Hz, ∼30-35 dB at 4Hz, and ∼35-40 dB above 5Hz. Between 2 and 10 Hz the QSPA Guralp borehole sensor shows data intervals with noise levels below the Peterson (1993) Low Noise Model (PLNM) (with minimum noise at ∼3Hz ∼12dB below PLNM), making the site among the quietest ever measured. The improvement in the long period (T > 20 s) band is ∼5-10 dB. Most significant noise reduction is seen in the vertical channels, although the horizontal data are improved as well.