Broadband Seismology in Antarctica: New Insights on Mountain Building and Glacial Processes
Douglas A. Wiens
Professor, Dept. of Earth and Planetary Sciences
Washington University, St. Louis, MO
When: Friday, January 12, 2007, 10:30 AM
Where: J.J. Pickle Research Campus, Bldg. 196, Rm 1.603
Host: Jay Pulliam, UTIG
Until the last five years, only a few broadband seismographs had been deployed in Antarctica, and these were located seismically noisy bases. The development of autonomous technology capable of operating at cold temperatures has recently opened the possibility of routinely deploying seismographs throughout Antarctica. The deployment of 44 broadband seismographs during the 2001-2003 TAMSEIS experiment demonstrates the potential of seismology in imaging the structure of the Antarctic continent. The seismographs were deployed from the Ross Sea to the East Antarctic Plateau in order to investigate the lithospheric structure beneath the Trans-Antarctic Mountains (TAM) and East Antarctica (EA).
Combined receiver function and Rayleigh wave phase velocity inversion, as well as P and S wave tomography constrain models for the development of the TAM. The crustal thickness increases rapidly from 20 ± 2 km in the Ross Sea to 40 ± 2 km beneath the TAM, achieving the maximum thickness immediately beneath the TAM crest. Farther inland, the crust of EA is uniformly 35 ± 3 km thick over a lateral distance greater than 1300 km. The phase velocities and body wave tomography indicate high velocity cratonic upper mantle beneath EA, low velocity beneath WA, and a transition beneath the TAM crest. These results are in agreement with models suggesting that warm buoyant Ross Sea upper mantle extends beneath the edge of the TAM, inducing flexural uplift of the mountains. However, a small crustal root exists, suggesting partial Airy support of the mountain range and the possibility that crustal thickening existed prior to rangefront extension and rifting. Both Rayleigh wave phase velocities and SKS splitting results show a large uniform region of azimuthal anisotropy within the uppermost mantle of EA, with fast axes oriented about N55E. The shallow depth of the anisotropy indicated by the surface waves suggest it results from remnant upper mantle lattice preferred orientation from past deformational episodes, rather than the current upper mantle flow pattern. The mapping of upper mantle anisotropic directions offers a possible method for delineating geologic terrains in ice covered East Antarctica.
We also discovered many 25-150 s Rayleigh wave packets propagating across the array from a source 1200 km away near Whillans Ice Stream, where stick-slip behavior has been previously identified. Comparison of the seismic phases with GPS records of Whillans slip from the Tides project demonstrates that the Rayleigh waves are radiated during the onset and perhaps additionally during the deceleration of the slip event. The entire slip event has the seismic moment of a Mw 6.5 earthquake, but the seismic radiation is small due to the degenerate source geometry and long time duration (~ 25 minutes). These observations, along with other slow glacial events detected in Greenland, suggest that broadband seismic observations can be used to efficiently detect and monitor a variety of glacial behavior over large continental regions.