SHEAR-WAVE VELOCITY AND ATTENUATION STRUCTURE BENEATH ANTARCTICA DETERMINED FROM SURFACE-WAVES

Fundamental and higher mode surface waves generated by nine earthquakes, which occurred in Antarctica and its nearby regions and were recorded at Scott Base (SBA), South Pole (SPA), Novolazarevskaya (NVL) and Mirnyy (MIR) seismic stations are used to determine the shear-wave velocity and attenuation structure beneath these regions. The Frequency-Time Analysis method is used to determine group velocities for periods ranging from 6 to 80 seconds for fundamental and higher mode Rayleigh and Love waves. Crustal thickness is found to be 41 km beneath the eastern part and 30 km beneath the western part of Antarctica. Rayleigh and Love wave group velocities show lower values in the eastern part as compared to the western part of Antarctica. A shear-wave velocity of 3.52 to 3.7 km s(-1) is estimated in the lower part of the crust (15-41 km from the surface) beneath eastern Antarctica. Similarly a shear-wave velocity of 3.48-3.6 km s(-1) is estimated in the lower part of the crust (15-30 km from the surface) beneath western Antarctica. Rayleigh and Love wave group velocities are found to be lower for eastern Antarctica compared to Australia, Canada, India and other shield models of the world. Love and Rayleigh wave attenuation coefficients are estimated at periods of 10-110 s using the spectral amplitude of these waves across the eastern and western parts of Antarctica. Backus & Gilbert inversion theory is applied to the surface-wave attenuation data to obtain average Q(beta)(-1) models for the crust and upper mantle beneath Antarctica. Inversion of Love and Rayleigh wave attenuation data shows a high-attenuation zone (Q = 125-200) at a depth of 10 to 40 km beneath eastern Antarctica. Similarly, a high-attenuation zone (Q = 65) occurs at a depth of 20 to 90 km beneath western Antarctica. The Q(beta)(-1) models show a lithospheric thickness of 80-100 km beneath western Antarctica. The base of the lithosphere is identified as the depth at which there is a significant change in the Q(beta)(-1) value. The Q(beta)(-1) models for Antarctica show that there is a decrease in Q(beta)(-1) value by an average of three factors in the asthenosphere as compared to lithosphere. In general, Q(beta)(-1) in the asthenosphere show a higher value (an average of 2-3 times) compared to the lithosphere. This implies that the asthenosphere beneath Antarctica is cooler compared to other shield structures of the world.