HR: 1340h
AN: S53B-0203    [Abstracts]
TI: Upper-mantle attenuation structure as revealed by surface-wave amplitudes
AU: * Dalton, C A
EM: cdalton@fas.harvard.edu
AF: Harvard University, 20 Oxford St., Cambridge, MA 02138 United States
AU: Ekstr\"om, G
EM: ekstrom@seismology.harvard.edu
AF: Harvard University, 20 Oxford St., Cambridge, MA 02138 United States
AB: Three-dimensional models of attenuation in the Earth's interior can provide important constraints on the thermal and chemical state of the planet, particularly when interpreted together with velocity models. Large lateral variations in attenuation (1/Q) will affect velocity dispersion in a way that also varies laterally and must be considered when constructing and comparing velocity models derived from seismic observations from different portions of the seismic frequency band. However, attenuation in the mantle is a difficult quantity to study due to the various factors, in addition to anelasticity, that can affect wave amplitude, including focusing and defocusing by elastic structure, uncertainty in the source scalar moment and radiation pattern, and inaccuracies in the instrument response. These extraneous effects must be accounted for before the amplitude data can be confidently interpreted in terms of anelastic structure. We present a 3-D model of shear attenuation in upper mantle derived from more than 250,000 measurements of fundamental-mode surface-wave amplitude that were recorded at 170 stations across the globe. The model shows a strong correlation with surface tectonics in the uppermost mantle that becomes weaker in the transition zone. The amplitude data are corrected for source, instrument, and focusing effects, a critical consideration that isolates the signal of attenuation in the data and enables the development of this improved model. We also invert measurements of phase delay together with the amplitudes to obtain 2-D maps of surface-wave attenuation and phase velocity. We show that as a result of focusing, the amplitude data contain significant and retrievable information about elastic velocity that should be included as an additional constraint in the development of phase-velocity maps.
DE: 8180 Tomography
DE: 7255 Surface waves and free oscillations
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting