HR: 17:00h
AN: T34A-05 INVITED     [Abstracts]
TI: Seismic-wave Attenuation in the Asthenosphere
AU: * Dalton, C A
EM: cdalton@fas.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138 United States
AU: Ekstrom, G
EM: ekstrom@seismology.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138 United States
AB: Seismic-wave attenuation (1/Q) is thought to be highly sensitive to variations in temperature, and joint interpretation of attenuation and velocity models should aid in distinguishing between thermal and chemical heterogeneity in the mantle. However, global attenuation tomography has thus far contributed little to our understanding of Earth structure, and the existing 3-D global Q models show only limited qualitative agreement. The primary reason for this is that factors other than attenuation influence wave amplitude. Principally, amplitudes are affected by focusing and defocusing due to lateral velocity variations, but uncertainties in the calculation of source excitation as well as inaccuracies and problems associated with the instrument response can also obscure the attenuation signal in the data. We have developed a method to remove these extraneous effects and isolate the signal due to attenuation. We invert a large data set of fundamental-mode Rayleigh wave amplitudes in the period range 50--250 seconds simultaneously for maps of attenuation, maps of phase velocity, and amplitude correction factors for each source and receiver in the data set. Measurements of phase delay are included in the inversion as an additional constraint on velocity structure. The maps of attenuation obtained by simultaneous inversion for elastic and anelastic models contain important features that are not robustly imaged when the effect of focusing is ignored. The shallow mantle (~100--300 km) is characterized by high attenuation along western North America and along the East Pacific Rise and other ridge systems, and low attenuation within stable continental interiors. Lateral variations in attenuation are ±60--80% at these depths, with differences most pronounced between the high-Q old continental regions and low-Q mid-ocean ridges. Such large variations require the presence of areas of very low Q, and correspondingly low velocity, in the asthenosphere and underscore the importance of lateral variability in physical dispersion. Our global maps of surface-wave attenuation exhibit a strong correlation with maps of phase velocity corrected for the effect of the crust, particularly for periods < 200 seconds. The correlation suggests that the variability in both Q and velocity in the shallow upper mantle has a common origin, which is most likely thermal. At the greatest depths sampled by our data (400--500 km) a different pattern, consisting of high attenuation in the southeastern Pacific and Red Sea regions and low attenuation along several subduction zones in the Pacific, dominates.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
DE: 8162 Rheology: mantle (8033)
SC: Tectonophysics [T]
MN: Fall Meeting 2005