HR: 08:45h
AN: S31E-04    [Abstracts]
TI: Global Attenuation Tomography and Implications for Upper-Mantle Thermal Structure
AU: * Dalton, C A
EM: dalton@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964,
AU: Ekström, G
EM: ekstrom@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964,
AU: Dziewonski, A M
EM: dziewons@eps.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138,
AB: Observation of seismic-wave attenuation provides a direct measure of the Earth's anelasticity. The sensitivity of attenuation to temperature, composition, partial melt, and water content is different from that of seismic velocity, and joint interpretation of elastic and anelastic models may be used to improve constraints on these properties throughout the Earth. Historically, the development of attenuation models has lagged behind velocity models. However, the availability of large seismic datasets and improved techniques to treat these data have recently led to better and higher-resolution attenuation models. We have developed a new 3-D global model of shear attenuation in the upper mantle. This new model, QRFSI12, is derived from > 30,000 fundamental-mode Rayleigh wave amplitude measurements at each period (period range 50-250 s). The amplitudes are inverted simultaneously for the coefficients of the 3-D model as well as frequency-dependent amplitude correction factors for each source and receiver. We have found that focusing by elastic heterogeneity can significantly influence surface-wave amplitudes and that this effect can be modeled at long periods using ray-theoretical approximations. We therefore subtract focusing effects from the data prior to inversion by using phase-velocity maps determined from jointly inverting amplitude and phase-delay datasets. In the shallow mantle, QRFSI12 exhibits a strong correlation with tectonic features, and different tectonic provinces are characterized by distinct attenuative properties. At depths > 250 km, the model is dominated by high attenuation beneath the southeastern Pacific and eastern Africa and low attenuation associated with subduction zones in the western Pacific. Comparison of QRFSI12 with global shear-velocity models shows a strong anti-correlation throughout the upper mantle. At 100-km depth, a clear trend of increasing velocity and decreasing attenuation with increasing age of the seafloor is apparent, and tectonically active continental areas are associated with slower velocities and higher attenuation than stable continental interiors. At depths of 150 and 200 km, oceanic regions exhibit a larger decrease in attenuation per fractional increase in velocity than stable continental regions do, suggesting differences in the mechanisms that influence the seismic properties within these two regions. Comparison with recent laboratory measurements (Faul and Jackson, 2005) of attenuation and velocity for olivine helps to quantify the extent to which temperature alone can explain the observed variability. We find that the mineral-physics predictions agree well with the global seismic models for the oceanic regions between 150- and 250-km depth, but that the cratonic areas cannot be fit.
DE: 3909 Elasticity and anelasticity
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7255 Surface waves and free oscillations
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: 2007 Fall Meeting