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GLOBAL MANTLE STRUCTURE 2006 IRIS 5-YEAR PROPOSAL Global Tomographic Model of Q in the Upper Mantle Obtained Using Long-period Waveform Data Yuancheng Gung, Barbara Romanowicz • Berkeley Seismological Laboratory We developed a degree 8 three-dimensional Q model (QRLW8, Gung and Romanowicz, 2004) of the upper mantle, derived from three component surface waveform data in the period range 60-400 sec. These waveform data were collected from stations of the IRIS/GSN and Geoscope networks. The inversion procedure involves two steps. In the first step, 3D whole-mantle velocity models are derived separately for elastic SH (transverse component) and SV (vertical and longitudinal component) velocity models, using both surface and body waveforms and the NACT approach (Non-linear asymptotic coupling theory, Li and Romanowicz, 1995). In the second step, the surface waveforms thus aligned in phase are inverted to obtain a 3-D Q model in the depth range 80-670 km. Various stability tests were performed to assess the quality of the resulting Q model, and in particular to assess possible contamination from focussing effects. We find that the 3D patterns obtained are stable, but the amplitude of the lateral variations in Q is not well constrained, because large damping is necessary to extract the weak Q signal from data. The model obtained agrees with previous results in that a strong correlation of Q with tectonics is observed in the first 250 km of the upper mantle, with high attenuation under oceanic regions and low attenuation Model QRLW8 derived from three component data at different depths in the upper mantle. Blue and red regions are regions of low and high attenuation, respectively. Black dots are hotspots according to the list by Richards et al. (1988). under continental shields. It is gradually replaced by a simpler pattern at larger depths. At the depths below 400 km, the Q distribution is generally dominated by two strong minima, one under the southern Pacific and one under Africa, which correlate strongly with the two minima observed in elastic tomographic models in the deep mantle, suggesting continuity into the upper mantle of the upwelling flow originating in these regions at the base of the mantle (Romanowicz and Gung, 2002). Moreover, most hotspots are located above regions of low Q in the transition zone. On the other hand, ridges are shallow features in both velocity and Q models. Gung, Y. and B. Romanowicz, Q tomography of the upper mantle using three-component long period waveforms, Geophys. J. Int., 157, 813-830, 2004. Romanowicz, B. and Y. Gung, Mega-upwellings from the core mantle boundary to the lithosphere: implications for heat flux, Science, 296, 513-516, 2002. NSF grants which supported this work: EAR- 9902777 and EAR-0308750 150
2006 IRIS 5-YEAR PROPOSAL GLOBAL MANTLE STRUCTURE Compressional-Wave Studies on the Frequency Dependence of and Lateral Variations in Mantle Attenuation Linda M. Warren • Carnegie Institution of Washington Peter M. Shearer • University of California, San Diego Figure 2. Lateral variations in upper-mantle compressional-wave attenuation generally correlate with surface tectonics. Figure 1. Absorption band Qα model for the Earth’s mantle. The top 220 km of the mantle is more attenuating than the lower mantle, and this contrast increases with frequency. The yellow region indicates the analyzed frequency band. We study the frequency dependence of and lateral variations in P-wave attenuation in the mantle by analyzing the spectra from >18,000 P and >14,000 PP arrivals. We select seismograms from the IRIS FARM database from large, shallow earthquakes at epicentral distances of 40°-80° for P waves and 80°-160° for PP waves and compute the spectrum for a 12.8-s-long window around each arrival. Each spectrum is the product of source, receiver, and propagation response functions as well as local source- and receiver-side effects, and we use a stacking procedure to isolate the propagation effects. Using separate absorption bands in the upper and lower mantles, we model the average depth and frequency dependence of mantle Q by combining measurements of the amplitude decay of the propagation log spectra between 0.16 and 0.86 Hz with long-period Qβ values of other workers. We find that the upper mantle is more attenuating than the lower mantle and that this contrast is greater at higher frequencies. At 1 Hz, the top 220 km of the mantle is ~6 times more attenuating than the lower mantle. In addition, our results indicate that the upper corner frequency of the absorption band is higher for the upper mantle than at greater depths; the lower layer is about twice as attenuating at 0.1 Hz than at 1 Hz, whereas upper mantle attenuation is relatively constant across this band. Since lower-mantle attenuation is small, we interpret deviations in spectral decay as lateral variations in upper mantle attenuation. The resulting map of more and less attenuating regions generally correlates with previously-published attenuation models and surface tectonics. Continents are usually less attenuating than the global average, whereas oceanic regions tend to be more attenuating. Warren, L.M., and P.M. Shearer, Investigating the frequency dependence of mantle Q by stacking P and PP spectra, J. Geophys. Res., 105, 25,391-25,402, 2000. Warren, L.M., and P.M. Shearer, Mapping lateral variations in upper mantle attenuation by stacking P and PP spectra, J. Geophys. Res., 107, 2342, doi:10.1029/2001JB001195, 2002. 151
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