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Q tomography of the upper mantle using three component long period waveforms
Q model derived from Rayleigh and Love Waveforms, degree 8
We present a degree 8 three-dimensional Q model (QRLW8) of the upper mantle, derived from three component surface waveform data in the period range 60-400 sec. 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). 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 are performed and the contamination from focusing effects is examined to assess the quality of QRLW8. We find that the 3-D 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 amplitude data contaminated by focusing effects.
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. It is replaced in the transition zone by a pattern correlated with hot spots, and,more strikingly, with the velocity structure at the base of the mantle in S velocity tomographic models. The shift in the pattern from the uppermost mantle to the transition zone occurs in areas with marked transverse isotropy (VSH>VSV), indicative of horizontal flow. Ridges are shallow features in both velocity and Q models.