Q tomography of the upper mantle using three‐component long‐period waveforms

SUMMARY We present a degree‐8 3‐D Q model (QRLW8) of the upper mantle, derived from three‐component surface waveform data in the period range 60–400 s. The inversion procedure involves two steps. In the first step, 3‐D 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 non‐linear asymptotic coupling theory (NACT) approach. 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 to assess the quality of the resulting Q model and, in particular, to assess possible contamination from focusing effects. 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 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 under continental shields. It is gradually replaced by a simpler pattern at larger depth. 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, yielding a strong degree‐2 pattern. Most hotspots are located above regions of low Q at this depth. Ridges are shallow features in both velocity and Q models.