Modeling of seismic Pn attributes using the spectral-element method for velocity and attenuation structure of the Tibetan lithosphere

Using seismic data from regional earthquakes recorded by the Hi-Climb experiment in Tibet, Pn attributes are used to constrain the velocity and attenuation structure of the Tibetan lithosphere. The seismic attributes considered include travel-times, peak envelope amplitudes, and instantaneous pulse frequencies. The numerical modeling is performed using the Spectral-element method (SEM) for which the results from the SEM which is validated by comparing with analytical and reflectivity results. The observed crustal and Pn amplitudes and pulse frequencies are then compared with numerical SEM calculations for Tibet models with laterally varying upper mantle velocities and attenuation. The results from the SEM modeling give two alternative models for the upper mantle beneath Tibet. One model has a constant upper mantle velocity gradient, similar to that of Griffin et al. (2010) and Phillips et al. (2007), with a variable attenuation model. The other model has a constant upper mantle attenuation and a laterally variable upper mantle velocity gradient similar to that of Myers et al. (2010). In either case, the Qiangtang terrane can be distinguished from the Lhasa terrane by a change of upper mantle velocity and Moho depth. The lower velocities of the uppermost mantle, as well as inefficient Sn propagation and lower Pn amplitudes, suggest hotter temperatures in the upper mantle beneath the Qiangtang terrane compared to the Lhasa terrane. Either lower Qp values or a lower upper mantle velocity gradient in the Qiangtang terrane can be used to explain the seismic attribute data presented here.