An Analysis of Recorded and Simulated SH Wave Reverberations in the Upper Mantle beneath the USArray

In addition to SS precursors, the traveltimes of long-period (T > 10 s), teleseismic topside shear-wave reflections may be useful to constrain the depths of discontinuities in the upper mantle. We focus our attention on waveforms from the USArray and the topside reflections Sv410 and Sv660 at the 410-km and 660-km, respectively.

Using waveform stacking and inversion approaches to isolate the reflections at the receiver side, we find that410-km and 660-km are mapped in the eastern US about 20 km shallower than in the western US. The east-west dichotomy in the410-km and 660-km depths is consistent with the shear-velocity contrast between the eastern and western US resolved with shear-wave tomography. Therefore, it is unclear that depth variations of the 410-km and the 660-km are required to explain the Sv410 and Sv660 arrival times recorded by the USArray.

In our presentation, we explore how well we can resolve undulations of the 410-km and 660-km in the presence of strong shear-velocity velocity variations in the upper mantle using long-period shear waves. In addition to our analysis of USArray records, we analyze synthetic waveforms computed using modified SPECFEM3D-Globe software to determine (i) the contribution to Sv410 and Sv660 traveltimes by shear-velocity variation, (ii) the validity of ray-theoretical corrections for upper mantle velocity structure, and (iii) the resolvability of harmonic 410-km and 660-km undulations at different spatial wavelengths.