High-resolution investigation of shear wave anisotropy in D″ beneath the Cocos plate

Broadband shear wave signals from 16 deep South American earthquakes are used in a high-resolution analysis of lowermost mantle structure beneath the Cocos Plate. Shear wave splitting of 162 ScS phases is generally compatible with vertical transverse isotropy (VTI) in the D″ region, with little path length or spatial dependence across the 200 × 800 km² area sampled. Assuming a 250 km thick D″ layer, the VTI has an average value of 0.63%. ScS-S differential travel time anomalies for the same data indicate a lateral gradient in positive D″ transverse component velocity perturbations, increasing northward. S-wave triplication arrivals from a shear velocity discontinuity at the top of D″ show splitting similar to the ScS phases, but with smaller magnitude. Vertical resolution remains limited, but the data are consistent with VTI material being concentrated in just the upper portion or at the top and bottom of D″.