A new probe of ULVZ S-wave velocity stucture: Array stacking of ScS waveforms

S-wave velocity reductions in thin layers above the core-mantle boundary (CMB) are resolved by modeling precursors to the tangential-components of core-reflected ScS phases. Unlike most phases used to study ultra-low velocity zone (ULVZ) structure, ScS provides direct sensitivity to shear velocity structure, V_S. Recordings of deep earthquakes in the Tonga-Fiji and South American subduction zones at broadband seismic networks in western North America are used to map small-scale ULVZ variability. A two-layer low-V_S structure exists under the central Pacific: an abrupt 0.8-2.0% decrease 60-86 km above the CMB is underlain by a 24-30 km thick ULVZ layer having reductions of 3.3-7.4% (with respect to PREM). No corresponding velocity reductions are found under the Cocos plate. The structure above the ULVZ may be associated with reverse transformation of post-perovskite to perovskite in a steep thermal gradient in a boundary layer above the CMB.