A new probe of ULVZ S‐wave velocity structure: 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.