Coexisting shear- and compressional-wave seismic velocity discontinuities beneath the central Pacific



Stacking of broadband shear waves (S) and short-period compressional waves (P) provides compelling evidence for the existence of a localized discontinuous increase in both S and P wave velocity 230 km above the core-mantle boundary (CMB) beneath the central Pacific. Modelling indicates a 1.7% increase in S velocity (VS) and a 0.75% increase in P velocity (VP). Similar features have been detected intermittently on a global basis, but seldom with S and P structure being resolved in the same location. Previous evidence for anisotropy near the CMB beneath the central Pacific suggests that the discontinuity may be related to anisotropic fabrics induced by flow in the thermal boundary layer above the core.