title: Ultra-Low velocity zones near the core-mantle boundary from broadband PKP precursors
abstract: Short- and long-period precursors of the PKP phase were used to study an ultra-low velocity zone (ULVZ) near the core-mantle boundary beneath the Western Pacific. Synthetic seismograms were computed from a hybrid method, which handles seismic wave propagation through two-dimensional complex structures. Long-period precursors were explained by Gaussian-shaped ULVZs of 60 to 80 kilometers height with P velocity drops of at least 7 percent over 100 to 300 kilometers. Short-period precursors suggest the presence of smaller scale anomalies accompanying these larger Gaussian-shaped structures. These fine structures may be areas of partial melt caused by vigorous small-scale convection or the instability of a thermal boundary layer at the mantle's base, or both.
authors.affiliation: Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.
authors.initials: L
authors.lastname: Wen
authors.initials: DV
authors.lastname: Helmberger