Lateral variations in P velocity and attenuation in the D″ layer, from diffracted P waves

Abstract

Abstract Diffracted P waves recorded at two stations along the same great circle are analysed to specify the lateral variations in velocity and velocity gradient in the D″ layer at the base of the mantle. Data come from long-period and broadband GEOSCOPE and (GDSN) stations. The variations in both apparent ray parameter and amplitude decay are studied as a function of the dominant period of the P wave. Regional variations in the ray parameter indicate a ±2.5% variation for the P velocity in D′, with high velocities beneath the North Pacific, Central America and northern Asia, and low velocities beneath the South Pacific, South Atlantic and northern Indonesia. The absence of correlation between these velocity variations and the lateral variations in P attenuation favour a chemical origin rather than a thermal origin for these heterogeneities. On the other hand, lateral variations in the velocity gradient are suggested from the frequency dependence of both ray parameter and P-amplitude decay.