Diffracted seismic waves and the dynamics of the core-mantle boundary

Diffracted P and S waves (Pd, Sd) traveling around the core-mantle boundary (CMB) of the Earth give us information about the velocity structure and therefore thermochemistry of D{dollar}sp{lcub}primeprime{rcub}{dollar}, the base of the Earth's mantle. By examining Pd and Sd arrivals we determined the apparent slowness for different regions at the base of the mantle. By comparing the data slownesses to those found from synthetic seismograms (generated using reflectivity and normal mode summation) we were able to quantify D{dollar}sp{lcub}primeprime{rcub}{dollar} average velocities.; Examinations found significant lateral heterogeneity at the base of the mantle, amounting to {dollar}>{dollar}3% for P velocities ({dollar}alpha{dollar}) and {dollar}>{dollar}4% for S velocities ({dollar}beta{dollar}). {dollar}alpha{dollar} and {dollar}beta{dollar} did not always vary in parallel, and the Poisson ratio ({dollar}nu{dollar}) varied regionally by 7.5%. This was demonstrated for the CMB under the Northern Pacific rim, where we found fast shear velocities but slow P velocities. For the CMB under Central Asia we found just the opposite. The region of fast shear velocities under the Northern Pacific rim, our fastest, is also the region where Lay and Helmberger (1983a) and others found evidence of shear wave triplications, interpreted as a high-velocity discontinuity.; The first half of the Tonga to Mid-East path (under New Guinea to the South China Sea) displayed our slowest velocities, and the second part (under Southeast Asia) displayed some of the fastest. It is interesting that these regions correspond to areas of core upwelling and downwelling (respectively) found by Voorhies (1986), and so represent the velocities that are expected on thermal or chemical grounds.; We present several possible geodynamic and thermodynamic explanations. A rapidly increasing thermal gradient in D{dollar}sp{lcub}primeprime{rcub}{dollar} could exaggerate any thermochemical differences between adjacent regions and lead to large variations in {dollar}nu{dollar}. Using a third-order Birch-Murnaghan equation of state, the velocity variations can be explained by a 300{dollar}spcirc{dollar} temperature variation for {dollar}alpha{dollar}, a 700{dollar}spcirc{dollar} variation for {dollar}beta{dollar}, and a 400{dollar}spcirc{dollar} variation for {dollar}nu{dollar}. Also, a 30% variation in the Mg/(Mg + Fe) ratio would adequately explain the range of variations for {dollar}alpha{dollar} and {dollar}beta{dollar}.