Focussing effects at the edge of the Large Low Shear Velocity Provinces

Tomographic images of the Earth's lowermost mantle are dominated by two equatorial and nearly antipodal regions of large-scale reductions in seismic S-wave velocities beneath the central Pacific and Africa. These Large Low Shear Velocity Provinces (LLSVPs) are much less constrained in tomographic P-wave models. This discrepancy, together with other geophysical data, led to the interpretation of LLSVPs as thermo-chemical piles, but models of purely thermal LLSVPs might also be able to explain the geophysical data. Data from seismic arrays and high-resolution processing techniques are able to precisely determine the slowness vector of incoming seismic energy and therefore to extract velocity and directivity information from the seismic data directly. Here we use records of P and Pdiff from the medium aperture, short-period, vertical component Yellowknife array (YKA) located in northern Canada and S/Sdiff from stations of the Canadian POLARIS network. Using seismicity from the western Pacific rim allows good sampling of the lowermost mantle in the region of the Pacific LLSVP and the northern Pacific. The slowness information extracted from the array data using the high-resolution F-statistic allows detailed mapping of the LLSVP boundary and indicates a sharp boundary and velocity reductions of several percent. The data also indicate a second region of strongly reduced seismic velocities to the north of the Pacific LLSVP beneath the Sea of Okhotsk that does not seem to be connected to the main LLSVP, and which is not consistently resolved in S-wave tomography models. We observe very strong focussing and defocussing effects along the LLSVP boundary that indicate strong and small-scale heterogeneities in the vicinity of the LLSVP boundary beyond what can be explained by LLSVP material. This detection allows further insight into the structure and dynamics of the LLSVP. Using seismic wave propagation simulations we are aiming to resolve both structure and shape of these newly detected heterogeneities.