Probing an Ultra-low velocity zone at the Core Mantle Boundary with 13 and S Waves

Recent studies of the core-mantle boundary (CMB) have revealed some very anomalous tructures interpreted in terms of ultra low velocity zones (ULVZ). However, there remains considerable uncertainties about their physical descriptions or even if they occur above or below the CMB. They have only been detected in isolated situations using rather special techniques; these includes: distortions in SKS with the development of SKPdS and SPdKS, broadband PKP precursors, distinct ScS and S beyond 100 degree, and rapid changes in differential travel times of neighboring phases. Here we report on a situation where ray paths associated with PKP precursors and SKPdS sample the same ULVZ structure. The structure lies beneath central Africa and has been detected from WWSSN analog data (SKPdS) discussed previously. This data set has been enhanced with a collection of digital records sampling an elongated NorthSouth zone roughly 800 km long. The entire SKPdS data set can be modeled with a ridge-shaped cross section with widths of 250 to 400 km and drops in P and S velocity of 10 and 30 percent. Fortunately, a new IRIS station (MSKU) located in Western Africa provided excellent PKP data from the New Britain Region events sampling the above structure. The PKP and strong precursors can be modeled by 2D synthetics generated from the same structure (used in modeling SKPdS) which provides a strong constraint on the definition characteristics of this particular ULVZ.