Very low shear wave velocity at the base of the mantle under the South Pacific Superswell

摘要

S-wave velocity heterogeneity at the base of the mantle beneath the South Pacific Superswell has been examined using SKKS-SKS and S(Sdiff)-SKS differential travel times. The residuals of SKKS-SKS and S(Sdiff)-SKS with respect to the preliminary reference Earth model (PREM) are compared to the residuals predicted using recently published three-dimensional S-wave velocity models. We have obtained a three-dimensional model that explains the S(Sdiff)-SKS residuals well, and applied this model to the correction of the SKKS-SKS residuals. The corrected SKKS-SKS residuals beneath the South Pacific Superswell remain large and positive. Forward modeling reveals that these residuals can be accounted for by a 5-7% low velocity region with a lateral extent of about 500 km in the lowermost ～200 km of the mantle. Although the magnitude of the velocity reduction and the thickness of the anomalous region trade off against one another and cannot be uniquely determined, the existence of a very low S-wave velocity region at the base of the mantle is a robust feature. This feature lies close to the ultra-low velocity zone (ULVZ) identified by P-velocity data beneath the southwestern Pacific, and is located just below a 2-3% low S-velocity body in the lower mantle at depths of 2300-2700 km beneath the South Pacific Superswell.