Constraint on the S-wave velocity at the base of the mantle

Abstract : Some recent seismic studies have suggested the presence of a thin ultra-low P-wave velocity layer (ULVZ) at the base of the mantle, which is interpreted to be due to presence of partial melting. Partial melting would lead to a strong decrease of the S-wave velocity for which there is no seismic evidence. Such a decrease in the S-wave velocity would produce a strong precursor to SKS phase from the conversion of S to P at the upper boundary of the layer. We analyze records of events from the subduction zones in the south-west Pacific region obtained at stations in North America. At the source side, the converted phases propagate in the region, where the ultra-low P-wave velocity has been found earlier. Our analysis demonstrates that either the S-wave velocity drop in this layer is much smaller than predicted by the hypothesis of melting, or the layer is so thin (less than about 10-15 km) that it can not be detected with our technique.