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

Abstract

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.

Deep Earth Structure: Lower Mantle and D″

“…Early observations of these ULVZs are summarized by and . Similarly, the absence of seismically detectable ULVZ does not preclude the presence of a very thin (<5 km) layer that is not resolved by seismic probes of the CMB, and only upper bounds on thickness and velocity drop can be determined (Stutzmann et al, 2000). In some cases, the ULVZ region appears to be enhanced to scales of 600-900 km lateral extent and several tens of kilometers of thickness, as under the central Pacific (Cottaar and Romanowicz, 2012) and below the region northeast of Tonga (Thorne et al, 2013).…”

A new probe of ULVZ S‐wave velocity structure: Array stacking of ScS waveforms

“…The ULVZ is typically attributed to partial melting, based on the large velocity reductions and observation of a 3:1 ratio in V S to V P reductions [Williams and Garnero, 1996]. Past studies of the ULVZ have used a variety of seismic phases: SP d KS [e.g., Garnero et al, 1998]; precursors to PcP or ScP [e.g., Rost and Revenaugh, 2003]; PKP scattering [e.g., Vidale and Hedlin, 1998], and precursors to SKS [Stutzmann et al, 2000]. These phases all provide coupled sensitivity to V p , V S and density.…”

SKS and SPdKS sensitivity to two‐dimensional ultralow‐velocity zones

“…The most common hypothesis is the presence of partial melt [ Williams and Garnero , 1996; Revenaugh and Meyer , 1997; Helmberger et al , 1998; Vidale and Hedlin , 1998; Williams et al , 1998; Zerr et al , 1998; Berryman , 2000; Wen , 2000; Ross et al , 2004]. Variations in chemical composition on the mantle side of the CMB [ Manga and Jeanloz , 1996; Stutzmann et al , 2000], “sediments” of finite rigidity collecting on the top of the outer core [ Buffett et al , 2000; Rost and Revenaugh , 2001], and a gradient in the mantle‐core transition (rather than the traditional sharp CMB) [ Garnero and Jeanloz , 2000a, 2000b] have also been proposed, alone and in combination. More recently, Mao et al [2006] proposed that iron enrichment in the postperovskite phase [ Murakami et al , 2004] might account for ULVZs.…”