Lateral Variations in Compressional/Shear Velocities at the Base of the Mantle

Abstract

Observations of core-diffracted P (Pdiff) and SH (SHdiff) waves recorded by the Missouri-to-Massachusetts (MOMA) seismic array show that the ratio of compressional (P) seismic velocities to horizontal shear (SH) velocities at the base of the mantle changes abruptly from beneath the mid-Pacific (VP/VS = 1.88, also the value predicted by reference Earth models) to beneath Alaska (VP/VS = 1.83). This change signifies a sudden lateral variation in material properties that may have a mineralogical or textural origin. A textural change could be a result of shear stresses induced during the arrival at the core of ancient lithosphere from the northern Pacific paleotrench.

Deep Earth Structure: Lower Mantle and D″

“…A candidate major component of the lower mantle that may develop anisotropy with SH velocity faster than SV velocity in a horizontally sheared boundary layer is MgO or ferropericlase (e.g., Karato, 1998;Karki et al, 1999;Kendall, 2000;Long et al, 2006;Mainprice et al, 2000;Marquardt et al, 2009a;Stixrude, 1998;Yamazaki and Karato, 2002), whereas low-velocity lamellae composed of partially melted crust or other chemical heterogeneities are also possible causes of such anisotropy (e.g., Fouch et al, 2001;Kendall and Silver, 1998;Moore et al, 2003;Wysession et al, 1999). A candidate major component of the lower mantle that may develop anisotropy with SH velocity faster than SV velocity in a horizontally sheared boundary layer is MgO or ferropericlase (e.g., Karato, 1998;Karki et al, 1999;Kendall, 2000;Long et al, 2006;Mainprice et al, 2000;Marquardt et al, 2009a;Stixrude, 1998;Yamazaki and Karato, 2002), whereas low-velocity lamellae composed of partially melted crust or other chemical heterogeneities are also possible causes of such anisotropy (e.g., Fouch et al, 2001;Kendall and Silver, 1998;Moore et al, 2003;Wysession et al, 1999).…”

Shear wave splitting and waveform complexity for lowermost mantle structures with low‐velocity lamellae and transverse isotropy

“…We restricted our modeling to low‐velocity lamellae with partial melt characteristics (δV P : δV S = 1:3) [ Williams and Garnero , 1996], given that there are no obvious candidate D″ materials with large velocity increases, as noted by Kendall and Silver [1998]. Wysession et al [1999] note that reconciling P and S velocities in D″ can further constrain the anisotropy, but there are relatively poor constraints on P velocity in most areas with clear S wave splitting, so we focus on the better characterized S wave behavior.…”

Apparent Splitting of S Waves Propagating Through an Isotropic Lowermost Mantle

“…Most observations of anisotropy suggest that V SH is higher than V SV in regions of D ${}^{\mathrm{\prime \prime }}$ where the shear velocity is relatively high (see Nowacki et al, 2011, for a recent review). These include the D ${}^{\mathrm{\prime \prime }}$ region beneath Alaska (e.g., Garnero & Lay, 1997; Wysession et al, 1999), the Caribbean (Kendall & Silver, 1996), the Indian Ocean (Ritsema, 2000), and Siberia (Thomas & Kendall, 2002). The pattern of anisotropy is more complex within the LLSVPs and the transition zones between LLSVPs and the high‐velocity regions of D ${}^{\mathrm{\prime \prime }}$.…”