Waveform modeling of shear wave splitting from anisotropic models in Iceland

Abstract

[1] No quantitative model of seismic anisotropy beneath Iceland has yet explained the observed shear wave splitting results in Iceland. In this study we explore the structure of mantle seismic anisotropy associated with plume ridge interaction beneath Iceland by modeling synthetic waveforms using a pseudo-spectral method. First, predicted SKS waveforms are shown to produce shear wave splitting results for two anisotropic layers that are in generally agreement with analytic solutions. Next, simple models in which anisotropy are imposed at different depths and geographic regions around Iceland are used to examine different hypothesized effects of plume-ridge interaction. The model that is designed to represent crystallographic fabric due to mantle deformation associated with lithosphere spreading in western Iceland and along-axis, channeled asthenospheric flow in eastern Iceland can predict the distinct shear wave splitting observations on the east and west sides of the island. Last, we analyze geodynamic models that simulate 3-D mantle flow and evolution of mineral fabric. The predicted fast directions of shear wave splitting from the models with realistic spreading rate (10 km/Myr) and broad range of plume viscosities are consistent with the observations in central and eastern Iceland. However, because these models are symmetric about the ridge, none of them can predict the observed shear wave splitting results in western Iceland.

Directions of seismic anisotropy in laboratory models of mantle plumes

“…Deformation and resulting a axis orientations in two‐dimensional calculations are unable to reflect a time‐evolving three‐dimensional flow field, which includes the azimuthal extension and deformation history that are the essential elements in strain and whisker alignment. Our results are, however, in agreement with recent three‐dimensional numerical models of shallow plume dispersion [ Fu et al ., ], which observe flow‐perpendicular alignment within similar depths of expanding plume material. For sheared conduit regions (Figure c, Experiment 2), the variable whisker pattern is also consistent with the weakly flow‐aligned LPO calculated by Kaminski and Ribe [] in their model of a sheared buoyant cylinder nearby a ridge.…”

“…The simplest, most common view for anisotropy within surfacing plumes is that the fast polarization direction will represent the local buoyant‐driven flow, being vertically oriented within the conduit and radially within the pooling, expanding plume head (Figure a) [e.g., Bjarnason et al ., ; Li and Detrick , ; Xue and Allen , ], which has been supported by LPO calculations within simple models of plume flow [ Blackman et al ., ; Rümpker and Silver , ]. More recent numerical models of LPO within plumes, however, suggest this may not hold entirely true [ Kaminski and Ribe , ; Fu et al ., ].…”

Investigating seismic anisotropy beneath the Reykjanes Ridge using models of mantle flow, crystallographic evolution, and surface wave propagation

“…Hence the along-axis-average radial anisotropy is negative in a small zone at about 100 km depth (Figure 8d, left). Above and below this depth, the a axes are oriented with a large component being parallel to the ridge axis [Fu et al, 2012]. Without azimuthal effects, the average radial anisotropy is positive above and below 100 km directly beneath the ridge; with azimuthal effects, the apparent radial anisotropy is negative over a greater depth range (Figure 8d, middle versus left).…”

“…The latter situation can occur, for example, as a result of water being extracted from the mantle during partial melting, which should substantially stiffen the mantle above the dry solidus [Hirth and Kohlstedt, 1996;Ito et al, 1999]. Only recently have these types of flow been tested against observations of shear wave splitting [Fu et al, 2012], whereas the effects on surface wave anisotropy have not been studied quantitatively.…”

“…[25] Relevant to anisotropy, the radially spreading plume produces an axisymmetric pattern of LPO in which the a axes preferentially align parallel to the flow where the radial shear is dominant (e.g., >$300 km radial distance from the plume center, at a depth of 100 km, Figure 6c), and perpendicular to the flow where the circumferential stretching of the spreading plume material is more important [Fu et al, 2012] (<300 km from the plume center, at a depth of 100 km, Figure 6c). When averaged along the direction of wave propagation, the apparent radial anisotropy of the input models ( Figure 8, left and middle) is positive within the 100 km thick lithosphere and weak in the spreading plume material at depths of 100-200 km.…”