Shear in the mantle associated with the motion of Earth's tectonic plates tends to align minerals within large volumes of mantle rock. One of the most common minerals in mantle rock is olivine, which is stiffer in some direction than in others. The directional variation of stiffness is called anisotropy. The partial alignment of olivine minerals in mantle rocks causes these rocks to be anisotropic. Anisotropy expresses itself in the tendency for seismic wavespeed to vary slightly on the direction of propagation, shear-waves to acquire elliptical polarization, and for incoming seismic waves to scatter into other seismic waves. Love wave are horizontally-polarized surface waves that scatter to elliptically-polarized Rayleigh waves when they encounter regional concentrations of anisotropy. These scattered waves, called Quasi-Love, or QL waves, are useful for detecting shear near plate boundaries, where the strong plates stretch the plastically-deformable mantle surrounding the plates as they sink at oceanic trenches, spread at rift zones, or crumple where continents collide. QL waves can be observed on individual seismograms, and complement other types of seismic data. The investigators found QL scattering in and around the Cascadia subduction zone at the NW border in seismic data from the land-based USArray, a component of Earthscope. At the Cascadia plate boundary the Juan de Fuca plate converges with the North American plate along the coast of the Pacific Northwest, and sinks into the mantle beneath the line of Cascade volcanoes. Offshore anisotropy beneath the Juan de Fuca plate appears to align in agreement with coastal anisotropy estimated from other data. This suggests that the mantle beneath the Juan de Fuca plate deforms from the lateral motion of the plate, rather than from "slab rollback," that is, a progressive retreat of the plate as it falls (of its own weight) into the plastically-deforming mantle.
The researchers are extending their analysis to the entire Cascadia subduction zone, stacking Quasi-Love scattered waves for stations along great-circle paths that track the progress of individual surface waves. The frequency dependence of the maximum scattering is sensitive to the depth range of anisotropy. QL-scattering proximal to the Cascadia subduction zone is significant, but lateral anisotropic gradients are present inland as well. The researchers are extending the stacking procedure inland from Cascadia to explore QL-crossing paths along the San Andreas system, the Basin and Range, and the Yellowstone hot spot.
For detailed interpretation of the data the researchers compute synthetic seismograms in simplified earth-model geometries as a quick-interpretation tool, and explore 3-D structure in anisotropy by adapting the SPECFEM-GLOBE seismic-wave simulator to explore the effects of hypothetical anisotropic geometries. The researchers will produce maps of the Western US, that highlight the locations where Love-to-Rayleigh scattering is strong, what symmetry orientation (and therefore shear orientation) appears to be consistent with the scattering, and whether the anisotropic gradients are asthenospheric (i.e. dynamic), lithospheric (perhaps fossil) or represent deeper flow in the upper mantle.
