CSEDI: Testing Hypotheses for the Origin of Seismic Anisotropy at the Base of the Mantle

University Of California-Berkeley, Berkeley CA

Investigators

Abstract

Seismic anisotropy (i.e. seismic waves travel in different directions at different speed) in the deeper earth was discovered in the mid-sixties and was soon interpreted in a qualitative way as a result of crystal alignment during convection. This concept became generally accepted. More recently strong anisotropy and heterogeneity was documented in the lowermost mantle adjacent to the metallic and liquid core. This enigmatic D? zone is both a thermal and chemical boundary layer which is the site of complex dynamic processes that are reflected in many intriguing seismic observations. The cause for seismic anisotropy at the base of the mantle has been the source of much speculation, but its dominant cause is still not known. Inspired by analogy with dynamic processes that are known to occur in the upper mantle, it is tempting and natural to explain it by invoking deformation processes associated with the direction of flow as lithospheric slabs reach the bottom of the mantle and minerals are subject of phase transformations with increasing pressure. The predominant observation of faster velocities for horizontally polarized shear waves than for vertically polarized ones (Vsh>Vsv) in D? suggests that minerals deform and reorient. In order to gain a better understanding of the origin of seismic anisotropy in D? tools and observations that have so far been developed independently by seismologists, and mineral physicists working with geodynamicists, should be combined. Specifically, this research will rely on five modelling ingredients: 1) Knowledge of microscopic deformation mechanisms of minerals at lower mantle conditions (specifically perovskite, postperovskite and magnesiowuestite), 2) single crystal elastic properties of the constituents at pressure and temperature; 3) plausible large scale dynamics models providing the macroscopic strain field in the lower and lowermost mantle; 4) polycrystal plasticity that predicts reorientation of crystals in a macroscopic deformation field; 5) from 1-2-3-4, a model of elastic anisotropy and its 3D variations can be constructed and, given an appropriate wave propagation code, synthetic seismograms can be computed for appropriate seismic phases and compared to actual observations. There are now precise ways to compute synthetic seismograms in a 3D anisotropic earth down to body wave frequencies. In order to obtain new insights into our understanding of deformation behavior at the base of Earth?s mantle, we rely on the different expertise in seismology and mineral physics of the two principal investigators. The research will provide suggestions for future seismic experiments targeted at better characterizing anisotropy in D? and the methodologies that are developed will become available to other researchers in the future.