Mapping elastic and anelastic structure in the global upper mantle

Virginia Polytechnic Institute And State University, Blacksburg VA

Investigators

Abstract

The elastic and anelastic seismic structure of the Earth's mantle provide two independent seismological constraints that are required to distinguish between temperature and compositional heterogeneities in the mantle. Seismic surface wave observations have been used to map the elastic and anelastic structure in the global upper mantle and large discrepancies exit among present-day anelastic earth models developed by different research groups. Wave diffractional effects, as well as the dual dependence of surface-wave observables upon elastic and anelastic variations in the mantle remain poorly understood and have not been properly taken into account in current surface wave tomographic studies. This research aims to better understand fundamental theoretical problems in mapping the earth's elastic and anelastic structure using seismic surface waves, develop theoretical foundations beyond traditional methods and to image the global 3-D elastic and anelastic structure in the upper mantle. This research includes both a theoretical and an observational component. The theoretical component focuses on full wave propagation simulations in 3-D earth models using the Spectral Element Method (SEM) to quantify (1) finite-frequency effects in surface wave propagation in the presence of 3-D elastic and anelastic anomalies; (2) the dual dependence of seismic traveltimes and amplitudes upon 3-D wavespeed and 3-D anelasticity structure, including 3-D anelastic dispersion and amplitude focusing and defocsuing effects and (3) resolution limitations of traditional seismic ray theory in joint tomographic inversions using traveltime and amplitude data. The observational component of this research includes (1) development of a global dataset of fundamental-mode surface-wave phase-delay and amplitude measurements using a multi-taper technique and (2) simultaneous inversion of 3-D velocity and anelasticity (Q) structure in the global upper mantle based upon the global dataset, fully accounting for finite-frequency effects as well as the dual dependence of seismic observables upon 3-D velocity and 3-D Q structure.