Revisiting the High-Velocity Anomalyies in the Great Basin, Exploring the Role of Seismic Anisotropy

Session: Advances in Seismic Imaging of Earth’s Mantle and Core and Implications for Convective Processes [Poster]
Type: Poster
Date: 4/29/2020
Time: 08:00 AM
Room: Ballroom
Description: 

While we know that seismic anisotropy is common in the upper mantle, most tomographic body-wave velocity models assume isotropy. This can potentially induce bias if the results are interpreted in terms of temperature and composition. Since inverting for anisotropy multiplies the number of unknowns in an already under-determined problem, we address this issue by applying hypothetical a priori anisotropy constraints to isotropic tomography. We focus on the Great Basin, where a high-velocity anomaly in isotropic P-wave tomography has been interpreted as “dripping” lithospheric material. This anomaly is located over an area with low shear-wave splitting times surrounded by higher splitting times, which has been inferred to indicate vertical flow. We explore two hypotheses: 1) Assuming anisotropic fast axes are subhorizontal, P-waves traveling through the low-splitting-time region will be delayed less than those traveling through its periphery, and this alone can cause the apparent high-velocity anomaly. 2) Subvertical fast axes in the low-splitting-time region advance passing P-waves, producing (or contributing to) the travel-time delay pattern that is observed and interpreted as high isotropic velocities. Results show that anisotropy from subhorizontal mantle flow alone is not sufficient to explain the observed delay times. Meanwhile, considering sub-vertical fast axes produces a model that fits travel time data at an equivalent level as the isotropic model, but has a reduced high-velocity anomaly. We conclude that the consideration of seismic anisotropy in the analysis of body-wave travel-time observations may alter our interpretations and understanding of ongoing geodynamic processes.

Presenting Author: Zhao Zhu

Authors