Weak Upper Mantle Anisotropy Revealed by Teleseismic P Wave Receiver Functions from the USArray

Session: Advances in Seismic Imaging of Earth’s Mantle and Core and Implications for Convective Processes
Type: Oral
Date: 4/29/2020
Time: 09:15 AM
Room: 120 + 130
Description: 

Upper mantle anisotropy has important implications for constraining mantle flow and composition by linking observational seismology to mineral physics. Measurements of S-wave splitting and directionally-dependent surface wave speeds have provided useful info on upper mantle anisotropy for several decades. Here, we consider the amplitudes of Pds phases (such as the P410s and P660s) on the transverse component of teleseismic P wave receiver functions to obtain independent estimates of the magnitude and orientation of seismic anisotropy in the deep upper mantle and transition zone. The contiguous U.S. portion of the USArray was split into four regions divided by the Rocky Mount Front and the latitude of the Mendocino Triple Junction. The SV and SH components receiver functions within each region were mapped into depths and stacked based on highly sampled back-azimuth groups. The amplitude ratio between the P410s and the P660s phases on the SH component and that on the SV component were adopted as a metric to estimate the deep anisotropy along with confidence intervals from bootstrap resampling. The observational results were compared with synthetic receiver functions to give preferred anisotropic structures. An average upper mantle composition pyrolite was assumed in the forward modeling, and the maximum P wave and S wave anisotropy were estimated based on recent mineral physics studies. Preliminary results show approximately 0.5% anisotropy beneath the Northwest U.S., where active subduction occurs. The preferred fast axis is 70˚ to 90˚ from North with a dip angle of 15˚ to 30˚. No clear Pds signal on the SH component was found in the other sub-regions, implying weak to absent depth-integrated anisotropy on average. These results suggest diminished anisotropy near the base of the upper mantle and in the transition zone compared to common evidence for stronger anisotropy in the lithosphere and asthenosphere. Ongoing research will further compare the new seismic results with predictions from hypothetical upper mantle composition and numerical models of mantle flow.

Presenting Author: Han Zhang

Authors