Shear-Wave Splitting in the Appalachians and the Urals: A Case for Multilayered Anisotropy
Vadim Levin
Yale University, New Haven, Connecticut
William Menke
Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY
Jeff Park
Yale University, New Haven, Connecticut
Observations of shear-wave splitting in the Northeastern US
Appalachians and in the foredeep of the Urals vary significantly with
the back azimuth and incidence angle of the incoming phase.
These variations suggest two or more layers within the upper mantle
with different anisotropic properties.
Synthetic seismograms for simple multilayered
anisotropic structures show that shear-wave splitting parameters tend
to vary substantially with the direction of approach. Relying on a
subset of back-azimuth and incidence angle may strongly bias the model
inferred, especially if the observations are averaged. On the other
hand, the azimuthal splitting pattern provides additional constraints
on vertical or lateral variation of anisotropic properties in the
Earth.
Using a new error-estimation technique for splitting, we find that
individual measurements from broadband data have
errors on the order of 
for the fast
direction, and 0.1-0.2s for the delay of split shear waves.
The azimuthal variation of splitting parameters is broadly
consistent throughout the Appalachian terranes in the Northeastern US,
especially for two long-running stations in the Northeast US -
HRV and PAL. Observations can be separated into two distinct populations, with mean fast-axis azimuths of 
and 
. Delay values within each population range from near-zero to 
s.
Azimuthal splitting variation for
ARU in the foredeep of Uralian mountains is characterized by sharp
transitions between different groups of observations.
Using synthetic seismograms in simple structures, we
develop one-dimensional anisotropic models under
stations HRV and ARU. The model for HRV includes two layers of
anisotropic material under an isotropic crust, with fast-axis azimuths
and 
for the bottom and the top layers,
respectively. The model for the upper mantle under ARU includes a
layer with a fast-axis at 
atop a layer with fast-axis
azimuth 
. Our modeling confirms the need for a layer of strong
anisotropy with a slow axis of symmetry in the lower crust under ARU,
reported by Levin and Park [1997a].
Our results suggest that both Urals and Appalachians possess a relict
anisotropy in the tectosphere, associated with past continental
collision and accretion, underlain by anisotropy with orientation
similar to the local absolute plate motion, suggesting an
asthenospheric component to the signal.