Observations of mantle seismic anisotropy using array techniques:
shear-wave splitting of beamformed SmKS phases
Abstract
Shear-wave splitting measurements are commonly used to resolve seismic
anisotropy in both the upper and lowermost mantle. Typically, such
techniques are applied to SmKS phases that have reflected (m-1) times
off the underside of the core-mantle boundary before being recorded.
Practical constraints for shear-wave splitting studies include the
limited number of suitable phases as well as the large fraction of
available data discarded because of poor signal-to-noise ratios (SNRs)
or large measurement uncertainties. Array techniques such as beamforming
are commonly used in observational seismology to enhance SNRs, but have
not been applied before to improve SmKS signal strength and coherency
for shear wave splitting studies. Here, we investigate how a beamforming
methodology, based on slowness and backazimuth vespagrams to determine
the most coherent incoming wave direction, can improve shear-wave
splitting measurement confidence intervals. Through the analysis of real
and synthetic seismograms, we show that (1) the splitting measurements
obtained from the beamformed seismograms (beams) reflect an average of
the single-station splitting parameters that contribute to the beam; (2)
the beams have (on average) more than twice as large SNRs than the
single-station seismograms that contribute to the beam; (3) the
increased SNRs allow the reliable measurement of shear wave splitting
parameters from beams down to average single-station SNRs of 1.3.
Beamforming may thus be helpful to more reliably measure splitting due
to upper mantle anisotropy. Moreover, we show that beamforming holds
potential to greatly improve detection of lowermost mantle anisotropy by
demonstrating differential SKS-SKKS splitting analysis using beamformed
USArray data.