Upper mantle anisotropy from long‐period P polarization

Abstract

Abstract. We introduce a method to infer upper mantle azimuthal anisotropy from the polarization, i.e., the direction of particle motion, of teleseismic longperiod P onsets. The horizontal polarization of the initial P particle motion can deviate by •10 ø from the great circle azimuth from station to source despite a high degree of linearity of motion. Recent global isotropic three-dimensional mantle models predict effects that are an order of magnitude smaller than our observations. Stations within regional distances of each other show consistent azimuthal deviation patterns, while the deviations seem to be independent of source depth and nearsource structure. We demonstrate that despite this receiver-side spatial coherence, our polarization data cannot be fit by a large-scale joint inversion for whole mantle structure. However, they can be reproduced by azimuthal anisotropy in the upper mantle and crust. Modeling with an anisotropic reflectivity code provides bounds on the magnitude and depth range of the anisotropy manifested in our data. Generally, our fast directions are consistent with anisotropic alignment due to lithospheric deformation in tectonically active regions and to absolute plate motion in shield areas. Our data provide valuable additional constraints in regions where discrepancies between results from different methods exist since the effect we observe is local rather than cumulative as in the case of travel time anisotropy and shear wave splitting. Additionally, our measurements allow us to identify stations with incorrectly oriented horizontal components.

Rayleigh wave phase velocities, small‐scale convection, and azimuthal anisotropy beneath southern California

“…The 40 broadband stations consist of several different types of seismometers. It often may not be properly appreciated, but just as stations frequently have misoriented horizontal components [Schulte-Pelkum et al, 2001], they also commonly have misreported or miscalibrated response functions. In every study we have performed to date that has involved multiple instrument types (MELT Experiment, northeastern United States/MOMA, Colorado/Rocky Mountain Front, Tanzania), we have found that the instrument response parameters reported for at least one of the instrument types has been incorrect.…”

“…Assuming 4% anisotropy for upper mantle materials, Polet and Kanamori [2002] estimated an anisotropic layer about 100-200 km thick, according to the range of delay times. Combining information from P wave polarization [Schulte-Pelkum et al, 2001], Pn times [Hearn, 1996], Rayleigh and Love velocities, and SKKS and SKS splitting, Davis [2003] concluded that anisotropy is distributed throughout the upper 200 km of the mantle up to the base of the crust. In our study, the strength of azimuthal anisotropy is $1.7% at periods shorter than 100 s and less than 1% at longer periods.…”

Crustal complexity in the Lachlan Orogen revealed from teleseismic receiver functions

“…Anisotropy with a dipping symmetry axis can produce a pattern identical to that caused by a dipping interface in an isotropic medium. It is difficult to distinguish between a dipping axis of symmetry and a dipping interface for a single station from RFs alone (Savage 1998) or from particle motion alone (Schulte-Pelkum et al 2001). …”

Global Imaging of the Earth's Deep Interior: Seismic Constraints on (An)isotropy, Density and Attenuation

“…Significant azimuthal anisotropy has been observed for Pn waves beneath oceans (Hess, 1964) and continents (Smith & Ekström, 1999) as well as for travel time residuals of teleseismic P waves (e.g., Babuška et al, 1998). There are clear observations of polarization anomalies of long period P-waves which have been used to infer upper mantle anisotropy (Schulte-Pelkum et al, 2001), but the biggest wealth of observations comes from SKS-splitting measurements (for a review see e.g., Long & Silver, 2009). Surface waves also exhibit a clear anisotropic behaviour.…”