| Measuring anisotropy in the crust and mantle is commonly performed to make
inferences on crust/upper mantle deformation, tectonic history or the presence of fluids.
However, separating the contribution of the crust and mantle to the anisotropic signature
remains a challenge. This is because common seismic techniques to determine
anisotropy (e.g., SKS splitting, surface waves) lack the resolution to distinguish
between the two, particular in regions where deep crustal earthquakes are lacking.
Receiver functions offer the chance to determine anisotropy in the crust alone, offering
both the depth resolution that shear-wave splitting lacks and the lateral resolution
that surface waves are unable to provide. Here I present a new anisotropic H-κ
stacking technique which constrains anisotropy in the crust. I show that in a medium
with horizontally transverse isotropy a strong variation in κ (VP-to-VS ratio) with
back azimuth is present which characterises the anisotropic medium. In a vertically
transverse isotropic medium no variation in κ with back azimuth is observed, but κ is
increased across all back azimuths. While, these results show that estimates of
κ are more difficult to relate to composition than previously thought, they offer
the opportunity to constrain anisotropy in the crust. Based on these observations
I develop a new anisotropic H-κ stacking technique which inverts H-κ data for
anisotropy. I apply these new techniques to data from the Afar Depression, Ethiopia
and extend the technique to invert for melt induced anisotropy solving for melt
fraction, aspect ratio and orientation of melt inclusions. I show that melt is stored in
interconnected stacked sills in the lower crust, which likely supply the recent volcanic
eruptions and dike intrusions. The crustal anisotropic signal can explain much of the
SKS-splitting results, suggesting minimal influence from the mantle. This results
show that it is essential to consider anisotropy when performing H-κ stacking on
receiver function data. The new technique presented here can be applied to any
anisotropic medium where it can provide constraints on the average crustal anisotropy. |