Constraining crustal anisotropy from receiver functions: A new approach

Tuesday, 16 December 2014
Ahmet Okeler and Miaki Ishii, Harvard University, Cambridge, MA, United States
It is one of the great challenges in seismology to resolve spatially varying crustal anisotropy, and body waves that undergo P-to-S conversion at the crust-mantle boundary provide the best sampling. Since such phases arrive within the P wave coda, they are identified using receiver functions. Sophisticated techniques have been developed to migrate or to invert receiver functions for crustal structure, however, crustal anisotropic properties are hardly investigated. Most of the receiver function studies targeting crustal anisotropy use forward modeling approaches that demand examination of several tens of thousand of models, which becomes daunting.

We have developed a technique to explore detailed anisotropic nature of the crust from splitting analysis of teleseismic receiver functions. Unlike the conventional methods, we consider crust as a transversely isotropic medium with tilted (not horizontal) symmetry axis and modify the cross-convolution method accordingly. For each station, our approach seeks the optimal set of fast polarization azimuth, tilt of the symmetry axis, dip of the Moho discontinuity, and measure of anisotropic strength by analyzing data from multiple sources simultaneously.

We test our approach using ray-theoretical synthetic seismograms generated by the RAYSUM algorithm. Estimated parameters are usually in good agreement with the input, however, the most robustly estimated value is always the fast splitting direction. Besides synthetics, we also applied our technique to the recordings of the ANZA network stations that are locate in southern California. Every station in this network shows significant amount of energy on the transverse component receiver functions indicating the presence of crustal anisotropy or dipping interface. In addition to lateral variations in fast polarization direction across this array, we also present our preliminary results for the neighboring states. At some stations, we detect converted energy from a strong mid-crustal interface with polarity reversals on transverse component, which implies the presence of multiple anisotropic layers within the crust. The new approach can be applied for studying multi-layered anisotropy, and provides a unique opportunity for investigating depth-varying crustal anisotropy.