Transdismensional Approach Applied to the Measurement of Higher Mode Surface Wave Dispersion

Measurements of seismic anisotropy can constrain patterns of deformation in the mantle. In recent years, there has been more and more evidence from shear-wave splitting and higher mode surface waves that the deep upper mantle and top of the lower mantle are seismically anisotropic, challenging common views on mantle deformation mechanisms. Higher mode surface waves carry unique, independent constraints on structure at greater depths than commonly used fundamental mode surface waves, and enhance resolution in the deep upper mantle, transition zone, and uppermost lower mantle. However, direct measurement of higher modes is challenging because their group velocities overlap significantly in a broad frequency range. We present a novel method to measure path-specific multimode Rayleigh wave dispersion and quantify their uncertainties. Previous measurement methods of higher modes have employed either regularized inverse techniques or have used model space searches with a model parametrization imposed a priori. Here, a transdimensional inversion method was combined with normal mode code MINEOS to measure multimode Rayleigh wave dispersion. This kind of sampling-based algorithm includes the number of parameters in the set of unknowns, but high dimensional models are naturally discouraged to avoid over-fitting the data. We perform non-linear waveform fitting to find path-specific 1-D shear wave speed profiles that fit long-period surface waveforms and determine dispersion curves. The uncertainties on the measurements are assessed from the posterior distribution. Fundamental modes and overtone dispersion curves measured with this method are in good agreement with previous results. Future work will include generating new regional Rayleigh phase velocity maps by using our new method, and 3-D velocity and anisotropy models inverted from these phase velocity maps will shed light on the patterns of deformation in the deeper mantle.