Understanding and Performing High-Resolution Surface-Wave Attenuation Measurements with USArray

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"Understanding and Performing High-Resolution Surface-Wave Attenuation Measurements with USArray"This project focuses on making use of seismic wave amplitudes to achieve high-resolution images of the Earth's crust and upper mantle. Velocity imaging using the travel times of surface waves has been extremely successful, but interpretations of these velocity images are non-unique. The amplitude analysis performed here constrains not only velocity structure, but also density and attenuation structure. This extra knowledge helps solve the non-uniqueness problem, allowing better constraints on key features of the North American continent's deformation history, evolution, and geochemical and tectonic structure. In particular, the high-resolution attenuation measurements are potentially sensitive to features such as magma and partial melt, water and other volatiles, as well as fault structures through the differing dependencies of these features on intrinsic attenuation as compared with seismic velocities.This work provides higher resolution images compared to previous attenuation and density imaging due to the confluence of vastly improved station coverage by USArray with new data processing techniques. On one hand, developments in theoretical and empirical techniques allow for utilization of ambient noise attenuation tomography, even when the noise sources are non-uniformly distributed. These ambient noise methods promise to allow for improved spatial resolution compared to earthquake-based studies due partly to the increased frequency band over which measurements are successful. On the other hand, developments in array-based phase-front tracking and amplitude mapping make it possible to use the dense station coverage of USArray to account for focusing and defocusing effects and to reduce the influence of source uncertainty. The convergence of this theoretical, numerical and empirical work allows for accurate measurements of surface-wave amplitudes, both for ambient noise and earthquakes, and allows for the density and attenuation imaging considered here.