Combined analysis of surface wave phase and amplitude data using finite frequency kernals: Towards a detailed mode of attenuation in the upper mantle

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Studies of the 3D structure of the Earth usually focus on the speed of wave propagation within the Earth, but the rate at which seismic energy is attenuated is also of fundamental interest. Seismic attenuation operates through thermally activated processes and is therefore a very sensitive indicator of temperature variations within the mantle. Thus, combining wave speed variations with attenuation variations gives a powerful tool for learning about the physical cause of seismic anomalies within the Earth. The fundamental datum for learning about seismic attenuation is the amplitude of seismic waves, but amplitudes are affected by so many things, and intrinsic attenuation is not even the strongest contributor to seismic amplitudes. Consequently, it has proven difficult to get a reliable picture of seismic attenuation within the Earth. We use extremely large datasets of the seismic amplitudes (and phase) of fundamental mode surface waves to address this problem. The approach we use is not new and involves trying to directly estimate the various factors affecting surface wave amplitudes. The new feature of this proposal is to use a more robust theoretical basis for assessing the effects of amplitude anomalies caused by focusing and defocusing effects on the seismic wavefield -- one of the main contributors to surface wave amplitude anomalies. Since phase and amplitude effects are intertwined through the effects of physical dispersion, our research will also provide improved understanding of the dispersion of seismic waves and consequently of the elastic structure of the upper mantle. Our datasets are sufficiently large to also address the effects of anisotropy on surface wave propagation -- in itself an important topic of investigation.This research focuses on improving our understanding of seismic wave propagation with an emphasis on understanding how seismic energy is attenuated in the Earth. Being able to accurately model the propagation of seismic waves in and on the Earth has many societal implications. We give three examples. Firstly, there is the issue of seismic hazard where being able to accurately model seismic amplitudes after a large earthquake is crucial. Secondly, there is the issue of being able to monitor a nuclear test ban reliably. Being able to locate and estimate the size of seismic events of any kind is critical to this endeavor. Finally, as part of this research, we will also be refining models of the seismic sources of many thousands of earthquakes. Improving our understanding of seismic sources is of obvious intrinsic interest to society.