Theoretical and Observational Studies of Surface Wave Attenuation From Ambient Noise

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Attenuation, or its inverse, quality factor (Q), is one of the most fundamental parameters of the Earth?s media. Measurement of attenuation at regional distances traditionally uses seismic waves generated by earthquakes, which generally requires either a good knowledge of the source or a special choice of geometries to cancel out source effects. Research based on seismic ambient noise correlation methodology has emerged as one of most rapidly expanding branches of seismology. The approach has been demonstrated to be highly effective at extracting seismic velocities. Extracting amplitude information is more challenging. The greatest challenge is that the Earth?s ambient noise field is highly anisotropic, non-uniform, and variable with time. Here, we propose to explore the methodologies and procedures for extracting surface wave attenuation from empirical Green functions (EGFs) constructed from seismic ambient noise. Our approaches are to combine sound theoretical understanding and practical considerations with real data. We will use data from the USArray and ChinArray as testing beds with the goal to produce surface wave attenuation maps of the regions. Preliminary results from theoretical derivations and numerical simulations show that even in the case of incompletely diffuse noise fields, we can robustly recover not only travel times, but also ray arrival amplitudes, the ambient field?s specific intensity, the strength and density of its scatterers if any, site amplification factors, and most importantly attenuation. We propose two approaches with detailed formulations: linear array methods and more general methods for 2D station networks, each to be developed through applications to numerically simulated data, and to real data. Extraction of attenuation from ambient noise is a frontier research problem. In particular, innovations are required to address the issue of non-uniform, anisotropic, and time-variable ambient noise distribution on Earth. Breakthroughs in this area will be transformative in producing a new way for mapping the Earth?s attenuation beyond traditional earthquake based methods and in our new understanding of the rapidly expanding ambient noise seismology. The approach overcomes some of the disadvantages of traditional earthquake-based method in mapping surface wave attenuation (such as earthquake source information and earthquake and station distribution). New results from attenuation maps and site factors with ambient noise have broad implications for studies of Earth?s interior, earthquake sources, and earthquake hazards.