Surface Wave Attenuation in the Tibetan Plateau from Ambient Noise.

摘要

We explore the methodologies to extract amplitude information from the empirical Green functions (EGF) derived from ambient noise correlations and to map the attenuation of the surface waves. Our approaches are to combine theoretical derivations, numerical simulations, and practical considerations. Theoretical insights show that even in the case of incompletely diffuse noise fields, we can robustly recover not only travel times, but also ray arrival amplitudes and attenuation. We investigate two approaches with detailed formulations: linear array methods and more general methods for 2D station networks. Our numerical simulations validate that amplitudes and attenuations can indeed be extracted from noise correlations for a linear array or for a more general 2D array. We developed a temporal flattening procedure, which is effective in speeding up convergence while preserving relative amplitudes. For real data, we developed an asynchronous temporal flattening procedure that does not require all stations to have data at the same time. Tests on real data suggest attenuations extracted from our noise-based methods are comparable with those from earthquakes. A preliminary attenuation map of Chinese continent is obtained based on earthquake data. Internal scattering can be important in contaminating the amplitude of the main arrivals, which need to be considered when making amplitude measurements. Our studies show the great promise of retrieving amplitude and attenuation information from ambient noise correlations and suggest practical methods for applications to real data.