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Toward Unified Corrections of Regional Phases for Amplitude and Travel Time Effects of Heterogeneous Structure


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We analyze variability of the Pn seismic phase associated with regionally varying seismic velocity structure, seeking to establish self-consistent correction of geometric spreading effects on Pn and regional travel time calculations. This effort has included calculations of frequency-dependent Pn geometric spreading behavior for specific regionally varying 1D structures. We consider a suite of models with varying velocity gradients to establish basic behavior and then explore 2D models with lateral variations in gradient structure. Finite-difference calculations have also explored the effects of small-scale heterogeneity imposed on regional structures to evaluate the stability of frequency-dependent spreading corrections, extending such calculations from 1 Hz as analyzed under a previous contract to 6-10 Hz. We compare the results for various models with calculations for standard assumptions of frequency- independent power law geometric spreading correction models commonly used to correct Pn amplitudes. Pn data analysis for geometric spreading and attenuation from 2 to 10 Hz for the North Korean nuclear tests at stations in northeast China is reported, and the relative effects of oceanic paths on observations of Pn in Japan near 1000 km distance is modeled with 2D laterally varying structures. We compute synthetics for structures in Eurasia based on RSTT (Regional Seismic Travel Time) models and empirically based models that match the observed Pn spreading, contrasting the behavior with standard power-law spreading assumptions. The regional models with Earth flattening and variable lid gradients provide reasonable attenuation estimates and reduced frequency dependence of attenuation. A 2D tomographic Pn velocity gradient model is determined for Eurasia.



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