Application of regional phase amplitude tomography to seismic verification

We have applied tomographic techniques to amplitude data to quantify region al phase path effects for use in source discrimination studies. Tomography complements interpolation (kriging) methods by extending our ability to app ly path corrections into regions devoid of calibration events, as well as r aising levels of confidence in the corrections because of their more physic al basis. Our tomography technique solves for resolvable combinations of at tenuation, source-generation, site and spreading terms, First difference re gularization is used to remove singularities and reduce noise effects. In initial tests the technique was applied to a data set of 1488, 1.0 Hz, P -g/L-g amplitude ratios from 13 stations for paths inside a 30 degrees by 4 0 degrees box covering western China and surrounding regions. Tomography re duced variance 60%, relative to the power-law distance correction tradition ally applied to amplitude ratios. Relative P-g/L-g attenuation varied with geologic region, with low values in Tibet, intermediate values in basins an d high values for platforms and older crust. Spatial patterns were consiste nt with previous path effect studies in Asia, especially local earthquake c oda-Q. Relative spreading was consistent with expected values for P-g and L -g. Relative site terms were similar to one another, yet some tradeoff with attenuation was evident. Tomography residuals followed systematic trends with distance, which may re sult from the evolution from direct to coda phases, focusing, model tradeof f or data windowing effects. Examination of the residuals using a kriging i nterpolator showed coherent geographical variations, indicating unmodeled p ath effects. The residual patterns often follow geological boundaries, whic h could result from attenuating zones or minor blockages that are too thin to be resolved, or that have anisotropic effect on regional phases. These r esults will guide efforts to reparameterize tomography models to more effec tively represent regional wave attenuation and blockage. The interpolated r esiduals also can be combined with predictions of the tomographic model to account for path effects in discrimination studies on a station by station basis.