Calibration of Attenuation Structure in Eurasia to Improve Discrimination and Yield.

摘要

It is well known that one-dimensional models do a poor job of predicting both regional amplitudes and travel-times over large and tectonically complicated regions. As a result regional discrimination methods (e.g., high-frequency P/S, Ms:mb) and magnitude estimates (e.g. coda magnitude) can perform poorly when applied over broad regions. The careful calibration of the earth's attenuation structure is critical to the universal application of event discrimination and yield estimation methods down to very small magnitudes. We have developed and are continuing to improve methods that use the direct amplitudes of the major regional phases (Pn, Pg, Sn, Lg) to determine the attenuation structure of the lithosphere in the Middle East and East Asia. The amplitudes are inverted simultaneously for attenuation parameters (Qp, Qs) of the crust and upper mantle, event source terms, and station site terms, which ensures that parameters, such as seismic moment and apparent stress, are consistent across each phase. We are using these models to correct observed amplitudes for path-dependent variations due to earth structure. Amplitude corrections can be demonstrated to improve high-frequency regional P/S discriminants (e.g., Pn/Lg, Pg/Lg, Pn/Sn) by reducing scatter in the earthquake population and increasing separation from explosions. Better path corrections allow the extension of discrimination to lower frequencies so long as true source differences between events exist at those frequencies. We are applying similar methodologies to coda amplitudes with the goal of improving magnitude and yield estimation. While coda waves average over large regions and have less variation than direct phases, they too are subject to path-dependent variations in amplitudes. Furthermore, they can be sensitive to changes in the dominant phase (e.g., Sn vs. Lg) over broad regions. We report on our efforts for the 2-D calibration of coda amplitudes in the Middle East.