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Analysis of short-period regional phase path effects associated with topography in Eurasia

Institute of Tectonics University of California, Santa Cruz, California 95064

Abstract

Propagation of regional seismic phases is highly dependent on path effects, but we have a limited understanding of these effects and no general procedure for accounting for path variation influence on regional waveforms. Thus, there is strong regional variability in the effectiveness of regional wave discriminants used to identify small earthquakes and nuclear explosions. Motivated by many observations of correlation between surface geology and regional phase behavior, we empirically explore the relationship between short-period regional and upper mantle distance signal energy and statistics of topography along different travel paths using data for underground nuclear explosions at Semipalatinsk, Kazakhstan. We find strong linear correlations of the logarithmic rms amplitude ratio S_n/L_g (and to a lesser extent, P/L_g) with mean altitude, rms roughness, rms slope, and skewness of topography along the paths to receivers in Eurasia. This indicates that energy partitioning in the regional wave field is controlled by wave-guide structure and attenuation variations that are manifested in surface topography. This suggests that it is feasible in many cases to directly calibrate regional discriminants for path effects in terms of observable surface topography, as a surrogate for overall path properties. The relationships also help to understand the nature of regional phase propagation.

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