Statistical analysis of irregular wave-guide influences on regional seismic discriminants in China: Additional results for Pn/Sn, Pn/Lg, and Pg/Sn

Reducing scatter in measurements of regional-phase amplitude ratios is desi rable for seismic discrimination applications and also provides insight int o crustal structure controls on energy partitioning of regional seismic wav es, Our previous analysis (Fan and Lay, 1998) of the regional seismic discr iminant, Pg/Lg, indicated that variations in crustal structure cause path-s pecific fluctuations of those amplitude ratios for earthquakes recorded at broadband station WMQ in western China. In this study, we extend our multiv ariate regression analysis to Pn/Sn, Pn/Lg, and Pg/Sn amplitude ratios reco rded at WMQ in frequency bands of 0.75 to 1.5 Hz, 1.5 to 3.0 Hz, and 3.0 to 6.0 Hz for the suite of path-specific parameters: path length, mean path e levation, variance of topography along the path, rms topographic slope vari ations, mean crustal thickness, and mean sediment thickness. Optimal three- and four-parameter models all achieve reductions in variance of the measur ements relative to conventional distance corrections. At low frequency, the improvements for ratios involving Pg can be more than a factor of 2. For a ll short-period amplitude ratios, mean path elevation seems to play an impo rtant role. While crustal thickness and sediment thickness affect ratios in volving Pg, topographic variance and surface slope variations have more inf luence on ratios involving Po. Strong crustal variations associated with th e structure of the Tibetan Plateau are responsible for much of the amplitud e variations.