A comparison of methodologies for representing path effects on regional P/S discriminants

Short-period regional P/S amplitude ratios hold much promise for discrimina ting low-magnitude explosions from earthquakes in a Comprehensive Test Ban Treaty (CTBT) monitoring context. However, propagation path effects lead to variability in regional-phase amplitudes that, if not accounted for, can r educe or eliminate the ability of P/S ratios to identify the seismic source . Here we compare four different methodologies that account for the effect of heterogeneous structure on P/S amplitude variance: (1) distance correcti ons, (2) path-specific crustal waveguide parameter regressions, (3) cap ave raging (running mean smoothing), and (4) kriging, The predictability of eac h method is established by cross-validation (leave-one-out) analysis. We ap ply these techniques to Pn/Lg, Pg/Lg, and Pn/Sn observations in three frequ ency bands from 0.75 to 6.0 Hz at station ABKT (Alibek, Turkmenistan), site of a primary station of the International Monitoring System (IMS), Paths t o ABKT sample diverse crustal structures (e.g, various topographic, sedimen tary, and geologic structures), leading to great variability in the observe d P/S amplitude ratios. For these data to be useful for isolating source ch aracteristics, the scatter needs to be reduced by accounting for the path e ffects, and the resulting distribution needs to be Gaussian for most existi ng spatial interpolation and discrimination strategies to have valid applic ation. Each method reduces the scatter of the P/S amplitude measurements wi th varying degrees of success; however, kriging has the distinct advantages of providing the greatest variance reduction and a continuous correction s urface with an estimate of the model uncertainty. The largest reductions in scatter are found for the lowest frequency P/S ratios (<3.0 Hz).