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Correcting regional seismic discriminants for path effects in western China

Earth and Environmental Sciences Division Los Alamos National Laboratory, Los Alamos, New Mexico 87545(505) 665-8495 (505) 667-4739 hartse{at}lanl.gov

Abstract

The effect of path on regional seismic wave propagation can be significant. In an effort to improve discriminant performance, we explore the effect of path upon P_g/L_g ratios. Our primary objective is to find path corrections that reduce scatter within earthquake and explosion ratio populations, while at the same time increasing the separation between the two populations. We emphasize the 1.5- to 3-Hz and 2- to 4-Hz bands, as P_g and L_g in these bands can often be observed at smaller magnitudes and greater distances than higher-frequency bands, which have previously been shown to be reliable discriminants. For data, we use 271 earthquakes from northwest China, 25 nuclear explosions from the Kazakh test site (KTS), and one nuclear explosion from the Lop Nor test site recorded at station WMQ. We also use 185 earthquakes from the same region and seven nuclear explosion from the Lop Nor test site recorded at station AAK. Event-station distances range from 200 to 1400 km, earthquake magnitudes range between m_b 2.5 and 6.2, and explosion magnitudes range between m_b 4.5 and 6.5. In addition to ratio-distance trends, we examine P_g/L_g ratio-parameter trends related to topography, basin thickness, and crustal thickness. The parameters we consider are mean, roughness, gradient mean, and gradient roughness of the topography, basin thickness, and crustal thickness along each event-station path. We also consider the same parameters after weighting by path length. Through linear regressions, we found path corrections that reduce scatter within event populations, and we also found path corrections that increase the separation between earthquakes and KTS explosions recorded at WMQ. We obtained the best improvement in discrimination performance at WMQ by removing the trends of topography roughness, mean topography, and the gradient of basin thickness after weighting the parameters by path length. For AAK, we found that removing the trends of mean topography and the basement roughness improved discrimination performance over the uncorrected case. However, unlike WMQ, weighting these parameters by path length degraded discriminant performance. Because we see no predictable or repeatable trends for "adjacent" central Asian stations and overlapping regions of interest, we recommend an even more empirical approach to correcting for the effect of path. Where earthquakes are abundant, such as the Tian Shan, contouring a grid of ratio residuals (for each band of interest) may be a simpler method of finding appropriate path corrections.

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