The ability to discriminate between underground explosions and earthquakes is an important operation for global security and test‐ban‐treaty monitoring, and seismology plays a key role in this endeavor. The use of seismic P‐ to S‐wave amplitude ratios has been demonstrated to be an effective discriminant at regional distances for a variety of emplacement conditions and test sites around the world. Increasing interest in detecting and discriminating smaller events, both for purposes of nuclear monitoring and seismic hazard estimation, requires the use of data with shorter event‐to‐station paths. At these local distances (⁠≤200  km⁠), the use of P/S ratios for event discrimination is poorly studied compared with regional distances (200–1500 km). The path averaging that allows for relatively simple corrections to P/S ratios to remove bias from along‐path structural differences at regional distances may be insufficient to remove enough scatter from the ratios to allow for accurate event identification at local distances. We use data from a series of chemical explosions detonated as part of the Source Physics Experiment at the Nevada National Security Site in southern Nevada and surrounding seismicity to investigate the discrimination performance of P/S ratios at local distances. We find that discrimination at individual stations is highly variable, regardless of frequency. Network averaging of ratios improves discrimination performance above 6 Hz, as a result of the reduction of scatter among the earthquake ratios. However, azimuthal and distance‐limited averaging suggests that explosion ratios are affected by a complicated source and path relationship that may lead to substantial variability in local P/S ratios at different locations.

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