An Overview of P/S Methods of Discriminating Explosions from Earthquakes with Insights from the Source Physics Experiments

Regional distance (200-1600 km), high-frequency seismic P/S wave amplitude ratios have proven empirically effective at identifying explosions among a background of natural earthquakes at a wide range of locations. However the physical basis for the generation of explosion S-waves, and therefore the predictability of this P/S technique as a function of path, frequency and event properties such as size, depth, and geology, remains incompletely understood. A goal of current research, such as the Source Physics Experiments (SPE), is to improve our physical understanding of the mechanisms of explosion S-wave generation and advance our ability to numerically model and predict them. At regional distances the lithospheric waveguide usually allows P/S discriminants to be applied over broad areas using relatively simple 1- or 2-D path corrections. In addition to the necessary path corrections, the main caveats to P/S discriminant effectiveness are: 1) the signals need to be sufficiently high frequency (usually > 2 Hz, sometimes higher) and, 2) the path attenuation must allow source generated S-waves to still be visible at the frequencies and recording stations of interest. Here we use the largest SPE chemical explosions (5-ton TNT equivalent SPE-5 and the upcoming 2.2-ton SPE-6) to explore the frequency dependent behavior of the P/S discriminant over a range of distances, from very near source out to the regional distances, where it has proved most effective. We contrast the explosions with nearby natural earthquakes and in particular are seeking to understand why the discriminant seems to work best at higher frequencies. SPE-5 discriminates well above 4 Hz at regional distances, similar to what we observe for much larger historic Nevada nuclear tests recorded at the same stations. This result is in contrast with some of the ideas about explosion S-wave generation, and is leading to improvements in our physical understanding of this important discriminant.