Evaluation of ML-MC as a Possible Depth Discriminant at Local Distances

We find that the difference between local magnitude (ML) and coda duration magnitude (MC) is effective as a local-distance depth discriminant for a wide variety of seismic sources: naturally occurring earthquakes in Utah, the Yellowstone region, and Italy; mining induced earthquakes in Utah; hydrologically induced earthquakes in Oklahoma and Kansas; mining or weapons testing and disposal explosions in Utah; and buried, single-fired chemical explosions in Nevada, Washington, and Wyoming. The primary implication is that combining ML-MC observations with traditional high-frequency P/S amplitude ratios will likely enhance current capabilities in identifying low-yield (M<3) underground nuclear explosions. Future work should focus on (1) testing and calibrating ML-MC in regions of special monitoring interest, (2) quantifying the increase in discrimination capability that derives from combining ML-MC with high-frequency P/S ratios, and (3) modeling high-frequency wave propagation at local distances in realistic Earth models to determine the relative importance of various mechanisms that are likely involved in generating long and strong coda waves for very shallow seismic sources.