We use recent results from deterministic numerical modeling of earthquakes and explosions to develop an improved physical basis for earthquake/explosion discrimination. We use six three-dimensional finite difference simulations of earthquakes for the earthquake source models and four empirical and four numerical models for explosions in different materials. Four factors contribute to the success of the mp:Ms discriminant: source spectra, focal mechanism, near-source elastic properties, and interference of pP. Spectral differences contribute about 0.5 magnitude unit to the separation between earthquake and explosion populations for large events, but this difference vanishes for events with mp4.0, using a frequency band of 0.5--3.0 Hz. For small events the VFM discriminant is complementary to mb:Ms in several ways: VFM is improved by pP interference, while mb:Ms is degraded; VFM is more effective for hard rock explosions, while mb:Ms will preferentially discriminate explosions in low-velocity materials; and VFM is insensitive to fault orientation. VFM is more sensitive to noise and regional attenuation differences than mb:Ms. Simultaneous use of both methods should allow improved discrimination of small events. |