||Kirtland Air Force Base, NM 871175776
There is a need for improved methods for discriminating nuclear explosions from earthquakes and other natural and man-made seismic sources, and characterizing suspicious seismic sources. The technical challenges are driven by the need to monitor smaller events. Recordings of smaller events are generally sparser, made at closer distances (local and near regional), have lower signal-to-noise ratios, and have maximum signal amplitudes at higher frequencies than recordings of larger events. These differences all present challenges. Of the two most effective discriminants, phase ratio discriminants can fail at local distances and in areas with high attenuation, while the Ms:mb discriminant may not work for small shallow events (Steven and Day, 1985). In addition, monitoring requirements evolve in response to geopolitical events, which drives a need to monitor in new regions that are often poorly calibrated both empirically and with respect to the resolution of available Earth models. The need to use higher frequency and lower signal-to-noise ratio waveforms for smaller events exacerbates the problems that arise from insufficient Earth model resolution. This drives a need for discriminants that are robust to high attenuation and limited means for removing propagation effects from seismic waveforms.
These requirements drive a need for extensions of existing discriminants to make them more effective for the challenging monitoring conditions described above, and for new discriminants that are effective for small events recorded sparsely at local and near regional distances. New methods will require a firm physical basis to support their application in new areas. Inversion for regional moment tensors is currently an effective automated means (e.g. Ekström et al., 2012) of identifying earthquake focal mechanisms and for separating populations of different events types (Ford et al., 2009) at regional distances. Statistically rigorous uncertainty estimates however are needed, as are methods to improve resolution for smaller events, for which propagation effects are more difficult to predict and remove for the higher frequency waveforms. Methods such as the inclusion of specific parametric measurements as a priori constraints to moment tensor inversion (Ford et al., 2012) and phase matched filtering to enhance signal-to-noise (unpublished work at AFRL) have been effective approaches that merit further development and application. Further development and application of such methods are sought.
Steven JL, Day S: The physical basis of mb: Ms and variable frequency magnitude methods for earthquake/explosion discrimination, Journal of Geophysical Research 90: B4, DOI: 10.1029/JB090iB04p03009 1985
Ekström G, Nettles M, Dziewonsk AM: The global CMT project 2004-2010: Centroid-moment tensors for 13,017 earthquakes, Physics of the Earth and Planetary Interiors 200-201: 1-9, 2012. doi:10.1016/j.pepi.2012.04.002
Ford SR, Walter WR, Dreger DS: Event discrimination using regional moment tensors with teleseismic-P contraints, Bulletin of the Seismological Society of America 102: 867-872, doi:10.1785/0120110227, 2012
Ford SR, Dreger DS, Walter WR: Identifying isotropic events using a regioinal moment tensor inversion, Journal of Geophysical Research B01306, doi:10.1029/2008JB005743, 2009
Seismology; Explosion; Earthquake; Moment tensor; Phase matched filter; Discrimination;