Grant No DE-FG02-03ER83720 Report for US Department of Engery

Effective and reliable nuclear monitoring requires discrimination between small magnitude explosions and earthquakes based on the use of limited regional data. Lg is generally the largest seismic phase from both explosion and earthquake sources recorded at regional distances. For small events, Lg may sometimes be the only well-recorded seismic phase so that discriminants based only on the use of Lg are especially desirable. Recent research has provided significantly better understanding of Lg by demonstrating that the explosion-generated Rg makes significant contribution to the low-frequency S or Lg from explosions. Near-source scattering of explosion-generated Rg appears to be a viable mechanism for generating low-frequency(< 2 Hz) Lg waves from explosions. Detailed knowledge of the complex scattering process is, however, still incomplete and is in fact the subject of several ongoing studies. Our analysis of regional data from nuclear explosions from both Nevada Test Site (NTS) and Kazakh Test Site (KTS) and nearby earthquakes in Phase I has suggested that there are several reliable source discrimination methods only based on the use of Lg at regional distances. These discriminants should be. especially useful for small magnitude seismic events for which Lg may be the only well-recorded seismic phase. Our results suggest four possible regional discriminants: (a) frequency-amplitude-time analysis of spectrograms, (b) Lg(low frequency)/Lg(high frequency), (c) Lg spectral slopes, and (d) skewness of Lg spectra. Remarkable similarity of discrimination results from both NTS and KTS nuclear explosions and nearby earthquakes, with entirely different geological settings, indicates that our results should be applicable to other regions of the world