M_(Rg): A magnitude scale for 1 s Rayleigh waves at local distances with focus on yield estimation

摘要

Yield estimation of small explosions at local distances represents a challenge for the nuclear explosion monitoring community. We have examined the feasibility of using short-period surface-wave magnitudes, called M_(Rg), to estimate explosion yields at local distances (< 100 km). We have modified the Russell (2006) _s formula, which was derived for periods of 8-25 s for distances beyond 50 km, for application at local distances and 1 s period. We have studied short-period surface-wave attenuation in diverse lithologies in order to incorporate an attenuation term in the magnitude scale, which is suitable for Rg. We have also incorporated multiple excitation corrections for Rg based on the near-source seismic velocities, which greatly affect the source-region amplitudes for Rg. It is important to note that in the formula the excitation is estimated from the measured Rg group velocity. We have also derived a new Butterworth filter cutout definition for filtering Rg near 1 s period at distances between 2 and 100 km. We used the new formula to estimate M_(Rg) for 39 small (37 ≤ Y ≤ 12;270 kg TNT equivalent) and shallow (<120 m) explosions detonated in North America in lithologies ranging from alluvium to granite. Regressions of the magnitudes with yield result in the equation M_(Rg) log_(10) (Y) - 3.03 for chemical explosions with Y < 12; 270 kg. An F factor with 95% confidence was determined to be 2.25, giving lower and upper bounds on the yield estimates of Y=2:25 and Y × 2:25, respectively. We applied the relationship M_(Rg) log_(10) (Y) - 3.33 (assuming factor of 2 equivalence between chemical and nuclear) to nuclear explosions detonated at the Degelen and Shagan, Kazakhstan, test sites. The estimated yields based on Rg magnitudes were often within 20% of the true yield and had smaller F factor than the estimated yields for United States chemical explosions.