Evaluation of Short-Period, Near-Regional Ms Scales for the Nevada Test Site

Surface wave magnitude (Ms) estimation for small events recorded at near-regional distances will often require a magnitude scale designed for Rayleigh waves with periods less than 10 sec. We have examined the performance of applying two previously published Ms scales on 7-sec Rayleigh waves recorded at distances less than 500 km. First, we modified the Marshall and Basham (1972) Ms scale, originally defined for periods greater than 10 sec, to estimate surface wave magnitudes for short-period Rayleigh waves from earthquakes and explosions on or near the Nevada Test Site. We refer to this modification as M(M+B)(s) (7) , and we have used short-period, high-quality dispersion curves to determine empirical path corrections for the 7-sec Rayleigh waves. We have also examined the performance of the Rezapour and Pearce (1998) formula, developed using theoretical distance corrections and surface wave observations with periods greater than 10 sec, for 7sec Rayleigh waves M(R+P)(s) (7) as recorded from the same dataset. The results demonstrate that both formulas can be used to estimate Ms for nuclear explosions and earthquakes over a wider magnitude distribution than is possible using conventional techniques developed for 20-sec Rayleigh waves. These Ms(7) values scale consistently with other Ms studies at regional and teleseismic distances with the variance described by a constant offset; however, the offset for the M (M+B)(s) (7) estimates is over one magnitude unit nearer the teleseismic values than the M(R+P)(s) (7) estimates. Using our technique, it is possible to employ a near-regional single-station or sparse network to estimate surface wave magnitudes, thus allowing quantification of the size of both small earthquakes and explosions. Finally, we used a jackknife technique to determine the falsealarm rates for the M(M+B)(s) (7)-mb discriminant for this region and found that the probability of misclassifying an earthquake as an explosion is 10%, while the probability of classifying an explosion as an earthquake was determined to be 1.2%. The misclassification probabilities are slightly higher for the M(R+P)(s) (7) estimates. Our future research will be aimed at examining the transportability of these methods.