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Published August 2003 | Published
Journal Article Open

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


Surface wave magnitude (M_s) 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 M_s scales on 7-sec Rayleigh waves recorded at distances less than 500 km. First, we modified the Marshall and Basham (1972) M_s 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+B) M_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 7-sec Rayleigh waves ^(R+P) (M_S(7)) as recorded from the same dataset. The results demonstrate that both formulas can be used to estimate M_s for nuclear explosions and earthquakes over a wider magnitude distribution than is possible using conventional techniques developed for 20-sec Rayleigh waves. These M_s(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+B) M_s(7) estimates is over one magnitude unit nearer the teleseismic values than the ^(R+P) M_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 false-alarm rates for the ^(M+B) M_s(7)-m_b 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 ^(R+P) M_s(7) estimates. Our future research will be aimed at examining the transportability of these methods.

Additional Information

© 2003 Seismological Society of America. Manuscript received 6 December 2002. We are indebted to Howard Patton for his assistance in database acquisition and his comments concerning various aspects of the research. We also wish to thank Bill Walter for help in acquiring the MNV dataset. We express our gratitude to Marv Denny, Jeff Stevens, Steve Taylor, Nancy Cunningham, Delaine Reiter, Shelly Johnson, and James Lewkowicz for insightful discussions about the manuscript and research. Constructive comments from an anonymous reviewer were greatly appreciated. We thank the developers of the Generic Mapping Tools software (Wessel and Smith, 1998), Computer Programs in Seismology (Herrmann, 2002), and Matlab, which were all used to generate and present the results of our research. This research was sponsored by the Defense Threat Reduction Agency under Contract Number DTRA01-01-C-0080.

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