Development of a Time-Domain, Variable-Period Surface-Wave Magnitude Measurement Procedure for Application at Regional and Teleseismic Distances, Part II: Application and Ms-mb Performance

Abstract

The Russell surface-wave magnitude formula, developed in Part I of this two-part article, and the M s (VMAX) measurement technique, discussed in this article, provide a new method for estimating variable-period surface-wave magnitudes at regional and teleseismic distances. The M s (VMAX) measurement method consists of applying Butterworth bandpass filters to data at center periods between 8 and 25 sec. The filters are designed to help remove the effects of nondispersed Airy phases at regional and teleseismic distances. We search for the maximum amplitude in all of the variable-period bands and then use the Russell formula to calculate a surface-wave magnitude.In this companion article, we demonstrate the capabilities of the method by using applications to three different datasets. The first application utilizes a dataset that consists of large earthquakes in the Mediterranean region. The results indicate that the M s (VMAX) technique provides regional and teleseismic surface-wave magnitude estimates that are in general agreement except for a small distance dependence of ‫200.0מ‬ magnitude units per degree. We also find that the M s (VMAX) estimates are less than 0.1 magnitude unit different than those from other formulas applied at teleseismic distances such as Rezapour and Pearce (1998) and Vanȇk et al. (1962).In the second and third applications of the method, we demonstrate that measurements of M s (VMAX) versus m b provide adequate separation of the explosion and earthquake populations at the Nevada and Lop Nor Test Sites. At the Nevada Test Site, our technique resulted in the misclassification of two earthquakes in the explosion population. We also determined that the new technique reduces the scatter in the magnitude estimates by 25% when compared with our previous studies using a calibrated regional magnitude formula. For the Lop Nor Test Site, we had no misclassified explosions or earthquakes; however, the data were less comprehensive.A preliminary analysis of Eurasian earthquake and explosion data suggest that similar slopes are obtained for observed M s (VMAX) versus m b data with m b Ͻ5. Thus the data are not converging at lower magnitudes. These results suggest that the discrimination of explosions from earthquakes can be achieved at lower magnitudes using the Russell (2006) formula and the M s (VMAX) measurement technique.

Regional Multistation Discriminants: Magnitude, Distance, and Amplitude Corrections, and Sources of Error

“…Therefore, M w is actually acting as a surrogate for a rapid change in material properties occurring near the water table encountered by larger and more deeply buried explosions. The apparent differences in compression to shear energy scaling for explosions and earthquakes are also observed in published (and widely used) developments of the m b versus M s discriminant (see Stevens and Day, 1985;Taylor, 1996;Bonner et al, 2006). The apparent correlation between Z and M w is fundamentally due to sampling bias for the explosion population-there are no large shallow explosions and no small deep explosions in this data set.…”

The Surface Wave Magnitude for the 9 October 2006 North Korean Nuclear Explosion

“…We compared our magnitude to previous M s ∶m b research (Bonner, Pasyanos, et al, 2006;Bonner, Russell, et al, 2006) for Eurasia and found the NK event plots slightly above the Murphy et al (1997) event screening value, which is M s 2:90 for an IDC m b of 4.1 (Fig. 3).…”

“…We do note that some mining explosions do not discriminate well because of the reduced P-wave amplitudes associated with delay-firing practices (Bonner, Pasyanos, et al, 2006). Some have suggested the NK results could be evidence of a convergence of the earthquake and explosion M s ∶m b populations at small magnitudes (as postulated in Stevens and Day [1985]); however, Bonner, Russell, et al (2006) saw no evidence of the convergence at the Nevada Test Site (NTS) for events of similar and smaller m b . Others have suggested this is further evidence of the need to revise the current screening criteria used for earthquake and explosion identification.…”

“…While there are other similar empirical relationships in the literature (e.g., Sykes and Cifuentes, 1984;Woods, 1993), we chose Bache (1982) and Stevens and Murphy (2001) because they employed the Marshall and Basham (1972) and Rezapour and Pearce (1998) formulas, respectively, to estimate surface wave magnitudes. We have developed conversion factors that relate M s VMAX to both formulas (Bonner, Russell, et al, 2006). For example, M s VMAX estimates are on average 0.18 m.u.…”

“…For periods 8 ≤ T ≤ 25 sec, the equation is corrected to T 20 sec to account for source effects, attenuation, and dispersion. Bonner, Russell, et al (2006) refer to this technique as M s VMAX, for variableperiod, maximum amplitude magnitude estimates. There are several advantages to the M s VMAX method.…”