Discrimination Between Earthquakes and Underground Explosions Employing an Improved Ms Scale

Abstract

The M, Rayleigh wave magnitude formula is revised for purposes of eliminating the heretofore variable effects of near distances and propagation paths on the values computed from standard long-period seismograms. The improved formulation employs a revised distance correction function and a period-dependent path correction that normalizes M, to large teleseismic distance 20-s values. For purposes of earthquake-explosion discrimination, an empirical focal depth correction is derived on the basis of Rayleigh wave frequency content as a function of focal depth, which normalizes M , values to the surface focus equivalent, i.e. aids discrimination when it can be applied by increasing earthquake M , values and moving them away from the equivalent explosion population on M, : m, plots.The revised M , improves on previously achieved discrimination of North American events, and provides reliable discrimination between suites of Eurasian earthquakes and explosions. Having removed the dominant propagation path effects on M,, the residual differences in M , : mb among events are generally attributed to source environment and regional effects on m,,.The 42 Eurasian WWSSN stations employed are shown to have a discrimination threshold at the M,3.2 level. With the improved M , scale now equivalent to first order for North American and Eurasian continental propagation, available Nevada Test Site explosion yields are extrapolated to the Eurasian sites to illustrate that this M,3.2 discrimination threshold is equivalent to an Eurasian explosion of about 20 kt in hard rock. Given improved long-period instrumentation to reduce the Rayleigh wave detection threshold, the principal restriction on further studies of discrimination to lower levels of magnitude and yield will be the availability of earthquake occurrence information at the low magnitudes.

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

“…And these magnitudes are not biased with respect to teleseismic estimates using the same M s (VMAX) measurement technique. Additionally, the application of narrowband Butterworth-filtering techniques appropriately handles Airy phase phenomena that, before this study, had to be accounted for by using Marshall and Basham's (1972) empirical corrections. Finally, because the method is variable period and not restricted to near 20-sec period, the analyst is allowed to measure M s where the signal is largest.…”

Magnitudes of great shallow earthquakes from 1953 to 1977

“…This formula was derived for oceanic paths and for teleseismic distances. (The problems which result from magnitude determination at short distances or along continental paths have been discussed by Alewine (1972) and Marshall and Basham (1972) and will not be covered here.) The amplitude, An, in equation (A1) is a somewhat ill-defined quantity.…”

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

“…Instead, we considered a series of published M s = log Y relations for different test sites from previous researchers (e.g., Bache, 1982;Stevens and Murphy, 2001). 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).…”

“…Selby (2007) estimated the surface wave magnitude for this event as 2.83 using the Marshall and Basham (1972) formula, which is typically 0.10 m.u. smaller than M s VMAX, as will be discussed later in this article.…”

“…And the local and regional distance magnitude estimates are not biased with respect to teleseismic estimates using the M s VMAX measurement technique. Additionally, the application of narrowband Butterworth-filtering techniques appropriately handles Airy phase phenomena that, prior to this technique, had to be accounted for using Marshall and Basham's (1972) empirical path corrections. Finally, because the method is variable period and is not restricted to near a 20 sec period, the analyst is allowed to measure M s where the signal is largest.…”