Regional Magnitude Scaling, Transportability, and Ms:mb Discrimination at Small Magnitudes

Abstract

Data sets of mb(Pn) and mb(Lg) measurements are presented for three continental regions in order to investigate scaling relationships with moment magnitude W , and event discrimination at small magnitudes. Compilations of published measurements are provided for eastern North American and central Asian earthquakes, and new measurements are reported for earthquakes located in western United States. Statistical tests on W ,:mb relationships show that the mb(Lg) scale of Nurru (1973) is transportable between tectonic regions, and a single, unified W ,:mb(Lg) relationship satisfies observations for M 6.5 in all regions. A unified relationship is also developed for nuclear explosions detonated at the Nevada Test Site and test sites of the former Soviet Union. Regional mb for explosions scale at higher rates than for earthquakes, and of significance is the finding that mb(Pn) for explosions scales at a higher rate than mb(Lg). A model is proposed where differences in scaling rates are related to effects of spectral overshoot and near-field Rg scattering on the generation of Pn and Lg waves by explosions. For earthquakes, mb(Pn) and mb(Lg) scale similarly, showing rates near 1.0 or 2/3 • log1oMo (seismic moment). W ,:mb(Lg) scaling results are converted to unified Ms:mb(Lg) relationships using scaling laws between log Mo and Ms. For earthquakes with Ms greater than 3.0, the scaling rate is 0.69 • Ms, which is the same as it is for nuclear explosions if M., is proportional to 1.12 • log Mo, as determined by NTS observations. Thus, earthquake and explosion populations are parallel and separated by 0.68 mb units for large events. For small events (Ms < 3.0), populations may converge or diverge depending on the tectonic region in which earthquakes occur and the scaling rate of explosions at small yields. Earthquakes scale as 0.64 and 0.75 on M:mb(Lg) plots for stable and tectonic regions, respectively. While the scaling rate for explosions is —0.69, this value is uncertain due to paucity of Mo observations at small yields. Measurements of [mb(P) — mb(Lg)] for earthquakes in the western United States have an average value of —0.33 ±.03 mb units, in good agreement with Nuttli’s estimate of mb bias for NTS. This result suggests that Nuttli’s method for estimating test site bias can be extended to earthquakes to make estimates of bias on regional scales. In addition, a new approach for quick assessments of regional bias is proposed where Ms:mb(P) observations are compared with MM:mb(Lg) relationships. Catalog Ms:mb(P) data suggest that mb bias is significant for tectonic regions of southern Asia, averaging about —0.4 mb units.Key wordsSeismic magnitudesregional phasestransportabilityevent discrimination