Ms:mb Relationships for Small Magnitude Events: Observations and Physical Basis for mb Based on Regional Phases

摘要

To address the challenge of small event monitoring for the Comprehensive Nuclear-Test-Ban Treaty (CTBT), there is great interest to extend the successful teleseismic mb-Ms discriminant to regional-distance applications. Among the outstanding issues are (1) how well can we expect the discriminant to perform for regional data as a function of source size and (2) what is the physical basis behind its performance. In a recent paper Patton (2000), Mw: mb scaling relationships for Pn and Lg waves with 1-s periods were developed for earthquakes and explosions located in distinct tectonic regions and geologic materials around the world. Among a number of findings of that study is the result that mb(Pn) for explosions scales at a significantly higher rate than mb(Lg) in the Mw range approx. 3.5 - 6.0. On the other hand, Pn and Lg scaling rates do not differ for earthquakes. On plots of Ms versus mb(Pn), the scaling results suggest that earthquake and explosion populations converge at Ms approx. 1 -2, while populations may or may not converge on plots of Ms versus mb(Lg), and show better separation at small magnitudes. Observations of regional Ms for small-yield explosions at the Nevada Test Site (NTS) confirm the scaling predictions. Thus, mb based on regional shear phases may serve as a better mb-Ms discriminant than mb based on regional compressional phases, a surprising conclusion. These findings certainly cannot be explained by simple theories of the explosion source, and I propose a physical model combining well- established scaling models and the latest understanding of Lg generation by underground nuclear explosions.