Seismic Moment Estimation and the Scaling of the Long-Period Explosion Source Spectrum
- Steven R. Taylor,
- Howard J. Patton and
- Paul G. Richards
Published Online: 18 MAR 2013
Copyright 1991 by the American Geophysical Union.
Explosion Source Phenomenology
How to Cite
Patton, H. J. (1991) Seismic Moment Estimation and the Scaling of the Long-Period Explosion Source Spectrum, in Explosion Source Phenomenology (eds S. R. Taylor, H. J. Patton and P. G. Richards), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM065p0171
- Published Online: 18 MAR 2013
- Published Print: 1 JAN 1991
Print ISBN: 9780875900315
Online ISBN: 9781118663820
- Underground nuclear explosions—Detection—Congresses;
One of the outstanding problems in our understanding of the explosion source is the long-period scaling of the source spectrum. The discrepancy between observed scaling rates and predicted scaling rate based on the Mueller and Murphy model was first noted by Murphy  and is reviewed in this paper. Causes of the discrepancy are discussed, and the focus of this paper is on non-isotropic source effects associated with two classes of mechanisms: tectonic release and explosion-driven block motions. A brief review of evidence in support of both mechanisms is presented. Tectonic release is characterized by mainly strike-slip motions on faults at shot level or deeper. Driven block motions are associated with substantial vertical displacements above shot level, often in directions opposite to the naturally occurring faulting in the Basin and Range. Quadrupole Love-wave patterns, non-circle Rayleigh-wave amplitude patterns, and complete reversal of Rayleigh waveforms are clear indicators of non-isotropic source effects, yet under most circumstances, fundamental-mode surface-wave observations are insufficient to determine unambiguously the mechanism of shear-wave generation. Recent advances have been made inverting combined datasets of fundamental-mode and higher-mode surface waves recorded at regional distances. Since spall is a viable source of higher modes, the moment tensor inversion results depend on estimates of spall mass and ballistic period to constrain the spall source model. Along with reduced scatter on moment-yield plots and scaling rates in better agreement with the Mueller and Murphy model, the inversion results gave deviatoric tensors that suggest the mechanism of non-isotropic source effects is not the same for all explosions. Explosions with yield above 300 kt showed strike-slip mechanisms consistent with regional tectonics, while smaller explosions showed dip-slip reverse mechanisms. Generalizing these results, I propose a two-mechanism model for non-isotropic source effects, and this model explains the salient features of Ms scaling that were identified by Murphy as the source of discrepancy. Analysis of more explosions is needed to test this model, but before this is possible, knowledge of spall phenomenology must improve in order to build confidence in the moment tensor results.