Research Required to Support Comprehensive Nuclear Test Ban Treaty Monitoring (1997) / Chapter Skim
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Appendix D: Seismic Magnitudes and Source Strengths
Appendix D: Seismic Magnitudes and Source Strengths
Pages 113-120
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From page 113... ...
Important contributions to this discriminant also come from the inefficient excitation of Rayleigh waves from an isotropic source and the rebound of the explosion cavity which produces a peak in the amplitude spectrum. 113 Significant challenges have arisen as the monitoring community has attempted to extend this and related teleseismic discrimination methods to smaller events recorded at regional distances: the relative source dimensions change, fewer stations record signals from the events, the paths are more variable and complex, and the complexity introduces differences in numbers and characteristics of the signals (e.g., frequency content and amplitude)
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From page 114... ...
and 5.7 for the Yucca Mountain earthquake. Note that there is an order-of-magnitude larger surface wave for the earthquake compared to the explosion despite their comparable short period amplitudes that result in similar local magnitudes.
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From page 115... ...
This proves difficult because the strength of the regional Rayleigh waves depends on many factors: source type, source depth, local structure, and attenuation. The approach taken for extending Ms to regional distances by Woods and Harkrider (1995)
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From page 116... ...
A similar study by Stevens for the Semipalatinsk, East Kazakhstan, test site involving primarily teleseismic data obtained b = 11.86. Thus, NTS appears less effective in producing surface wave amplitudes per unit of source strength Mo or yield than does the Soviet test site.
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From page 117... ...
An alternative to the empirical approach of estimating source strengths involves waveform modeling methodologies. Examples of routine earthquake processing with such approaches include Harvard's solutions for long-period body and surface waves (Dziewonski et al., 1981)
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From page 118... ...
This tectonic release creates a secondary seismic source that can modify the energy radiation pattern as well as the spectral character of the source. These effects can vary the short- to long-period magnitude ratio of explosions.
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From page 119... ...
ROLE OF SIZE ESTIMATION IN TREATY MONITORING The initial stages of underground explosion monitoring were driven almost entirely by empirical relationships involving these magnitudes. It was noted 30 years ago that P waves from underground shots become quite simple at distance ranges beyond 30° (3300 km)
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From page 120... ...
However, regional distance signals have more complex propagation effects, and as a result, source strength estimation may be better performed by quantitative modeling procedures that synthesize complete seismic signals, explicitly accounting for RESEARCH REQUIRED TO SUPPORT CTBT MONITORING propagation complexities, or use features such as the coda of the wave, which are less dependent on specific paths. Modern instrumentation, coupled with recent modeling techniques, allow complete long-period (>30-second)
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