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Seismic source functions from free‐field ground motions recorded on SPE: Implications for source models of small, shallow explosions
Author(s) -
Rougier Esteban,
Patton Howard J.
Publication year - 2015
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2014jb011773
Subject(s) - radius , moment (physics) , amplitude , physics , seismic moment , scaling , mechanics , geology , geometry , optics , classical mechanics , seismology , mathematics , fault (geology) , computer security , computer science
Reduced displacement potentials (RDPs) for chemical explosions of the Source Physics Experiments (SPE) in granite at the Nevada Nuclear Security Site are estimated from free‐field ground motion recordings. Far‐field P wave source functions are proportional to the time derivative of RDPs. Frequency domain comparisons between measured source functions and model predictions show that high‐frequency amplitudes roll off as ω − 2 , but models fail to predict the observed seismic moment, corner frequency, and spectral overshoot. All three features are fit satisfactorily for the SPE‐2 test after cavity radius R c is reduced by 12%, elastic radius is reduced by 58%, and peak‐to‐static pressure ratio on the elastic radius is increased by 100%, all with respect to the Mueller‐Murphy model modified with the Denny‐Johnson R c scaling law. A large discrepancy is found between the cavity volume inferred from RDPs and the volume estimated from laser scans of the emplacement hole. The measurements imply a scaled R c of ~5 m/kt 1/3 , more than a factor of 2 smaller than nuclear explosions. Less than 25% of the seismic moment can be attributed to cavity formation. A breakdown of the incompressibility assumption due to shear dilatancy of the source medium around the cavity is the likely explanation. New formulas are developed for volume changes due to medium bulking (or compaction). A 0.04% decrease of average density inside the elastic radius accounts for the missing volumetric moment. Assuming incompressibility, established R c scaling laws predicted the moment reasonable well, but it was only fortuitous because dilation of the source medium compensated for the small cavity volume.