Comparison of Seismic Methods for Fault Characterization at the Nevada Test Site, Nevada

As part of a multi-method geophysical study, we collected compressional- (P) and shear- (S) wave seismic data along three transects that cross the Yucca Fault or associated splays at the Nevada Test Site, Nevada. In this presentation, we focus on the peculiarities of the seismic data and on the processing schemes that we used to produce tomographic images of the fault. The faults offset caliche-rich alluvial materials in the shallow subsurface and show varying ranges of displacement; the Yucca Fault extends to pre-Cenozoic basement, where it shows 200-400 m of offset, while the smaller faults are thought to be shallow features with significantly smaller offsets (less than one meter). The Yucca Fault has a prominent (approximately 4 m) surface scarp, whereas the other faults show little surface expression. The caliche layer presents challenges for processing seismic data of both wave types. In the S-wave data we see significant converted P-wave energy that makes accurate interpretation of true S-wave arrivals nearly impossible. The P-wave data show generally low noise due to efficient propagation in the partially cemented alluvium; interpretable reflections are not evident, however. Inversion of the P-wave first arrival times yields nonsensical results using a standard commercial seismic tomography algorithm that employs a smoothed grid velocity model. Inversion of these first arrivals using a method that employs a blocky velocity model defined with a smaller number of parameters yields more reasonable velocity models that fit with known geology. In the model corresponding with the transect that crosses the Yucca Fault, we see a distinct low-velocity zone (approximately 50 m wide) at the surface exposure of the fault and extending to the base of the model (60 m). We interpret that these low velocities indicate a zone of mechanically deformed material, although the true width of the deformed zone is likely narrower than the width indicated by the velocity model. Along the other transects, we see lesser velocity anomalies corresponding with smaller magnitudes of mechanical deformation on the faults that have less offset.