Underground nuclear explosions in Yucca Flat produce fractures in the overlying alluvium. These fractures not only form the expected radial and concentric patterns, but also are commonly aligned along certain preferential directions. These preferential fracture trends form two groups: those that show a spatial relationship with the Yucca and related faults, and those that do not. The fracture trends unrelated to the Yucca fault parallel joint trends in the bedrock at the adjacent edge of the valley; the most conspicuous preferentially oriented fracture trends in the alluvium appear to parallel the set of most closely spaced joints in the bedrock. This similarity in trend of fractures and joints may result from differential movement, during an explosion, along joints in the underlying bedrock that parallel joints exposed in the valley sides. Fractures above explosions detonated in tuff beneath Yucca Flat show a greater degree of preferred orientation than those above explosions detonated in the alluvium.
Some explosion-produced fractures related to the Yucca fault, a recently active north-trending fault in Yucca Flat, form an en echelon pattern along the fault trace. Other fractures and fracture zones branch from the fault trace, or lie adjacent and nearly parallel to it. One explosion caused vertical displacement of at least 18 inches, east side relatively down, across the main fractures along the fault. These fractures exhibit all the features of grouped fault scarps in alluvium as described by Gilbert in 1890 and Slemmons in 1957 and are, therefore, considered to be the result of faulting. The faulting is believed to be caused by pre-existing strain being released by the action of nuclear explosions, as the east side is always relatively down and the fracturing along the fault extensive.