ABSTRACT
Twenty-nine Palms Marine Corps Base is positioned in a step-over region between the West Bullion Fault and Mesquite Lake Fault. Both of these belong to a series of transform faults located in the Mojave Desert called the Eastern California Shear Zone (ECSZ) that trends northwest to southeast. Previous studies have found that these faults carry 15% of the motion of San Andreas and have had a cumulative slip of 65 km over the past 10 Ma.

Both the West Bullion and Mesquite Lake are primary faults with right lateral strike slip and cause extension within a step-over region ultimately leading to the formation of secondary faulting with slight vertical displacement. The initial reason for this investigation was due to  the base at Twenty-nine Palms proposing to construct building additions, however per the Alquist Priolo Act of 1972 buildings may not be constructed on actively ruptured areas; making it necessary to find the  trace and assess the secondary faulting occurring in the area. By mapping geomorphic structures in Google Earth and excavating 11 seismic trenches,  the Geotechnical team at the Naval Facilities Engineering Command (NAVFAC) and I were able to determine the trace of the fault and the amount of displacement. It was found that in some areas of the mapped fault trace there was clear displacement in measurements of up to 14 inches, where in other areas there was strictly a series of very condensed fractures extending over a broad area. This suggests that in certain areas of the base the displacement was extensive enough to halt the construction of some buildings, while in other areas there was not enough surface rupture potential to pose a threat to the building additions.

The vast complexity of faults in the ECSZ makes it difficult to fully comprehend the hazardous potential of seismic events in the area. It is necessary to make a comparison of the step-over area found on the base with others known in the ECSZ to make a better prediction as to the potential damage that may threaten any future or existing buildings on the base.