Evidence for tectonic movement of the Las Positas Fault, Alameda County, California

On October 29, 1979, a new exposure of the Las Positas fault zone near Livermore, California, was created by excavation of a creek bank along Arroyo Seco. The face of the bank was cleaned by geologists of Lawrence Livermore Laboratory (Livermore, California) to obtain a better and more complete view of the fault. The excavation was undertaken by Lawrence Livermore Laboratory as part of a larger review of the earthquake hazards of the laboratory site. The Las Positas fault zone extends along the south side of Livermore Valley, passing just south of Lawrence Livermore Laboratory. The exposure revealed new evidence concerning not only the width of faulting in the Las Positas fault zone, but also its recency, style, and origin.