Earthquake history, microstructure, and fault frictional properties of near-surface sediments on the northern San Andreas Fault at Alder Creek, California

Brittle deformation of young unconsolidated sediments related to recent faulting events is not well understood. Deformation bands in Late Holocene fluvial sands overlying the active trace of the San Andreas Fault (SAF) were studied to examine the relationship between faulting and deformation band development. Methods used include: trenching and logging the known 1906 SAF rupture trace, taking detailed oriented deformation band and control samples, experimental deformation of sand samples away from the active SAF trace to determine fault mechanical properties, and comparison of microstructures in deformation bands found in the SAF to experimental microstructures within the experimentally deformed samples. Deformation bands and experimentally deformed sand exhibit very similar preferential grain orientations that are not observed in control samples. Deformation band and control sand grain orientations are statistically unique populations. Deformation bands have lower porosity and smaller grain-size compared with control sands. The sand has a high coefficient of friction that increases with accelerated strain-rate at near-surface conditions. Experimental work shows velocity-strengthening characteristics. I conclude from frictional and fault mechanical properties of sand at the study site that new deformation band development most likely do not reoccupy older deformation band shear planes. Deformation band formation is a lithologically and compositionally controlled process that changes the grain-size, grain orientation, and strength of the original sediment.