Deformation Band Shear Zones Formed in Unconsolidated Sediment From Repeated Late Holocene Coseismic Deformation Along the 1906 Rupture Trace of the San Andreas Fault

Two trenches were excavated across the 1906 rupture trace of the San Andreas Fault (SAF) at Alder Creek, near Manchester, CA, in Mendocino County for the purpose of structural and microstructural analysis of deformed late Holocene unconsolidated sediment. The site records coseismic rupture in the form of upward terminations of deformation band faults and written historical accounts of the 1906 rupture directly adjacent to the site. Based on the age of the deposits exposed and paleoseismic record of the northern SAF, as many as three to five surface faulting events have occurred, including ~5 m of dextral displacement during the 1906 earthquake. Deformation band shear zones, 2-20mm thick, form an upward-branching 1-2m. wide array of splay faults developed in unconsolidated silt and very fine- to medium-grained sand. Vertical displacement on individual deformation band faults is generally <5cm. Microstructural characteristics (porosity, grain size, grain orientation) of oriented samples collected at ~ 2 m in depth were measured in thin section using image analysis of SEM backscatter images. Porosity estimated from SEM images is slightly lower in deformation bands (39.0±1.8%) than in sand in the same horizon several m. from the fault (42.9±0.9%) or in 1-5 mm wide sand lenses bracketed by deformation bands (42.9±2.0%). Deformation band samples contain significantly more very fine and fine sand, and less medium and coarse sand, than samples collected from the same horizon several m. from the fault. Grain size distributions record grain size reduction from grain fracturing within deformation bands. Also, fractured grains in sand adjacent to deformation bands and angular to acicular small grains within deformation bands attest to grain breakage accompanying development of deformation bands. All horizontal samples show a strong preferred orientation of elongate grains. Clustering of grain long axis orientations is more pronounced in deformation band faults than in sand several m. from the fault, but both display preferred orientation of elongate grains parallel or subparallel to the fault zone. This study demonstrates that deformation bands can form in near-surface unconsolidated late Holocene sediments from repeated coseismic displacement along discrete shear planes. These structures record significant porosity reduction, grain rotation, and grain fracturing as a product of surface-fault rupture on the SAF. We speculate that these microstructural characteristics of fault zones in well-sorted unconsolidated granular deposits may be diagnostic of coseismic rupture. Further study of microstructures and the deformation mechanisms that operate during their formation may provide insight into loading conditions associated with earthquake propagation in unconsolidated surface deposits.