Comparison of Microstructures Developed by Coseismic Slip and by Aseismic Creep in Unconsolidated Late Holocene Sediments Along Both the 1906 Rupture and the Creeping Segments of the San Andreas Fault, CA

Evaluation of microstructures in unconsolidated near-surface sediments provides promising initial results for differentiating between earthquake rupture-related (coseismic) structures and creep-related (aseismic) structures formed by the San Andreas Fault (SAF). Paleoseismic trenches excavated at Alder Creek (AC), on the 1906 rupture trace, and at Flook Ranch (FR), on the creeping segment of the SAF, allow comparison of structural and microstructural characteristics of faulted late Holocene sediment. At AC, trenches exposed the 1906 rupture as a well-defined 1 to 3 m wide zone of NW-striking faults in fluvial gravel, sand, and silt. The fault zone is defined by: prominent 2-20mm thick deformation band shear zones; offset, warped and tilted stratigraphy; anomalous bedrock highs; discontinuous fractures; groundwater seeps; vertical aligned clasts; and 0.5-1.0mm thick clay gouge. At FR a 20-m trench exposed abundant branching faults in interbedded fluvial silt, gravel, and fine sand. The trench crossed an ~12m wide linear swale bounded by 0.5-m-high scarps. Three-m-wide branching fault and fracture zones underlie the scarps, while more widely spaced faults accommodate strain both within and outside of the swale. Faults extend into extensional fractures that rotate clockwise up-section (20° to 30°) relative to the main NW-striking SAF. Some faults have discontinuous 1-5mm clay stringers; some have 1-20 cm fissure fill. Microstructural characteristics (grain orientation, grain size, porosity) of samples were measured using image analysis of SEM backscatter images. All fault zone samples have preferred grain orientations; grain long axes are parallel or subparallel to the fault in both vertical and horizontal samples. However, grain size distributions and porosities of fault zone samples differ between AC and FR. At AC, 2-20mm wide deformation bands are significantly more fine-grained (more fine and very fine sand, less medium and coarse sand) than sand in the same stratigraphic unit several m. from the fault. In contrast, grain size distributions in faults at FR are indistinguishable from those in samples within the same horizon between faults. Porosity is lower in AC deformation bands (39.0±1.8%) than in sand in the same horizon several m. from the fault (42.9±0.9%) or in 1-5mm wide sand lenses bracketed by deformation bands (42.9±2.0%). In contrast, porosity is lower both in (23.6±10.4%) and adjacent to (<3cm from) FR faults (28.9±5.4%) than it is in the same horizon 1 m. from the fault (37.1±3.1%). Microstructures provide insight into near-surface distribution and accommodation of strain in unconsolidated sediments along active faults. Microstructures reveal differences between near-surface fault zone sediment from the 1906 eq rupture segment and that from the creeping segment of the SAF: (1) deformation band shear zones formed at AC but are absent at FR; (2) shear localization in deformation bands at AC resulted in narrow zones of grain breakage but only minor porosity loss; (3) faulting at FR resulted in porosity loss in and adjacent to faults without grain breakage. Differences in microstructure between samples from the AC and FR sites may reflect differences in strain accommodation for stick-slip vs. creeping faults.