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Microstructures developed by coseismic and aseismic faulting in near-surface sediments, San Andreas fault, California

¹ Department of Geology, Humboldt State University, Arcata, California 95521, USA
² William Lettis and Associates Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA
³ Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA
⁴ Lawrence and Associates, 2001 Market Street, Room 523, Redding, California 96001, USA
⁵ Department of Geology, Humboldt State University, Arcata, California 95521, USA

Evaluation of microstructures in unlithified near-surface sediments provides promising results for differentiating between earthquake rupture-related (coseismic) and creep-related (aseismic) structures formed by the San Andreas fault. Paleoseismic trenches excavated at two sites, Flook Ranch on the creeping section, and Alder Creek on the 1906 rupture trace, show contrasting fault-zone structures and microstructures in near-surface, late Holocene sand. At Alder Creek, the 1–2-m-wide fault zone consists of both faults and 2–10-mm-thick deformation bands. Deformation bands have preferred grain orientations 30° counterclockwise from the fault (viewed in the slip-parallel direction), broken and disaggregated grains, smaller than average grain size, and lower porosity than control samples. In contrast, at Flook Ranch, two 4–6-m-wide fault zones consist of multiple faults but lack deformation bands. Silty sand in the fault zone at Flook Ranch has preferred grain orientations 10° clockwise from the fault, lacks broken grains, and has comparable grain size but lower porosity than control samples. These microstructures record different deformation mechanisms in near-surface sediment: cataclasis at Alder Creek, and distributed deformation at Flook Ranch. Deformation bands on the 1906 rupture trace of the San Andreas fault at Alder Creek demonstrate that these structures, with their grain rotation, grain breakage, and localized porosity variations, can form coseismically in unlithified sediment. The grain bridge model accounts for fundamental microstructural characteristics of Alder Creek deformation bands, and it provides a connection between these microstructures and laboratory studies of stick-slip instability. Deformation bands are easily recognizable in field and trench exposures and may be a useful indicator of coseismic slip.

Key Words: fault zone • microstructure • San Andreas fault • paleoseismology • fault creep

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