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

Abstract

Research Article| July 01, 2007 Microstructures developed by coseismic and aseismic faulting in near-surface sediments, San Andreas fault, California Susan M. Cashman; Susan M. Cashman 1Department of Geology, Humboldt State University, Arcata, California 95521, USA Search for other works by this author on: GSW Google Scholar John N. Baldwin; John N. Baldwin 2William Lettis and Associates Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA Search for other works by this author on: GSW Google Scholar Katharine V. Cashman; Katharine V. Cashman 3Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA Search for other works by this author on: GSW Google Scholar Karl Swanson; Karl Swanson 4Lawrence and Associates, 2001 Market Street, Room 523, Redding, California 96001, USA Search for other works by this author on: GSW Google Scholar Ryan Crawford Ryan Crawford 5Department of Geology, Humboldt State University, Arcata, California 95521, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Susan M. Cashman 1Department of Geology, Humboldt State University, Arcata, California 95521, USA John N. Baldwin 2William Lettis and Associates Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA Katharine V. Cashman 3Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA Karl Swanson 4Lawrence and Associates, 2001 Market Street, Room 523, Redding, California 96001, USA Ryan Crawford 5Department of Geology, Humboldt State University, Arcata, California 95521, USA Publisher: Geological Society of America Received: 30 Nov 2006 Revision Received: 18 Feb 2007 Accepted: 21 Feb 2007 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2007) 35 (7): 611–614. https://doi.org/10.1130/G23545A.1 Article history Received: 30 Nov 2006 Revision Received: 18 Feb 2007 Accepted: 21 Feb 2007 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Susan M. Cashman, John N. Baldwin, Katharine V. Cashman, Karl Swanson, Ryan Crawford; Microstructures developed by coseismic and aseismic faulting in near-surface sediments, San Andreas fault, California. Geology 2007;; 35 (7): 611–614. doi: https://doi.org/10.1130/G23545A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.