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Shallow Structure and Geomorphology along the Offshore Northern San Andreas Fault, Tomales Point to Fort Ross, California

2019; Seismological Society of America; Volume: 109; Issue: 3 Linguagem: Inglês

10.1785/0120180158

1943-3573

Samuel Y. Johnson, Jeffrey W. Beeson,

Geological formations and processes

Research Article| March 26, 2019 Shallow Structure and Geomorphology along the Offshore Northern San Andreas Fault, Tomales Point to Fort Ross, California Samuel Y. Johnson; Samuel Y. Johnson Corresponding Author aU.S. Geological Survey, Pacific Coastal and Marine Science Center, 2885 Mission Street, Santa Cruz, California 95060 U.S.A., sjohnson@usgs.gov Search for other works by this author on: GSW Google Scholar Jeffrey W. Beeson Jeffrey W. Beeson bFugro USA Marine, Inc., P.O. Box 740010, Houston, Texas 77274 U.S.A., j.beeson@fugro.com Search for other works by this author on: GSW Google Scholar Author and Article Information Samuel Y. Johnson Corresponding Author aU.S. Geological Survey, Pacific Coastal and Marine Science Center, 2885 Mission Street, Santa Cruz, California 95060 U.S.A., sjohnson@usgs.gov Jeffrey W. Beeson bFugro USA Marine, Inc., P.O. Box 740010, Houston, Texas 77274 U.S.A., j.beeson@fugro.com Publisher: Seismological Society of America First Online: 27 Mar 2019 Online Issn: 1943-3573 Print Issn: 0037-1106 © Seismological Society of America Bulletin of the Seismological Society of America (2019) 109 (3): 833–854. https://doi.org/10.1785/0120180158 Article history First Online: 27 Mar 2019 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Samuel Y. Johnson, Jeffrey W. Beeson; Shallow Structure and Geomorphology along the Offshore Northern San Andreas Fault, Tomales Point to Fort Ross, California. Bulletin of the Seismological Society of America 2019;; 109 (3): 833–854. doi: https://doi.org/10.1785/0120180158 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 SocietyBulletin of the Seismological Society of America Search Advanced Search Abstract We mapped a poorly documented 35‐km‐long section of the northern San Andreas fault (NSAF) zone between Tomales Point and Fort Ross, California. Mapping is largely based on high‐resolution seismic‐reflection profiles (38 fault crossings), multibeam bathymetry, and onshore geology. NSAF strike in this section is nearly parallel to plate motion, characterized by a slight (⁠∼2°⁠) northerly (transtensional) bend in the south between Tomales Bay and the Bodega isthmus, and a northwesterly (transpressional) ∼5° bend in the north between the Bodega isthmus and Fort Ross. The southern transtensional bend is the northern part of the now‐submerged, linear, ∼50‐km‐long and 1–2‐km‐wide, Tomales–Bodega valley. The valley floor is cut by a complex zone of subparallel, variably continuous fault strands, and the deformed valley fill is an inferred mix of late Quaternary marine and nonmarine strata. In the northern part of this elongate valley, Holocene fault offset occurred on two fault strands about 740 m apart. The northern transpressional bend is characterized by narrow, elongate, asymmetric basins containing as much as 56 m of inferred latest Pleistocene to Holocene sediment.Between Bodega Head and Fort Ross, the gently dipping (⁠∼0.8°⁠) shelf includes two large (4.8 and 5.9 km2⁠) zones of sediment failure that we speculatively correlate with the 1906 San Francisco NSAF earthquake. Similar sediment‐failure zones should be common along offshore reaches of the NSAF and other nearshore fault zones but have apparent limited preservation potential. Onland geomorphic impacts of the mainly offshore NSAF include: (1) northward upwarping of uplifted marine terraces in the transpressional zone north of Bodega Bay; and (2) blocking of littoral sediment transport by uplifts on the west flank of the NSAF at Bodega Head and Tomales Point, resulting in rapidly accreting beaches and large coastal sand dune complexes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.