Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone

Research Article| November 01, 2008 Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone Harvey M. Kelsey; Harvey M. Kelsey 1Department of Geology, Humboldt State University, Arcata, California 95521, USA †E-mail: hmk1@humboldt.edu Search for other works by this author on: GSW Google Scholar Brian L. Sherrod; Brian L. Sherrod 2U.S. Geological Survey, Earth and Space Sciences, University of Washington, Seattle, Washington 98193-1310, USA Search for other works by this author on: GSW Google Scholar Alan R. Nelson; Alan R. Nelson 3U.S. Geological Survey, MS 966, Box 25046, Denver, Colorado 80225-0046, USA Search for other works by this author on: GSW Google Scholar Thomas M. Brocher Thomas M. Brocher 4U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Harvey M. Kelsey 1Department of Geology, Humboldt State University, Arcata, California 95521, USA Brian L. Sherrod 2U.S. Geological Survey, Earth and Space Sciences, University of Washington, Seattle, Washington 98193-1310, USA Alan R. Nelson 3U.S. Geological Survey, MS 966, Box 25046, Denver, Colorado 80225-0046, USA Thomas M. Brocher 4U.S. Geological Survey, 345 Middlefield Road, MS 977, Menlo Park, California 94025, USA †E-mail: hmk1@humboldt.edu Publisher: Geological Society of America Received: 25 Jun 2007 Revision Received: 18 Jan 2008 Accepted: 22 Jan 2008 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2008 Geological Society of America GSA Bulletin (2008) 120 (11-12): 1581–1597. https://doi.org/10.1130/B26282.1 Article history Received: 25 Jun 2007 Revision Received: 18 Jan 2008 Accepted: 22 Jan 2008 First Online: 02 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 Harvey M. Kelsey, Brian L. Sherrod, Alan R. Nelson, Thomas M. Brocher; Earthquakes generated from bedding plane-parallel reverse faults above an active wedge thrust, Seattle fault zone. GSA Bulletin 2008;; 120 (11-12): 1581–1597. doi: https://doi.org/10.1130/B26282.1 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 SocietyGSA Bulletin Search Advanced Search Abstract A key question in earthquake hazard analysis is whether individual faults within fault zones represent independent seismic sources. For the Seattle fault zone, an upper plate structure within the Cascadia convergent margin, evaluating seismic hazard requires understanding how north-side-up, bedding-plane reverse faults, which generate late Holocene fault scarps, interact with the north-vergent master-ramp thrust and overlying backthrust of the fault zone. A regional uplift at A.D. 900–930 involved an earthquake that nucleated at depth and included slip on both the master-ramp thrust and the back-thrust. This earthquake also included slip on some of the <6-km-deep north-side-up, bedding-plane reverse faults. At locales where the north-side-up reverse faults intersect the Puget Sound coast, an earthquake a few centuries earlier than the A.D. 900–930 regional uplift only uplifted areas within hundreds of meters north of the reverse faults. We infer that the bedding-plane reverse faults are seismogenic because shore platforms near the reverse faults have been abruptly uplifted during earthquakes when other shorelines in the Seattle fault zone were unaffected. Faults of the Seattle fault zone therefore can both produce regional uplift earthquakes, with or without surface displacement on the reverse faults, and produce earthquakes that rupture the bedding-plane reverse faults causing fault scarps and uplift localized to hundreds of meters north of these faults. This latter type of earthquake has occurred at least twice and perhaps three times in the late Holocene, and all these earthquakes preceded the regional coseismic uplift of A.D. 900–930. To account for the paleoseismic observations, we propose that the Seattle fault zone is a wedge thrust, with the leading edge being a fault-bend, wedge thrust fold. The active axial surface of the wedge thrust fold is pinned at the tip of the wedge, and a steeply north-dipping sequence of Tertiary sediment forms the south limb of the wedge thrust fold. Some of these steeply north-dipping, bedding-plane surfaces are seismogenic reverse faults that produce scarps. Earthquakes on the wedge thrust produce the regional coseismic uplift events, and earthquakes within the fault-bend fold cause the local uplift earthquakes. Thus, bedding-plane faults can rupture during earthquakes when the wedge thrust does not rupture but instead continues to accumulate seismic energy. 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