Great Cascadia earthquakes and tsunamis of the past 6700 years, Coquille River estuary, southern coastal Oregon

Abstract

Research Article| October 01, 2003 Great Cascadia earthquakes and tsunamis of the past 6700 years, Coquille River estuary, southern coastal Oregon Robert C. Witter; Robert C. Witter 1William Lettis & Associates, Incorporated, 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA Search for other works by this author on: GSW Google Scholar Harvey M. Kelsey; Harvey M. Kelsey 2Department of Geology, Humboldt State University, Arcata, California 95521, USA Search for other works by this author on: GSW Google Scholar Eileen Hemphill-Haley Eileen Hemphill-Haley 3EHH Consulting Micropaleontology, 1871 Pickett Road, McKinleyville, California 95519, USA Search for other works by this author on: GSW Google Scholar GSA Bulletin (2003) 115 (10): 1289–1306. https://doi.org/10.1130/B25189.1 Article history received: 07 Jun 2002 rev-recd: 02 Jan 2003 accepted: 14 Apr 2003 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 Robert C. Witter, Harvey M. Kelsey, Eileen Hemphill-Haley; Great Cascadia earthquakes and tsunamis of the past 6700 years, Coquille River estuary, southern coastal Oregon. GSA Bulletin 2003;; 115 (10): 1289–1306. doi: https://doi.org/10.1130/B25189.1 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 Cascadia subduction zone earthquakes dropped tidal marshes and low-lying forests to tidal flat elevations 12 times in the last 6700 cal yr B.P. at the Coquille River estuary in southwestern Oregon. The youngest buried soil, preserved in tidal marsh deposits near the estuary mouth, records the A.D. 1700 earthquake that ruptured the entire Cascadia margin. Eleven other buried marsh and upland soils found in tributary valleys of the estuary provide repeated evidence for rapid, lasting relative sea-level rise interpreted as coseismic subsidence. Additional stratigraphic criteria supporting a coseismic origin for soil burial include: lateral soil correlation over hundreds of meters, fossil diatom assemblages that indicate a maximum of 1.2–3.0 m of submergence, and sand deposits overlying buried soils consistent with earthquake-induced tsunamis that traveled 10 km up the estuary.Twelve earthquakes occurred in the last 6500–6720 cal yr B.P., recurring on average every 570–590 yr. Intervals between earthquakes varied from a few hundred years to over 1000. Comparisons of the Coquille record to earthquake histories from adjacent sites in Oregon, southwestern Washington, and northwestern California suggest that at least two earthquakes in the last 4000 yr did not rupture the entire length of the subduction zone. An earthquake 760–1140 cal yr B.P. in southwestern Washington may have ruptured as far south as Coos Bay but probably stopped before it reached the Coquille estuary because no buried soil records the event, and tidal marsh conditions were set to record an earthquake. An earthquake limited to a southern segment of the Cascadia margin 1940–2130 cal yr B.P. probably did not rupture north of the Coquille estuary.An analysis of relative sea-level histories from either side of the Coquille fault failed to find conclusive evidence for late Holocene vertical deformation. However, we cannot preclude recent upper-plate faulting. If the fault is active, as geomorphic features suggest, then constraints on the highest possible elevation of mean tide level allow a maximum vertical slip rate of 0.2–0.4 mm/yr in the past 6200–6310 cal yr B.P. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.