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

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 earthquakeinduced 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 Co

Holocene Relative Sea-Level Changes from Near-, Intermediate-, and Far-Field Locations

“…Deglacial RSL histories of the Pacific coast [75][76][77][78][79][80][81][82][83][84][85] are supplemented by studies of the earthquake and tsunami history of the Cascadia subduction zone along the coasts of British Columbia, Washington, Oregon, and northern California [86][87][88][89][90][91][92][93].…”

Detection limits of tidal-wetland sequences to identify variable rupture modes of megathrust earthquakes

“…While these criteria were proposed with respect to tidal marsh sequences adjacent to the Cascadia subduction zone, they have proved valuable for numerous studies since (e.g. Clark et al, 2015;Dura et al, 2015;Dura et al, 2016;Dura et al, 2011;Engelhart et al, 2013;Garrett et al, 2015b;Grand Pre et al, 2012;Hamilton and Shennan, 2005a;Hayward et al, 2015;Kelsey et al, 2015;Leonard et al, 2004;McCalpin and Carver, 2009;Nelson et al, 2009;Nelson et al, 2006;Shennan et al, 2009;Witter et al, 2003).In this review, we first introduce general principles regarding indicators of relative sea-level change in tidal wetlands and the conditions in which paleoseismic indicators must be distinct from those resulting from non-seismic processes.Section 2 summarises the tectonic setting and Holocene chronology of great earthquakes in the region of the 1964 Alaska M w 9.2 earthquake since we use evidence from this region to revisit the criteria recommended by Nelson et al (1996) for identifying coseismic subsidence and use the evidence to test working hypothesis of variable rupture modes during the late Holocene.In section 3, we consider developments over the past 20 years in popular methods of reconstructing relative sea-level change and their application to paleoseismic records from tidal wetlands.Section 4 presents evidence from sites across southcentral Alaska to illustrate different detection limits and how these constrain alternative interpretations for marsh submergence and emergence.In section 5, we evaluate the predicted surface displacement of different rupture modes against the reconstructions of marsh submergence and emergence based on field data.In the final sections, we consider the broader implications of detection limits of tidal wetland sequences at subduction zones around the world and suggest an expansion of the Nelson et al(1996) criteria. …”

Tsunami recurrence in the eastern Alaska-Aleutian arc: A Holocene stratigraphic record from Chirikof Island, Alaska

“…The most comprehensive assessments are based on earthquake and tsunami histories that span multiple cycles of strain accumulation and release on the megathrust faults of subduction zones, that is, over many hundreds to thousands of years. Even in Japan, where extensive written records of earthquakes and tsunamis extend back more than a millennium, reconstructing the history of the greatest earthquakes and tsunamis requires thorough study of coastal stratigraphic archives (e.g., Minoura et al, 2001;Witter et al, 2003;Cisternas et al, 2005;Bourgeois et al, 2006;Shennan and Hamilton, 2006;Nanayama et al, 2007;Jankaew et al, 2008;Monecke et al, 2008;Sawai et al, 2012).…”