Identifying coseismic subsidence in tidal‐wetland stratigraphic sequences at the Cascadia subduction zone of western North America

Abstract

Tidal-wetland stratigraphy reveals that great plate boundary earthquakes have caused hundreds of kilometers of coast to subside at the Cascadia subduction zone. However, determining earthquake recurrence intervals and mapping the coastal extent of past great earthquake ruptures in this region are complicated by the effects of many sedimento!ogic, hydrographic, and oceanographic processes that occur on the coasts of tectonically passive as well as active continental margins. Tidal-wetland stratigraphy at many Cascadia estuaries differs little from that at similar sites on passive-margin coasts where stratigraphic sequences form through nonseismic processes unrelated to coseismic land level changes. Methods developed through study of similar stratigraphic sequences in Europe provide a framework for investigating the Cascadia estuarine record. Five kinds of criteria must be evaluated when inferring regional coastal subsidence due to great plate boundary earthquakes: the suddenness and amount of submergence, the lateral extent of submerged tidal-wetland soils, the coincidence of submergence with tsunami deposits, and the degree of synchroneity of submergence events at widely spaced sites. Evaluation of such criteria at the Cascadia subduction zone indicates regional coastal subsidence during at least two great earthquakes. Evidence for a coseismic origin remains equivocal, however, for the many peat-mud contacts in Cascadia stratigraphic sequences that lack (1) contrasts in lithology or fossils indicative of more than half a meter of submergence, (2) well-studied tsunami deposits, or (3) precise ages needed for regional correlation. Paleoecologic studies of fossil assemblages are particularly important in estimating the size of sudden sea level changes recorded by abrupt peat-mud contacts and in helping to distinguish erosional and gradually formed contacts from coseismic contacts. Reconstruction of a history of great earthquakes for the Cascadia subduction zone will require rigorous application of the above criteria and many detailed investigations. , 1994a, b; Darienzo et al., 1994]. In some estuaries in Oregon and northern California, less widely exposed buried soils may have subsided during earthquakes on ]. However, identifying evidence of coseismic subsidence in the Cascadia tidal-wetland stratigraphic record is complicated by the effects of many sedimentologic, hydrographic, and oceanographic processes found on the coasts of tectonically passive as well as active continental margins. At many estuaries along the west coast of central North America, tidal-wetland stratigraphy differs little from the stratigraphy produced by these ubiquitous processes on the temperate-latitude coasts of passive margins [Nelson and Personius, 1991; Nelson, 1992b]. 1991; Clague and BobrowskyIn this paper, we summarize factors causing relative sea level (RSL) changes along subduction zone coasts, outline a research framework and approach for inferring these types of changes, developed through studies along the passive-margin coasts ...

Seismic and non-seismic influences on coastal change in Alaska--Fieldtrip guide and conference abstracts, 5th International Conference of IGCP 588

“…If these sediments were preserved then sampled in an investigation employing only a single core, they could be misinterpreted as indicating rapid relative sea-level change or coseismic subsidence during an earthquake. A systematic stratigraphic survey, as outlined in the criteria of Nelson et al (1996) will prevent such a misinterpretation.…”

“… In analyzing coastal sediment sequences, four critical criteria help determine a coseismic signal and discriminate from nonseismic processes that might cause abrupt wetland submergence or emergence: lateral extent of peat-mud couplets (subsidence) or mud-peat couplets (uplift) with sharp contacts; suddenness of subsidence or emergence; amount of vertical motion; and synchroneity with other sites (Nelson et al, 1996). Presence of tsunami sediments may also help in certain geographic settings.…”

“…The aims for this stop are:  Introduce the " Nelson et al (1996) criteria" for distinguishing between coseismic and nonseismic relative sea-level change.  Outline the principles of using modern and fossil biological indicators to reconstruct elevation change.…”

Relative sea-level response to Little Ice Age ice mass change in south central Alaska: Reconciling model predictions and geological evidence

“…Research in the Pacific North West, Alaska and Japan produces various models of the cycle of land uplift and subsidence during and between great earthquakes through the Holocene (termed earthquake deformation cycles) with each applicable to specific subduction zones (Atwater, 1987;1992;Atwater et al, 1995;Atwater and Hemphill-Haley, 1997;Hamilton and Shennan, 2005a;Kelsey and Bockheim, 1994;Kelsey et al, 2002;Long and Shennan, 1994;1998;Nelson et al, 1995;1996a;1996b;Savage and Thatcher, 1992;Sawai, 2001;Shennan et al, 1996;Thatcher, 1984). Using the criteria established by Nelson et al (1996b) to test for regional co-seismic submergence accompanying a great earthquake and radiocarbon dating in situ horizontally bedded plant macrofossils from the top of fossil peat layers, seven great earthquakes have been identified in the last 4000 years in upper Cook Inlet (Hamilton and Shennan, 2005a;2005b;Shennan and Hamilton, 2006;Shennan et al, 2008).…”

“…Using the criteria established by Nelson et al (1996b) to test for regional co-seismic submergence accompanying a great earthquake and radiocarbon dating in situ horizontally bedded plant macrofossils from the top of fossil peat layers, seven great earthquakes have been identified in the last 4000 years in upper Cook Inlet (Hamilton and Shennan, 2005a;2005b;Shennan and Hamilton, 2006;Shennan et al, 2008). Similarities between each earthquake cycle leads to an EDC model for upper Cook Inlet with coseismic submergence and RSL rise, followed by rapid post-seismic uplift and RSL fall in the decades after the earthquake (Figure 2).…”