Summary of Coastal Geologic Evidence for past Great Earthquakes at the Cascadia Subduction Zone

Abstract

Earthquakes in the past few thousand years have left signs of land-level change, tsunamis, and shaking along the Pacific coast at the Cascadia subduction zone. Sudden lowering of land accounts for many of the buried marsh and forest soils at estuaries between southern British Columbia and northern California. Sand layers on some of these soils imply that tsunamis were triggered by some of the events that lowered the land. Liquefaction features show that inland shaking accompanied sudden coastal subsidence at the Washington-Oregon border about 300 years ago. The combined evidence for subsidence, tsunamis, and shaking shows that earthquakes of magnitude 8 or larger have occurred on the boundary between the overriding North America plate and the downgoing Juan de Fuca and Gorda plates. Intervals between the earthquakes are poorly known because of uncertainties about the number and ages of the earthquakes. Current estimates for individual intervals at specific coastal sites range from a few centuries to about one thousand years.

Tectonic deformation in western Washington from continuous GPS measurements

“…Comparison of subduction zones around the world focused attention on the great seismic potential of the CSZ [Heaton and Kanamori, 1984] and today the CSZ is widely recognized as a source of major megathrust earthquakes in the recent geologic past. The most compelling evidence comes from paleoseismological investigations of buried coastal marshes, off-shore turbidites and tsunami records [Atwater et al, 1995]. …”

Reducing Seismic Risk to Highway Mobility: Assessment and Design Examples for Pile Foundations Affected by Lateral Spreading

“…In terms of seismic hazard, the Pacific Northwest shares similar conditions from a Cascadia Subduction Zone (CSZ) earthquake source with the expected earthquake magnitude of 9.0 (M w ) and return period of 300 years (Atwater et al 1995, Atwater andHemphill-Halley 1997 The purpose of the literature review for stone columns was to identify gaps in our knowledge as a basis for research. From the literature review, no consensus was found on the contribution of the shear stress redistribution mechanism of stone columns to mitigating liquefaction.…”

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).…”