Geological Evidence

Past great earthquakes along the Cascadia subduction zone are preserved in the geological record through the effects of sudden land-level changes, tsunamis, and strong ground shaking.

The clearest evidence for past earthquakes comes from tidal marshes along the outer coasts of Vancouver Island, Washington, Oregon, and northern California. Outcrops, excavations, and cores at more than a dozen estuaries along the length of the Cascadia subduction zone have revealed buried peat layers, interpreted to be former intertidal marsh surfaces that were submerged by abrupt coastal subsidence during past great earthquakes (Figs. 4, 5a; Atwater et al., 1995, and references therein). These layers consist of vegetation identical to that growing at the tidal marshes today. Analysis of plant and animal fossils shows that submergence and burial of marshes at many estuaries resulted from at least 0.5 m of sudden subsidence. Radiocarbon and tree-ring ages suggest that the last earthquake occurred about 300 years ago and that hundreds of kilometres of coast subsided during this event (Nelson et al., 1995). Ruptures lengths along the subduction zone that are implied by such widespread subsidence are consistent with magnitude 8 or larger earthquakes, but not with smaller events. The pattern of subsidence during the last earthquake is similar to the widespread subsidence of historic great earthquakes along subduction zones in Chile, south-central Alaska, and Japan.

Sheets of sand deposited by tsunamis that rushed into the subsided coastal zone mantle some of the buried peat layers (Fig. 4; Clague and Bobrowsky, 1994). Stems and leaves of fossil plants rooted in the uppermost buried soil are covered by, or extend into, the overlying sand, providing evidence of rapid burial (Fig. 5b). This strongly suggests that the tsunami was triggered by the same earthquake that caused the coast to subside.

Liquefaction features, which provide evidence for ground shaking during the last great earthquake at the Cascadia subduction zone, have been observed in banks of the lower Columbia River between Washington and Oregon up to 60 km inland from the coast. The features are about 300 years old and include sand dykes and sand blows that formed when liquefied sediment was injected upward along fractures and expelled onto a subsided surface (Fig. 5c; Atwater, 1994). Sand dykes and blows are also common on the Fraser River delta directly south of Vancouver (Clague et al., 1992; Naesgaard et al., 1992). Some of the most spectacular and best dated of these features, discovered recently on Annacis Island, are about 1700 years old, which is about the time of one of the great Cascadia subduction earthquakes (Clague et al., 1997).

Shaking from earthquakes at the Cascadia subduction zone also has been inferred from Holocene turbidites in deep-sea channels off the Pacific coast of Washington and Oregon. Turbidites are deposits of sediment-laden, swift-moving, bottom-flowing currents that travel down a subaqueous slope. They originate in various ways, such as by storm waves, tsunamis, and earthquake-induced sliding. Adams (1990) argued that turbidites in five separate channels were deposited simultaneously during great subduction earthquakes and that there have been 13 such events in the last 7600 years, an average of one earthquake about every 600 years.