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5.4.2: Earthquakes in the Estuaries

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    It was in the bays and estuaries along the coastline that the most conclusive evidence for great earthquakes was found by Brian Atwater and his American and Canadian colleagues, as stated in the Introduction (Figures 4-10a, b, 4-11). From Port Alberni, at the end of a deep fjord on the west coast of Vancouver Island, to Sixes River in southern Oregon, and at many bays and estuaries in between (Figure 4-12), the sediments give evidence of sudden drops in the land level. The marshes and forests were found to be overlain directly by gray clay with marine microfossils (Figure 4-10), which Atwater could explain only by the sudden subsidence of the coastline. Some of these drops appeared to have been accompanied by great waves from the sea that deposited sand on the marsh deposits (Figure 8-12). The last of these waves struck about three hundred years ago. Atwater’s explanation was the catastrophic explanation: great earthquakes in the subduction zone.

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    Scientists from Japan, England, and New Zealand, including specialists in the ecology of marshes and estuaries, critically scrutinized this evidence to look for defects in Atwater’s earthquake hypothesis and to search for another, less apocalyptic, explanation. They were unable to find support for a non-seismic explanation for any of seven marsh soils buried at Willapa Bay in southwest Washington. For some of the buried marsh deposits, however, the evidence is ambiguous. These could have other origins such as gigantic Pacific storms or changes in the configuration of the estuary itself. But all soon agreed that the burial of marshes that took place three hundred and sixteen hundred years ago, at least, was caused by sudden submergences of the coast at the time of two great Cascadia earthquakes (Figure 4-10 and 4-11). Later, other burials would also be blamed on earthquakes.