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6.2.2: Other Tsunamis in the Pacific Northwest

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    6044
  • Overview

    The 1964 tsunami was the most damaging to strike the Pacific Northwest in recorded history, but it was not the only one. Tide-gauge records show that subduction-zone earthquakes in the Aleutian Islands generated tsunamis that were detected in Tofino and Victoria, B.C., Neah Bay, Washington, and Crescent City, California. A larger tsunami resulted from the 1960 earthquake in southern Chile of M9.5, the largest of the twentieth century (Figure 4-17). Wave heights for the Aleutian-based tsunamis were about one-fourth those in 1964, and those accompanying the Chilean earthquake were about half the wave heights in 1964. For all pre-1964 tsunamis, Crescent City had the greatest wave heights, as it did in 1964, evidence that there is something special about the configuration of the seafloor off Crescent City that causes focusing of tsunami waves as they enter the shallow water.

     

    The next tsunami arrived on March 11, 2011, as a result of the subduction-zone earthquake of M 9 off Japan. That earthquake was similar to the next earthquake expected on the Cascadia Subduction Zone. Its directivity caused greater effects off the California coast than in areas farther north, but it indicated that the truly great earthquakes would have effects thousands of miles from their sources. The damage was largely restricted to small boat basins in Brookings and Depoe Bay, Oregon, and Crescent City, California. Figure 9-6 is a photo of the tsunami wave near Emeryville, California, north of Oakland.

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    The twentieth-century tsunamis were relatively small compared with the tsunamis that struck the northeastern Japan coast on March 11, 2011, or those that accompanied the last Cascadia Subduction-Zone earthquake in January 1700. As pointed out in Chapter 4, this tsunami even did damage in Japan, more damage than the 2011 Tohoku-oki tsunami did in western North America. The buried peat deposits at Willapa Bay, Washington, and other areas are directly overlain by layers of laminated sand that were derived from the sea (Figure 9-5a). Figure 9-5b is a photo of a deposit from the 1700 earthquake that overlies Native American fire pits. The sand thickness and grain size diminish away from the sea. Tsunami sands from inlets on northwestern Vancouver Island preserve a record of both the A.D. 1700 tsunami and the tsunami that came from Alaska in 1964. In southern Oregon, Bradley Lake formed behind a sandbar, showing evidence of giant waves sweeping across the sandbar and depositing sand and marine diatoms in the lake. Thus it is likely that the next Cascadia Subduction Zone earthquake will be accompanied by a devastating tsunami. The damage will be worse in Oregon, Washington, and Vancouver Island because the tsunami will strike areas that have just subsided several feet as a consequence of the earthquake, just as coastal areas of Japan subsided during the 2011 Tohoku-oki earthquake and tsunami.

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    The earthquake on the Seattle Fault also produced a tsunami that was recorded at the base of Magnolia Bluff in Seattle, at the mouth of the Snohomish River near Everett, and on the south end of Whidbey Island. NOAA, in addition to planning for a tsunami on the Cascadia Subduction Zone, also is planning for a tsunami accompanying rupture of the Seattle Fault.

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