The Pacific Northwest may experience enormous earthquakes (Magnitude 8.0 to 9.0)

Question

The Pacific Northwest may experience enormous earthquakes (Magnitude 8.0 to 9.0) about every 300 to 500 years. These earthquakes occur along the contact between the Juan de Fuca Plate and the North American Plate. (Unlike the Satsop earthquake in 1999, the Duvall earthquake in 1996, or the Vashon earthquake in 1995, which occurred within the continental plate, or the Olympia (Nisqually) earthquake in 2001, which occurred within the oceanic plate.) The last really big earthquake in the Northwest occurred on January 26, 1700 A.D. What types of geologic evidence might we look for in the Pacific Northwest that records the occurrence of this earthquake? (Remember, the cities of Seattle and Bellevue didn't exist at that time.)

Explanation / Answer

The most important clue linking the tsunami in Japan and the earthquake in the Pacific Northwest comes from studies of tree rings (dendrochronology), which show that red cedar trees killed by lowering of coastal forests into the tidal zone by the earthquake have outermost growth rings that formed in 1699, the last growing season before the tsunami

Local indigenous American oral traditions describing a large quake also exist, although these do not specify the date.The tsunami swept across the Pacific also causing destruction along the Pacific coast of Japan. It is the accurate descriptions of the tsunami and the accurate time keeping by the Japanese that allows us to confidently know the size and exact time of this great earthquake.

There are many areas in the Pacific Northwest with drowned groves of trees that also show evidence of the earthquake. This earthquake also left signatures in the geological record as the outer coastal regions subsided and drowned coastal marshlands and forests that were subsequently covered with younger sediments. The recognition of definitive signatures in the geological record tells us the January 26, 1700 event was not a unique event, but has repeated many times at irregular intervals of hundreds of years. Geological evidence indicates that 13 great earthquakes have occurred in the last 6000 years.
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