DUNES AND EARTHQUAKES IN COASTAL SW WASHINGTON STATE

The Columbia River Littoral Cell (CRLC) system is composed of sand predominantly from one source, the Columbia River. A study of the dunes on the Washington portion of the CRLC show an accreting shoreline with a series of shore parallel dunes formed over the last 3000 to 5000 years. This dune system developed over scarps eroded into the beaches during subduction earthquake events. Three of these erosional scarps are easily recognized on ground penetrating radar (GPR) records and can be correlated between cell segments. The scarps have been tentatively dated at 300 years BP,1200 radiocarbon years BP and 2500 radiocarbon years BP. The spacing between the scarps decreases and the age of the oldest dune in each segment becomes younger as one proceeds northward from the Columbia River. These observations confirm a prehistoric record of net northerly longshore transport of sand which formed the accretional coastline. They also confirm that this process was punctuated by severe erosion episodes generated by great subduction zone earthquakes which affected the entire SW Washington coastline. In addition, in at least one case (the 300 ybp event) the tsunami associated with this event provided enough eolian sand to bury a forested back dune.