POSSIBLE CO-SEISMIC LANDSLIDE AT RIALTO BEACH, PACIFIC COAST OF WASHINGTON

An enigmatic low-elevation terrace at Rialto Beach on the Pacific coast of Washington contains evidence of landsliding that may overlap in time with earthquakes. Models suggest the Rialto Beach area is currently undergoing interseismic subsidence, and simple elastic rebound theory would predict that areas experiencing interseismic subsidence would experience coseismic uplift. At our study location, dense, oxidized sand and gravel, with an upper surface 1 to 2 meters higher than the modern beach surface, are interpreted as older beach deposits with a maximum age of 555 to 652 cal years BP. The higher elevation of the older beach surface could reflect uplift or deposition at a time of higher sea level. The older beach deposits are overlain by alternating sequences of coarse and fine-grained debris flow/landslide deposits <1 to 3 meters thick. The lowest, wood-rich, fine-grained facies could represent a back-beach marsh and/or paleosol. Multiple 14C dates on detrital wood in the debris flow layers date between 150 to 315 cal years BP. We conclude that at least one deep-seated rotational landslide occurred that disturbed the forest on the terrace and slope and deposited multiple pulses of coarse and fine-grained materials. Spruce and alder trees then grew on the landslide debris and their trunks were subsequently inundated by a later debris flow (s) possibly from an adjacent steep gully. Dendrochronology is underway on logs collected from the debris flow deposits and in the alder ghost forest to refine the age of the event (s) and determine if the landslide or debris flows were triggered by shaking from the 1700 AD Cascadia Subduction Zone earthquake. Debris flows and landslides are common on the Olympic Peninsula due to deleterious orientations of discontinuities, weak deposits, and heavy precipitation. In addition to dating, modeling may be needed to determine if earthquake shaking could have triggered these events.