SEISMICALLY INDUCED SUBMARINE LANDSLIDES IN PUGET SOUND OVER THE LAST 1100 YEARS

Debris flows identified in sediment cores off the toes of large and very large submarine landslides throughout Puget Sound record at least three landslide episodes over the last 1050 years. Debris flow and landslide dates correlate well with previous paleoseismic studies, as well as seismites in Lake Washington and the offshore Cascadia turbidite record, though we can not link specific events on faults to the submarine landslide record. The youngest event, from 280 to 90 calBP, is identified off the toes of landslides from Port Gardner, Mukilteo, Alki Pt., Port Orchard, Beals Point, Poverty Bay, Dumas Bay, and Fox Island, and corresponds with the 2σ confidence interval in Lake Washington (400-130 calBP) and in the Cascadia channel (310-100 calBP) for the 1700 A.D. Cascadia earthquake, though similarly timed shaking from surface faults cannot be ruled out. The penultimate landsliding episode, from 650 to 510 calBP, is recorded at Port Gardner, Port Madison, Beals Point, Three Tree Point, Quartermaster Harbor, Poverty Bay, and Fox Island, and the age corresponds with the penultimate seismite in Lake Washington (680-520 calBP) as well as the penultimate event in the Juan de Fuca Channel (660-520 calBP). The oldest episode identified, from 1050 to 820 calBP, at Edgewater, Port Orchard, Poverty Bay, and Fox Island, may be related to the most recent surface rupture on the Tacoma Fault (1180-790 calBP) or the most recent surface rupture on the Seattle Fault (1050-1020 calBP). The age range also corresponds with the third youngest event in the Juan de Fuca Channel (1090-900 calBP) and overlaps two events present in Lake Washington (900-680 and 1050-1020 calBP). Landslide and debris flows may be from the same large shaking event or may represent separate events throughout the region similarly spaced in time. Submarine landslide distribution is controlled by the location of surficial faults, raising the possibility of surface fault activity being higher at similar times to Cascadia subduction zone activity, however, we can not definitively make that distinction.