Landslide generated tsunamis: A smoking gun

2 Landslides, both subaerial and submarine, commonly occur in the Puget Lowland and in Puget Sound. Large coastal landslides can produce tsunamis, thus creating potential hazards in areas that are not in close geographical proximity to the actual landslide. The western coast of Camano Island in northern Puget Sound is characterized by „scalloped ‟ indentations, the largest of which (1500 m x 275 m) is north of Onamac Point; such features are often the result of slope failure. To determine if the Onamac Point morphology was the result of a landslide, a marine geophysical survey and bottom sampling operations were undertaken aboard R/V Clifford A. Barnes. Seismic reflection data imaged a large slide mass, consisting of a series of concentric arcuate blocks of deformed glacial sediments, on the bottom of Saratoga Passage in 85 m of water. The volume of material involved in the slide was estimated in two ways. An isopach (thickness) map was prepared using the seismic reflection data, yielding a volume estimate of 17.2 x 10 6 m 3. Next, the island topography and nearshore bathymetry of the Onamac Point area were used to create a pre-slide or paleosurface. Differencing this surface and the present topography gave a volume of approximately 17.8 x 10 6