LARGE TSUNAMI IN THE SALISH SEA: GEOLOGICAL EVIDENCE FROM LILY POINT, POINT ROBERTS, WASHINGTON STATE AND POTENTIAL GENERATION MECHANISMS

New evidence for a large tsunami at Lily Point at Point Roberts in Washington State in the southern Georgia Strait portion of the Salish Sea, includes rip-ups, differential fluidization, injection squirts, wave-induced liquefaction, distinctive cross-bedding, massive graded-bedding, overlapping unconformities and flame structures. Sediment composition and organic matter allow estimation of timing, sequence, and size of waves. These can only be explained by a tsunami arriving from the south during the last interglacial period.

A large nearby earthquake-induced tsunami and/or major undersea landslide off the Fraser delta with wave heights of 10 to 50 m or more would explain the observed features. Minimal evidence for tsunami deposits found nearby suggest that the tsunami potentially resulted from repeated ruptures of a nearby large fault or delta failure. Evidence suggests this type of large event is rare. However, near Boundary Bay, the recurrence period may have been a few thousand years during the Holocene.

This paper examines several sources for the tsunami, including a potential large landslide off the north side of Orcas Island. However, the modeled size is insufficient to explain the Lily Point deposit features unless the landslide was much larger than modeled. The deposits also suggest more recent large waves, as well as a large traction flow which likely moved along a paleo shoreline towards the northwest. These features are all likely from the same event or series of events and preserved due to higher sea levels during the last interglacial period. The Lily Point deposit features cannot be explained by large glacial outburst floods down the Fraser River at the end of the ice age, nor by a large lahar from ancient eruptions of Mt Baker due to deposit age and composition. Absent a meteorite impact, it’s more likely a large earthquake tsunami originating along the Fraser Delta fault or similar nearby unmapped fault, caused the Lily Point deposits. The potential for future tsunamis demonstrates our need to improve our understanding of faults capable of causing tsunamis. The region may need an early warning system for landslides and active faults, along with public education and vertical evacuation areas for tsunamis of tens of meters.