Reconstructing the Tsunami Record on an Emerging Coast: a Case Study of Kanim Lake, Vancouver Island, British Columbia, Canada
Keywords:Cascadia subduction zone, diatoms, lakes, relative sea level, tsunami deposits, Vancouver Island
A pilot study was conducted at Kanim Lake on the emerging coast of western Vancouver Island, British Columbia, to assess the efficacy of using lake sediments to determine tsunami run-up and recurrence. Sediment sequences in lakes near the coast can complement tsunami records derived from deposits underlying intertidal marshes. Marshes on emerging coasts are uncommon, of limited areal extent, and, most importantly, their deposits have a short lifespan. Tsunami deposits in lakes are less susceptible to bioturbation and erosion and, generally, can be more accurately dated than similar deposits in marshes and other terrestrial settings. An inferred tsunami deposit in Kanim Lake has distinctive lithological characteristics and contains marine and brackish-water microfossils. Kanim Lake also illustrates some of the limitations in using lakes to reconstruct tsunami run-up and recurrence. Although the lake has been in the potential run-up zone for tsunamis triggered by great earthquakes on the nearby Cascadia subduction zone for the last 3,500 - 4,000 years, it apparently has been inundated by only one tsunami in this period. This event probably occurred about 2,800 years ago. Tsunamis since that time have failed to reach Kanim Lake as the lake basin has continued to rise through the run-up zone and the distance to the sea has increased. The development of dense forest stands on the progressively widening reach between the sea and the lake has probably been the most important factor in limiting tsunami access to the site.