Foraminiferal evidence for the amount of coseismic subsidence during a late holocene earthquake on Vancouver Island, West Coast of Canada

Abstract

Foraminiferal data from two sites, 6 km apart, on the shores of an inlet near Tofino on the west coast of Vancouver Island, British Columbia, allow estimates to be made of the amount of coseismic subsidence during a large earthquake 100–400 years ago. The sampled sediment succession at the two sites is similar; peat representing a former marsh surface is abruptly overlain by intertidal mud grading upward into peat of the present marsh. At one of the sites, a layer of sand, interpreted to be a tsunami deposit, locally separates the buried peat from the overlying intertidal mud. The abrupt peat-mud contact records sudden crustal subsidence during the earthquake. The paleoelevation of each fossil sample was estimated by comparing its foraminiferal assemblage with modern assemblages of known elevation. The modern assemblages were obtained from surface samples collected along transects across the marsh near the fossil sample sites. Comparisons were made statistically using transfer functions. Estimates of coseismic subsidence, based on differences in paleoelevations just above and below the top of the buried peat, range from 20 cm to 1 m, with the most likely value in the 55–70 cm range. Post-seismic crustal rebound began soon after the earthquake and may have been largely complete a few decades later.