Amount of subsidence during a late Holocene earthquake—evidence from fossil tidal marsh foraminifera at Vancouver Island, west coast of Canada

Fossil foraminifera in intertidal deposits beneath a marsh near Tofino on west-central Vancouver Island, British Columbia, provide a basis for estimating the amount of subsidence during a large earthquake less than 400 years ago. We compared the fossil foraminiferal assemblages with present-day assemblages along a surveyed transect across the marsh, and we estimated paleo-elevation by means of both subjective and quantitative methods. The comparisons suggest that the site subsided at least 0.2 m and perhaps more than 1 m during the earthquake. Most of the subsidence was tectonic, for the deposits rest on a Pleistocene substrate unlikely to have compacted much when shaken. Soon after the earthquake, the land began to rebound tectonically. Most of the 0.2–1 m of recorded subsidence was recovered during deposition of the first 6 cm of sediment above the buried marsh surface, probably within a few decades after the earthquake. Because of the speed at which the land rebounded, some recovery may have occurred before the first post-earthquake sediments were deposited. If so, the actual amount of subsidence is larger than the foraminiferal data indicate.