Geological evidence for past large earthquakes in southwest British Columbia

Abstract

Large earthquakes have left evidence of land-level change, tsunamis, and strong shaking in coastal sediments in southwest British Columbia. Sudden lowering of the land during earthquakes accounts for buried tidal marsh and forest soils on Vancouver Island and near Vancouver. Some sand layers in peaty and muddy marsh sediments on western Vancouver Island were deposited by tsunamis triggered by Pacific earthquakes. Liquefaction features, including dykes and blows of mainly sandy sediments, provide direct evidence for ground shaking, but have been documented thus far only on the Fraser River delta south of Vancouver. Much of the evidence for subsidence, tsunamis, and shaking is attributed to great earthquakes at the boundary between the North America and Juan de Fuca plates, but some of the evidence probably results from one or more earthquakes within the crust of North America. In addition, some tsunami deposits on Vancouver Island are products of great earthquakes in Alaska.