LIQUEFACTION FEATURES ON THE FRASER DELTA - EVIDENCE FOR PREHISTORIC EARTHQUAKES

Sand dykes and sills and vented sand boils on the Fraser River delta and adjacent Serpentine River floodplain in southwestern British Columbia record one or more major liquefaction events. The source of the dykes is a shallow subsurface saturated sand unit that was deposited in foreslope and distributary-channel environments during Holocene progradation of the Fraser delta. The dykes cut steeply through a crust of delta topset muds and locally flatten out as sills in overlying peaty sediments. At two sites, evidence was found for venting of sand onto a subaerial or intertidal surface. Liquefaction and upward movement of sand and water locally deformed the intruded sediments, causing some subsidence and uplift of the delta surface. All observed liquefaction features are younger than ca. 3500 BP, and at least some are younger than 2400 BP. It is not clear, however, whether they formed during one or several separate events. A review of possible causes suggests that the liquefaction features probably are seismically generated; they thus may provide die first direct evidence for moderate to large, prehistoric earthquakes in the Vancouver metropolitan area.