Seabed slope instability on the Fraser River delta

The Fraser River delta lies in one of the most earthquake-prone regions in Canada and is composed, in part, of loose sands that have a high seismic liquefaction potential. Engineering investigations at the western margin of the Fraser delta have demonstrated that seismic liquefaction is possible on the existing delta slope. There are several areas of past and present slope instability at the delta front. Present examples include chronic retrogressive slumping and debris flows at the mouth of the Main Channel and slow downslope creep movements in adjacent areas, where sediments are accumulating on the delta slope. Prehistoric mass movements may have been triggered by liquefaction during past earthquakes or may be related to recurrent slumping at the mouths of former distributary channels. In addition, failure may be linked to leaching of metastable marine sediments at the base of the Holocene deltaic sequence. Spontaneous failures occurring at present at the river mouth are due to the combined effects of rapid sedimentation and slope over-steepening, shallow gas held within the sediment mass, and tidal loading.