ESTIMATING LIQUEFACTION SUSCEPTIBILITY OF LATE PLEISTOCENE AND HOLOCENE DELTAIC DEPOSITS USING CONE PENETRATION TESTS AND THREE-DIMENSIONAL QUATERNARY GEOLOGY MAP DATA: IMPLICATIONS FOR PALEOSEISMOLOGICAL STUDIES

Regional earthquake hazard evaluations of deltaic deposits in three areas of southwest British Columbia are used to illustrate the utility of cone penetration tests and three-dimensional Quaternary geology data for evaluating liquefaction susceptibility. The west coast of BC is the most seismically active region in Canada and the SW corner of the province is home to about 2 million people. One method of mitigating the seismic hazard in the region, is through the use of earthquake hazard maps that show variation in liquefaction susceptibility. These maps are designed for emergency and land-use planning and are based on local site conditions such as the thickness, age and distribution of Quaternary stratigraphic units. Surficial geology maps, geotechnical borehole databases, water well logs and other subsurface data are used to define map units. Quantitative techniques such as cone penetration tests are then used to measure parameters required for liquefaction assessments of each unit. We combine the Seed method for determining liquefaction susceptibility with the probabilistic seismic model developed for the NBC, to provide a quantitative assessment of the liquefaction hazard that can be compared between different maps units and areas.

The Quaternary geology of deltaic deposits in the three illustrative case study areas is complex and there are a variety of stratigraphic units that contribute to the liquefaction hazard. Normally consolidated Late Pleistocene deltaic sands and gravels are the oldest deposits of concern for liquefaction. In general these deposits are relatively dense and have a low liquefaction susceptibility but late-stage surface channels with perched water tables locally pose a significant hazard. In contrast, many facies of Late Holocene delta deposits of the Fraser River such as top-set channel sand facies, are highly susceptible to liquefaction. An intermediate liquefaction hazard, between these two end members, is presented by parts of the Fraser River delta proximal to the mountains. Early Holocene deltaic sediments there are comparatively gravelly and have a lower liquefaction susceptibility than sandier downstream reaches. The liquefaction assessments confirm paleoseismological studies of the different deltaic facies and highlight prospective areas for further study.