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UBC Theses and Dissertations

Ground surface motions in the Fraser delta due to earthquakes Wallis, Douglas Montague


The purpose of this thesis is to investigate the potential ground surface motions in the Fraser Delta due to earthquakes. The geological history of the area is reviewed and information concerning the nature of the thick soil deposits that form the Delta is presented. The dynamic properties of the Delta soils are calculated and a model of the deposit is developed for use with existing computer analysis based on wave propagation theory. The computer analysis method involves the computation of the ground motions as a vertically propagating shear wave passes from bedrock, through soil layers, to the surface. The accuracy of the model was checked at three sites by comparison of the surface motions computed using a recorded bedrock object motion with the ground surface motion recorded during the same earthquake. The close correlation between the computed and recorded motions confirms the validity of the analysis method and the soil model. The surface motions resulting from larger, maximum probable, earthquakes are computed. It was found that the thick soil deposits of the Delta affect both the ground accelerations and the period of the peak structural accelerations. Under the influence of low magnitude earthquakes the maximum acceleration is larger on the surface of the deep soil deposits than it is on nearby bedrock outcrops, while for large magnitude earthquakes the reverse is true. During large magnitude earthquakes, short buildings with periods of 0.25 sec. will experience greater accelerations if they are founded on bedrock than they will if founded on the thick soil deposits of the Delta. Taller buildings with periods of about one second will experience far greater accelerations if they are founded on deep soil deposits than they would if founded on bedrock. Comparison of Standard Penetration Test data in the Fraser Delta with empirical relations indicates that under large magnitude earthquakes liquefaction is likely in the upper few meters of the Delta sand deposits, but is unlikely below the 6 or 9 meter depth.

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