The Saguenay Fjord, Quebec, Canada: integrating marine geotechnical and geophysical data for spatial seismic slope stability and hazard assessment

In 1996 a major flood occurred in the Saguenay region, Quebec, Canada, delivering several km3 of sediment to the Saguenay Fjord. Such sediments covered large areas of the, until then, largely contaminated fjord bottom, thus providing a natural capping layer. Recent swath bathymetry data have also shown that sediment landslides are widely present in the upper section of the Saguenay Fjord, and therefore, should a new event occur, it would probably expose the old contaminated sediments. Landslides in the Upper Saguenay Fjord are most probably due to earthquakes given its proximity to the Charlevoix seismic region and to that of the 1988 Saguenay earthquake. In consequence, this study tries to characterize the permanent ground deformations induced by different earthquake scenarios from which shallow sediment landslides could be triggered. The study follows a Newmark analysis in which, firstly, the seismic slope performance is assessed, secondly, the seismic hazard analyzed, and finally an evaluation of the seismic landslide hazard is made. The study is based on slope gradients obtained from EM1000 multibeam bathymetry data as well as water content and undrained shear strength measurements made in box and gravity cores. Ground motions integrating local site conditions were simulated using synthetic time histories. The study assumes the region of the 1988 Saguenay earthquake as the most likely source area for earthquakes capable of inducing large ground motions in the Upper Saguenay region. Accordingly, we have analyzed several shaking intensities to deduce that generalized sediment displacements will begin to occur when moment magnitudes exceed 6. Major displacements, failure, and subsequent landslides could occur only from earthquake moment magnitudes exceeding 6.75.