Slope failure of continental frontal ridges offshore Vancouver Island, British Columbia

Bathymetric data from the Northern Cascadia margin offshore Vancouver Island reveal several submarine landslide features on the seaward slopes of frontal ridges. The slides occur just landward of the deformation front of the subducting Juan de Fuca and Explorer plates. Possible trigger mechanisms for the slope failures include earthquakes, pore pressure changes induced by sea-level changes, and the dissociation of gas hydrates. Evidence of gas hydrate has been found beneath the frontal ridges. A bottom simulating reflection (BSR) has been identified in regional seismic data and logging data showed gas hydrate indicators including sonic velocity and high electrical resistivity. The influence of gas hydrate formation and dissociation on slope stability is of special interest since previous studies showed coincident depths of BSRs and failure planes. We investigate two slope failure events in detail using numerical modeling techniques such as finite and discrete element modeling. Hybrid techniques provide a means to model processes ranging from grain-scale interactions up to movements of the sliding body by addressing both the continuous and discontinuous aspects of the problem. These include the internal forces, the evaluation of material failure criterion, deformation, and interaction forces. Furthermore, tensile failure and crack propagation, for example caused by gas hydrate or by the gradual breakdown of the slope material, can be characterized. Particle flow using different shapes and properties can be simulated. By examining the effect of local sea-level changes, glacial rebound, and gas hydrate formation or dissociation on stresses and fluid pressures, the work involves modeling the failure conditions associated with a decrease in shear strength, an increase in pore pressure, and the possible development or re-opening of cracks. Beyond describing the trigger mechanism, we also have interest in reconstructing the dynamics of the slide events to explain their different morphologies (e.g., blocky or smooth) observed in the bathymetric data. The results of this study have potential to contribute to the assessment of local tsunami hazard by evaluating the present state of slope stability and by estimating the impact of the dynamic slope failure events on the movements of the water body.