Displacement mechanisms of slow-moving landslides in response to changes in pore water pressure and dynamic stress

Although slow-moving landslides represent a substantial hazard, their detailed mechanisms are still poorly understood. We have conducted a suite of innovative laboratory experiments using novel equipment to simulate a range of pore water pressure and dynamic stress scenarios on samples collected from a slow-moving landslide complex in New Zealand. We seek to understand how changes in pore water pressure and ground acceleration during earthquakes influence the movement patterns of slow-moving landslides. Our experiments show that during periods of elevated pore water pressure, displacement rates are influenced by two components: first, an absolute stress state component (normal effective stress state) and second, a transient stress state component (the rate of change of normal effective stress). During dynamic shear cycles, displacement rates are controlled by the extent to which the forces operating at the shear surface exceed the stress state at the yield acceleration point. The results indicate that during strong earthquake accelerations, strain will increase rapidly with relatively minor increases in the out of balance forces. Similar behaviour is seen for the generation of movement through increased pore water pressures. Our results show how the mechanisms of shear zone deformation control the movement patterns of many large, slow moving translational landslides, and how they may be mobilised by strong earthquakes and significant rain events.