Effects of shear band propagation on early waves generated by initial breakoff of tsunamigenic landslides

The landslide mechanism based on the phenomenon of the dynamic shear band propagation suggests that landslide run-out begins with a non-zero initial velocity. The initial uplift of the passive failure block during the landslide breakoff is similar to the reverse-fault, and therefore can generate an early tsunami wave that will be one of the first waves to start propagating and could be potentially detected by the deep ocean warning systems. This paper attempts to quantify this phenomenon using the Coupled Eulerian-Lagrangian (CEL) approach within ABAQUS and modeling both soil and water as Eulerian materials. This approach allows for capturing the entire process of failure initiation, propagation and breakoff, taking into account large landslide deformations, soil plasticity and interaction of the sliding layer with the seawater. The study shows that the highest breakoff velocities are achieved for the shortest lengths of the initial weak zones initiating the shear band propagation. It also shows that both interaction of the sliding layer with water and appearance of the large plastic/failure zones during the shear band propagation cause slower uplift velocities and accelerations. For the computed example of a typical mid-size landslide in normally consolidated clays, with the initial weak zone length of several hundred meters and the final landslide length of about 1 km, the breakoff velocities can reach up to 4 m/s within 15 s, leading to an early wave of 0.5-1.5 m height, caused purely by this breakoff. Such waves can be easily detected by existing Deep-ocean Assessment and Reporting of Tsunamis (DART) systems, potentially allowing for some early tsunami warning.