© 2015, Thomas Telford Services Ltd. Catastrophic landslides pose significant threats to life and property, and in the case of submarine landslides damage to offshore infrastructure. Although widely discussed, the triggering mechanisms and propagation criteria for catastrophic failure remain open issues. This study investigates a particular case of shear band initiation history: creation of a fully softened initial failure zone in a thin lens of a weaker material causing catastrophic failure of an infinite planar slope in sensitive clay under undrained conditions. The corresponding shear band propagation criteria were derived analytically using a process zone approach and validated numerically using a static large-deformation finite-element method. New analytical solutions taking account of elastic deformations within the shear band and in the entire sliding layer are established for both a linear strength degradation curve and an exponential strength degradation curve. Advantages of formulating propagation criteria in terms of the critical length of the fully softened initial failure zone (excluding process zones) are demonstrated, with the more realistic exponential degradation case producing a more stringent criterion than the linear degradation case.