Gigantic submarine landslides pose a significant threat to offshore infrastructures and may be initiated by an inconspicuous weakened zone, followed by catastrophic shear band propagation under the effect of strain softening. Much attention has been paid over the last two decades to unidirectional (parallel to the travel direction of sliding mass) shear band propagation (SBP) related exclusively to an initiation zone that is wide relative to the SBP direction. The paper investigates for the first time the mechanisms of more realistic planar shear band propagation via numerical modelling including finite element and finite difference analyses. Rather than propagating only along the travel direction for a 1D shear band in a plane-strain slope, the multidirectional propagation behaviour of a planar shear band depends significantly on the aspect ratio of the initiation zone. New criteria for catastrophic propagation of a planar shear band are proposed and are shown to be in good agreement with the numerical results. The criteria with respect to the initiation zone area can be incorporated easily within a GIS-based stability analysis of catastrophic submarine landslides, facilitating robust hazard analysis for offshore developments.