3D analysis of 2014 Oso landslide

The United States witnessed its deadliest landslide on March 22, 2014 in Oso, Washington. Forty-three people were killed and the direct financial damage exceeded 170 million dollars. The topography of the slope prior to this landslide was complex due to the occurrence of several historical landslides on the same slope and three-dimensional (3D) features of the slope. This paper utilizes an inverse 3D limit equilibrium analysis to propose a 3D failure mechanism and sequence for the 2014 Oso Landslide. The 3D mechanism consists of two main phases (Phase I and II) with three retrogressive slides that were identified during field reconnaissance and confirmed via the 3D limit equilibrium slope stability analyses. The 3D analysis suggests that Phase II of the Stark et al. (2017) mechanism consists of two separate retrogressive slides instead of a single slide. The two slides of Phase II are referred to as Phase IIa and Phase IIb herein for comparison with Stark et al. (2017). Photographs herein show the geometry and runout of Phases IIa and IIb. After Phase IIa, sloughing of the steep scarp in the upper plateau onto the Phase IIb slide mass continued for months. The 3D analysis includes the complexities of the ancient landslide bench along with surrounding slope geometry and features, which help explain the initiation and direction of slide movement.