Assessing Locations Susceptible to Landslide Initiation During Prolonged Intense Rainfall in the Lares, Utuado, and Naranjito Municipios of Puerto Rico

Abstract. Hurricane María induced about 70,000 landslides throughout Puerto Rico, USA, including thousands each in three municipalities situated in Puerto Rico’s rugged Cordillera Central range. By combining a nonlinear soil-depth model, presumed wettest-case pore pressures, and quasi-three-dimensional (3D) slope-stability analysis we developed a landslide susceptibility map that has very good performance and continuous susceptibility zones having smooth, buffered boundaries. Our landslide susceptibility map enables assessment of (1) potential ground-failure locations, and (2) areas of potential landslide sources to support a companion assessment of inundation and debris-flow runout. The quasi-3D factor of safety, F₃, showed strong inverse correlation to landslide density (high density at low F₃). Area under the curve (AUC) of True Positive Rate (TPR) versus False Positive Rate indicated success of F₃ in identifying head-scarp points (AUC=0.84) and source-area polygons (0.85 ≤ AUC ≤ 0.88). The susceptibility zones enclose specific percentages of observed landslides. Thus, zone boundaries use successive F₃ levels for increasing TPR of landslide head-scarp points, with zones bounded by F₃ at TPR=0.75, very high; F₃ at TPR=0.90, high; and the remainder moderate to low. The very high susceptibility zone, with 118 landslides/km², covered 23 % of the three municipalities. The high zone (51 landslides/km²) covered another 10 %.