Factors contributing to hurricane-induced landslides in Puerto Rico: Implications for hazards and sediment mobilization

On September 20, 2017, Hurricane Maria traversed Puerto Rico and caused landslides that killed people, isolated communities, and destroyed infrastructure. These landslides also mobilized vast quantities of sediment and most landslides transitioned into highly mobile debris flows. We used a full-island event inventory of 71,431 hurricane-induced landslides (identified with aerial imagery) coupled with field studies of 123 landslides to gain insight into location-specific controls on landslide characteristics, consequent hazards, and sediment mobilization.

Frequency-ratio analyses of the landslide inventory indicated that landslide initiation correlated increasingly with proximity to roads, proximity to streams, soil moisture, bedrock composition, land cover, hillslope curvature, slope inclination, and soil class. The arithmetic mean landslide concentration by geological formation was as high as 97 slides/km² and 20 of the 152 formations had >25 slides/km². Field studies revealed substantial variation in landslide geometry with underlying geological formation. For the 7 formations for which we studied ≥9 slides in the field, median initial landslide areas ranged from 125 m² to 334 m², and median initial volumes from 57 m³ to 1,319 m³; high mobility resulted in larger final landslide areas and volumes that we have not quantified.

Landslide concentration by formation was not necessarily indicative of consequent hazard or volume of sediment mobilized. For example, the Utuado Granodiorite (Ku) suffered the highest concentration (97 slides/km²) but had the lowest median volume (63 m³) and a relatively low median slide area of 189 m². The Avispa Formation (Kav; basalt and tuffaceous sandstone) had a landslide concentration of 40 slides/km², the highest median volume (1,319 m³), and a relatively high median area of 320 m². Upon initiation, landslides in both formations presented hazard to similarly proportional areas as they affected ~13,000 m²/km² and ~18,000 m²/km², respectively. However, landslides in Ku mobilized an order of magnitude less sediment than did landslides in Kav (~6,000 m³/km² and ~53,000 m³/km², respectively). Analyses of factors contributing to differing landslide volumes and consequent effects on hazards and geomorphology are ongoing.