ASSESSMENT OF POTENTIAL HURRICANE-INDUCED SHALLOW LANDSLIDE SOURCES FOR NARANJITO MUNICIPALITY, PUERTO RICO

Here we describe the landslide initiation (source area) part of a landslide susceptibility assessment of Naranjito Municipality, Puerto Rico. Determining landslide initiation potential is part of an effort to determine debris-flow runout hazard for areas of Puerto Rico affected by Hurricane Maria. A nonlinear area and slope dependent (NASD) soil depth model and the Transient Rainfall Infiltration and Grid‐Based Regional Slope‐Stability Analysis (TRIGRS) applied across a high-resolution digital landscape show where shallow landslides are likely to initiate during future hurricanes. Soil texture and strength data aided model calibration against detailed event mapping. Slope-stability analysis varying cohesion and friction defined valid ranges of these soil strength parameters for landslide source slope and depth distributions derived from mapping and field studies. Subdividing the map area into geologically based parameter zones distinguished areas where average soil depth or strength differed from other map units (granitic bedrock or karstic limestone, respectively). Statistical comparison of results from 9 different soil models coded in the REGOLITH soil-depth software, each sampled over its respective parameter space, yielded a best fit to field-measured soil depth in landslide scars. Full saturation with the water table at the ground surface and (topographic) slope-parallel flow embodied the most likely hydrologic conditions for landslide initiation. We refined the extent of potential shallow landslide source areas from TRIGRS using a simplified 3-D slope stability analysis, which eliminated many false positives and narrowed the factor of safety, FS, ranges containing 75% and 90% of scarp points. Receiver Operator Characteristics (ROC) analysis of computed FS ranked the predictive success in identifying head-scarp points. The True Positive Rate for FS=1 was reasonable using the chosen soil strength parameters. Susceptibility categories capture 75% (FS=1.18) and 90% (FS=1.31) of the points, with FS ≤ 1.18, high; 1.18 < FS ≤ 1.31, medium, and 1.31 < FS, low susceptibility. The high susceptibility category, with > 48 scarp points/km², covers ~30% of the area. The source area susceptibility results provide the input for assessment of runout zones for highly mobile Hurricane Maria landslides.