QUANTIFICATION OF THE EFFECT OF GEOLOGY, SOIL, AND LAND-USE ON LANDSLIDES IN PUERTO RICO CAUSED BY HURRICANE MARIA IN 2017

Hurricane María triggered over 40 thousand shallow landslides throughout Puerto Rico in September of 2017. Many of these mass wasting sites across the island destroyed roadways, affected homes, and even claimed lives. A noticeable concentration of mass wasting sites in areas underlain by certain susceptible geological units has been noted in preliminary observations by various parties. By using our new, complete-island digital landslide inventory, the distribution of landslides with respect to geologic unit, soil type, and land-use was further explored and quantified using the frequency-ratio method.

Quaternary deposits and limestone covers about 45% of the island, but only 16% of landslides occurred in these units. Volcanic and Volcaniclastic rocks make up the basement of the island arc and failed relatively more often than other units. Intrusive igneous rocks showed the highest concentration of slope failures, likely because of the chemical weathering pattern in the tropical setting. From the diverse array of soil types in Puerto Rico, 10 specific units show high correlation with landslides sites.

Soil type is highly controlled by geological substrate and these specific units coincide with bedrock types that were also exceptionally vulnerable during the extreme rainfall event. Low cohesion sandy soils, like those found around the intrusive plutonic units were especially susceptible to failure.

Areas classified as active and abandoned coffee plantations suffered landslides at a rate almost 4 times higher than in other areas of Puerto Rico. This correlation is likely due to vegetation removal and dense unsurfaced road networks. These factors have been the focus of study by other workers prior to Hurricane Maria. Urban areas, beaches, dry forests, and crop lands show almost no correlation with landslide inventory and serve to validate the data analysis process.

Based upon these and other analyses, our understanding of the landscape effects of extreme precipitation events such as Hurricane Maria, is improving and will help shape the preparation and planning for future tropical cyclone events in Puerto Rico and elsewhere.