TRACKING A LIMESTONE BEDROCK LANDSLIDE ON AN URBANIZED HILLSLOPE IN GUANICA, PUERTO RICO

The community of Alturas de Bélgica in Guánica, Puerto Rico is built on hills of Tertiary limestone in the southern region of the island. Aerial photos from the 1930s show hummocky topography and landslide characteristics in the area, and these units have been recognized to be vulnerable to large, sometimes disastrous mass movements, especially during extreme rainfall events and on dip slip slopes. An example of this is the 1985 Mameyes disaster that claimed over 125 lives and remains the deadliest landslide event within any United States jurisdiction. In addition, a geotechnical survey (Bernal, 2004) revealed a zone of movement approximately 20-30 feet deep on the same south-dipping slope that activated in 2024 in the Alturas de Bélgica community.

From May 7-8, 2024, over 8 inches of rainfall were recorded in the vicinity of Guánica, about 25% of the normal annual rainfall in this arid region. In the days after this rainfall event, movement was reported on the southern slope of the Alturas de Bélgica community, where damage to a road and homes was noticed. Over 20 families were displaced (El Vocero, 2024) and field observations indicate that the slide body covers an area over 10,000 m2.

In June 2024, we installed a timelapse camera at the headscarp of the landslide and 8 high-visibility markers. The image processing software ImageJ was used to measure movement. From June until the beginning of October, average movement was between 2-5 inches daily. From October to December, the movement was much less, occasionally up to 1 inch per day. The reduction in slope movement rate may be coincidental with nearby water infrastructure repairs, but this is not certain. Drone surveys were also conducted in June and October. Structure from Motion models and pre-event LiDAR data will be used to measure the volume displaced.

The active landslide movement provides an opportunity to evaluate the dynamics of bedrock failure under the influence of natural and anthropogenic factors. Our analysis does not show any direct correlation between movement rate after initiation and precipitation or local seismic activity. The mass movement highlights the complexity of such events in urban environments. Continued monitoring aims to serve to alert stakeholders if the movement rate increases at any point in the future.