The San Eduardo Landslide (Eastern Cordillera of Colombia): Reactivation of a deep-seated gravitational slope deformation

Abstract

The recent San Eduardo Landslide (SEL), reactivated on the nighttime of September 20, 2018, is hypothesized as the lowest part of a large-scale deep-seated gravitational slope deformation (DSGSD) covering an area not less than 45 km2. Based upon remote sensing analysis through aerial photography, accompanied by fieldwork, this work attempts to identify the morphostructural features developed after reactivation of the SEL, as well as relict features upslope indicating active sagging of the adjacent mountain slopes. A preliminary segmentation of the SEL into three zones (SEL-1 to SEL-3) is proposed in order to describe the spatial distribution of extensional (e.g., uphill-facing scarps, trenches), compressional (toe bulging), and shear zones within the slid mass. To complement the geomorphological approach, displacement vectors and motion rates were derived by comparing a 5-month time series of free-cloud Sentinel 2 imagery acquired from September 2018 to February 2019. This approach helps to identify a peak motion rate of > 4 m/day in zones SEL-2 (between September 18 and 28, 2018) and SEL-3 (between September 8, 2018, and October 8, 2018). By February 2019, the motion rate decreased to less than 30 cm/day. The potential mechanisms explaining the triggering/reactivation of the large slope deformation observed are the cumulative displacement on a shale-dominated slope, and the high topographic stresses expected for this high-relief area in the Colombian Eastern Cordillera. Finally, this work explores the geomorphological evidence of a second DSGSD (sackung type) in the area, which in turn suggests that gravitational deformation is probably a more common deformation mechanism in the range than previously considered.