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Linking Inca terraces with landslide occurrence in the Moquegua region, Peru

Ronda, Gonzalo
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2025-05-26
Abstract
Terracing techniques have been used globally to increase the area and quality of croplands in mountain communities. Since the times of the Incas, farmers in the remote Andes of Peru have constructed terraces to grow crops in a landscape characterized by steep slopes, semi-dry climate, and landslide geohazards. Recent investigations have concluded that terracing and irrigation techniques could enhance landslide risk due to the increase in water retention and percolation, and interception of surface flow, in unstable slopes leading to failure (e.g., García-Chevesich et al., 2021). In this study, three methodologies were used to assess the spatial coherence, causative relations, and geomechanical processes linking landslide presence and Inca terraces in a 250 km2 area located in the wet valleys of San Cristobal, Moquegua district, southern Peru. During an initial phase, an inventory including 170 landslides of the Area of Interest (AoI) was generated in Google Earth and field checked for quality assurance. Increased landslide density was found around main rivers and communities in the AoI, with a predominance of rotational and translational landslides. To assess spatial coherence, a tool was developed based on the confusion matrix approach. Precision and recall statistical parameters were quantified for areas close to main rivers and communities where larger spatial coherence was observed, yielding precision and recall values between 64% and 81%. On a larger scale, poor performance was obtained pointing to the existence of additional processes accountable for landslide presence. To investigate the role of other natural variables in landslide prediction, a logistic regression analysis was performed. The results showed that terrace presence is a statistically relevant factor when predicting landslide presence, apart from first-order natural variables like distance to rivers, curvature and geology. To explore potential geomechanical processes linking terraces and slope failures, FEM numerical modeling was conducted. The results suggested that both decreased permeability and increased surface irrigation, at 70 % of the average annual rainfall, are capable of inducing slope failure. Overall irrigated terraces appear to further promote slope instability in an area characterized by fluvial erosion, high relief and poor geologic materials, exposing local communities to increased landslide risk.
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