Characteristics of the kinematics and geomorphology of slow-moving landslides in the Colca valley, South-Peru

Abstract

Slow-moving landslides cause large amounts of damage in mountainous regions and are known to be reactivated by seasonal rainfall and seismicity, although the mechanics of reactivation associated with these 2 forcing factors is still debated. In this case study, the kinematics and geomorphology are constrained and related to possible forcing factors for a number of remote slow-moving landslides in the Colca valley (Peru), which threaten local villages and roads. To do so, the horizontal displacement of the landslides has been studied with a well-tested methodology that combines spaceborne stereo-photogrammetry, orthorectification, and sub-pixel image correlation of satellite images. A dataset of images acquired the Pléiades satellites have been used for their stereo-imagery and high resolution (0.7 m) and was complemented by one image of the SPOT6 satellite to improve temporal resolution. From this, three Digital Terrain Models (DTM) have been produced of which the quality has been determined by comparison with GPS and with the DTMs among each other, resulting in an accuracy of ±70 cm. Subsequently, ten horizontal displacement maps have been assessed by comparing with a permanent GPS installed on one of the landslides and by determining the values of a stable area, resulting in displacements with an error margin of 5-18 cm and 16-29 cm for correlation between Pléiades images and Pléiades-SPOT6 images respectively. Horizontal displacements of 1-3 meters in a period of thirteen months were constrained for the seven landslides that were identified from the displacement maps. An increase in displacement values towards the toe of the landslides suggests thinning of the landslide towards the toe or removal of material by the river. The major forcing factor for all landslides is seasonal rainfall, although two different responses have been recognized: 1) displacement due to high seasonal rainfall and 2) displacement due to both high and low seasonal rainfall. Two possible solutions can be suggested: I) sensitivity to rainfall is increased due to earthquake-related damaging on the nearest landslides so that rainwater infiltrates more easily, and II) lithological differences due to the paleo-sedimentary environment cause a higher sensitivity to rainfall in landslides containing finer material.