An active large rock slide in the Andean paraglacial environment: the Yerba Loca landslide, central Chile
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Sergio A. Sepúlveda I Alejandro Alfaro I Marisol Lara I Javiera Carrasco I Paula Olea-Encina I Sofía Rebolledo I Mario Garcés
An active large rock slide in the Andean paraglacial environment: the Yerba Loca landslide, central Chile
Abstract A ca. 2.5 million m3 landslide occurred in August 2018 in the Yerba Loca valley, Andes Main Cordillera (33° 15′ S), at about 4000 m a.s.l. The Yerba Loca landslide is a multirotational slide, with a main scarp and failure surface developed in a volcanic rock mass, with secondary scarps and tilted blocks disturbing the colluvial soil cover. No clear trigger could be identified, although the failure took place some weeks after the largest winter precipitation and a sequence of snowfall and snowmelt, in the context of a severe drought. Inspection of optical satellite images suggests that the landslide suffered slow deformation for at least 15 years, increasing in the months prior to the failure. To corroborate these precursor deformations, InSAR analyses were performed at two time and spatial scales. For over 3 years, deformation in the landslide area was detected, while the local, short-term analysis from the 7 months before failure shows line-of-sight deformation rates at the landslide site of over 10 cm/year. Deformation continues after the failure with decreasing speed, with indications of further activity and expansion of the failure zone. This implies a hazard of rock avalanche, debris flows and/or river damming and subsequent outburst floods that may endanger communities downstream. The Yerba Loca landslide is an example of rock slope failure in paraglacial conditions and the influence of climatic factors in the context of climate change for the central Andes. This event represents an opportunity for learning on landslide mechanisms, remote sensing monitoring and hazard assessment of slow, large volume landslides in the Andean highlands. Keywords Landslides . Paraglacial slopes . InSAR . Geohazards . Chile Introduction In September 2018, a new large volume landslide in a remote area of the Andes Cordillera at the latitude of Santiago de Chile (33° 15′ S, Fig. 1) was reported by a team of glaciology researchers who detected it during an overflight over the area. An analysis of satellite images by the Chilean Geological Survey (Sernageomin 2019) showed that the landslide took place in mid-August 2018, in the glacial Yerba Loca valley, a tributary of the Mapocho River that drains the high mountains into the capital city of Santiago. The valley is part of the Yerba Loca National Sanctuary, a tourist attraction for trekking and excursions to the La Paloma glacier located in the catchment headers. The area is characterised by high, rough mountains up to over 5000 m a.s.l., with receding glaciers in the catchment headers and current paraglacial and periglacial environments in the glaciated valleys, prone to landslide activity. A large number of pre-historic large volume landslide deposits in glacial valleys are observed around the Santiago region (Antinao and Gosse 2009; Deckart e
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