Landslides: a climatic marker in the Central Andes.

MOREIRAS S.M., VILLALBA R.

Oslo, Noruega

Congreso; 33th International Geological Congress; 2008

It is accepted that the main causes of landslide occurrence in the Central Andean region are related to weather and climate conditions and as such the understanding of these parameters’ influences may enable a greater knowledge of prediction and alarm systems. More than 480 rainfall-induced events have been reported along the valleys of the Las Cuevas and Mendoza rivers during the past century, with  (32ºLS), being debris-flows and rockfalls due to water filling cracks or saturation being the most common events. However, an unexpected low rainfall threshold (6.5 – 12.9 mm) was established from daily precipitation records of nearby meteorological stations, which could partially be explained by the reduced amount of annual precipitation (300 mm) and the abundant generation of debris in these mountain areas. Besides, the antecedent precipitation plays an important role. If a 5-day precipitation window previous to the landslide events is taken into account, the mean values of accumulated rainfall reach to 28 mm. Temporal variations in landslide occurrence are related to climatic anomalies linked to the Pacific (ENSO) and Atlantic Oceans. In the Cordillera, landslides triggered by rainfall are common during El Niño years, decreasing in number during La Niña events. In contrast, slope instability in the Precordillera, located east of the Cordillera, rises during wet periods induced by the incursion of wet air masses from the Atlantic. No significant differences were recorded between cold and warm ENSO events in the eastern Precordillera. Landslide occurrence has increased during the last three decades due to both precipitation and temperature increases. In addition, higher temperatures affecting OºC isotherm elevation have increased the proportion of rain in relation to snow precipitation, which in turn affect hill-slope instability. Thus, rainfall induced landslides have become more frequent in higher areas where snow avalanches predominated and landslides were rare in the past, meaning the snow avalanche/landslide ratio is decreasing in these areas. General Circulation Models (GCM) simulate an increased summer influence of the Atlantic wet air masses on the Precordillera during the XXI century. In contrast, for the same interval, the GCMs predict a reduction of winter snow across the main Cordillera (Principal and Frontal). According to our findings, landslide frequency would increase in the Precordillera during the XXI century, whereas the number of events may decrease in the main Cordillera. However, changes in the snowfall/rainfall ratio would encourage landslide activity in higher areas. Future changes in climatic regimes should be considered to properly evaluate the impact of landslides on regional socio-economic activities.