Landslides as climate indicators in Argentinean Central Andes (32º S).

-MERGILI, M., MOREIRAS, S.M., FELLIN, W. Y STÖTTER, J.

Congreso; IAEG XII CONGRESS; 2014

IAEG

In the Central Andes of Argentina, summer rainstorms are a major forcing of landslide occurrence; still landslides are also triggered by earthquakes. More than 500 rainfall-induced events have been identified along the valleys of Las Cuevas - Mendoza rivers (32ºS) in the past century. Correlation of landslides occurrence with low threshold values, climatic phenomena, high slopes, and the great debris accumulation favor debris flows and rockfall generation. Argentinean Central Andes are characterised by an arid climate and a high topography reaching 7 km in altitude (eg. Aconcagua mount 6,958 m asl.) with dense international tourist attraction. Topography of this mountain landscape decreases toward the eastern piedmont (700 m.a.sl.) forcing precipitation behavior. Daily precipitation records suggest a relative low rainfall threshold (6.5 ? 12.9 mm) for landslide occurrence in middle elevations ranging 1500 to 2700 m asl. during South Hemisphere summer(Dec-Feb). This low threshold could partially be explained by the reduced amount of annual precipitation (300 mm) and the abundant generation of debris in these mountain areas. However, meteorological records are scarce in the region limiting a precise determination of the threshold values. Moreover, antecedent precipitation is important. If a 5-day precipitation window previous to the landslide events is taken in account, the mean values of accumulated rainfall reach to 28 mm. On the other hand, snowfall precipitation takes place during South Hemisphere winter in higher topography; as a consequence greater slope instability is recorded in the following spring period (Moreiras et al., 2012). Herein, landslide triggering factors and temporal distribution varies in the different ranges of the Argentinean Central Andes. Likewise temporal variations in landslide occurrence are related to climatic anomalies linked to the Pacific (ENSO) and Atlantic Oceans. Severity of debris flows generated by snow melting in the Main Cordillera is greater in the following year of above mean annual precipitation linked to the warm phase of ENSO phenomena (Moreiras et al., 2012), while landslides triggered by rainfall are common during El Niño years in the Cordillera Frontal, decreasing in number during La Niña events. In contrast, slope instability in the Precordillera, located east of the Cordillera Frontal, rises during wet periods induced by the incursion of wet air masses from the Atlantic. Not significant differences were recorded between cold-warm ENSO events in the Precordillera (Moreiras, 2005). The relationships between landslide occurrence in Main Cordillera, Frontal Cordillera, and Precordillera with climatic anomalies induced by the Pacific and Atlantic oceans respectively, suggest the possibility of using the landslide chronologies in the Central Andes as a geo-indicator of global climate change. Rockfall and debris-flow recurrence has increased during the last three decades due to both precipitation and temperature increases.