Mass-Wasting Events and related morphology along a coastal portion of

BEIGT, DÉBORA; GUSTAVO VILLAROSA; EDUARDO GÓMEZ

Mendoza

Congreso; 18th ISC 2010 Sedimentology at the foot of the Andes; 2010

IAS

A bathymetric survey using a Phase Measuring Bathymetric Sonar System (?GeoSwath Bathymetry System Plus?, GeoAcoustic Ltd.) (CONICET-IADO) was carried out at selected coastal areas of Nahuel Huapi lake on April 2007. The equipment records the bed morphology and sedimentology with a high (submetric) resolution up to 100m depth approximately (Gomez and Maraschin, 2006).The aim of the survey was to study the origin of the tsunami that hit the coasts of S. C. de Bariloche on 22 May 1960, destroying the pier of San Carlos harbour.Analysing bathymetric data acquired at the vicinity of the harbour, a large subaqueous landslide and recent masswasting deposits were identified below 70- 80m water depth (Villarosa et al., 2009). Through sedimentary core information and high resolution seismic profiles, the authors linked the mass-failure and consequent tsunami to the 1960 Valdivia earthquake, suggesting that the failure was probably induced by the presence of a non-cohesive surface (tephra layer?) that acted as a sliding surface. Based on these previous results, a hypothesis about the probable existence of landslide-prone areas in the lake bottom was established. These areas, with steep slopes and/or materials prone to fluidization below, could be destabilized during a high magnitude earthquake, causingan extraordinary wave that would affect vulnerable coasts. The aim of this work is to present preliminary results of a landslide susceptibility study on costal areas of Nahuel Huapi lake, particularly at the urbanized coasts of S. C. de Bariloche, Dina Huapi and adjacent zones. Subaqueous morphology along a 9-km coast from Playa Bonita to the vicinity of Nireco stream outlet (S. C. de Bariloche) was studied. Digital elevation models (DEMs), slope maps, bathymetric maps and bathymetric profiles were performed. Stream outlets were located and mapped.Steep slopes, subaqueous landslides, debris flow channels and probable mass- wasting deposits were found. The general areas affected by mass-wasting events were identified and related to slope ranges. In order to verify the hypothesis about the presence of potentially unstable areas in this portion of the lake bottom, next step will be collecting sedimentary cores at selected sites to determine the characteristics of the subjacent sediments.