Remobilized tephra in lacustrine deltas as a potential factor in landslide susceptibility evaluation: two deltas from northern patagonia affected by the 2011 Cordón Caulle eruption

BEIGT, DÉBORA; VILLAROSA, GUSTAVO; OUTES, VALERIA; GÓMEZ, EDUARDO A.

PUERTO VARAS

Congreso; Cities on Volcanoes 9 ?Understanding volcanoes and society: the key for risk mitigation?; 2016

SERNAGEOMIN

Pireco and Totoral (PT) deltas are located at thenorthwestern coast of Nahuel Huapi lake (Northern Patagonia) at 40km SE fromthe Cordón Caulle (CC) 2011 eruption vent and 10km NW from the coastal city of Villa La Angostura. A 50-15-cm tephralayer (predominantly lapilli and coarse ash) was deposited over the PTwatershed during the CC 2011 eruption. No attention has yet been paid to thepotential impacts of the accumulation and remobilization of these tephradeposits over the watershed and its further sedimentation on the lacustrine deltas,where a significant deltaic plain progradation was observed soon after thetephra deposition. Slope instability at the lake floor is a concern for coastalpopulations due to previous tsunamigenic processes recorded in the lake (as the1960 subaqueous landslide-induced tsunami that affected Bariloche city). Thisstudy involved (1) a GIS-based volume estimation of the CC 2011 tephradeposited and remobilized (2012) over the PT watershed, (2) an analysis ofsedimentation rates at the deltas during the eruption, and (3) a study of thedelta fronts morphology and evolution from 2011 to 2014. A tephra volume of ~57x 106m3 was deposited over the watershed in 2011. By the beginning of 2012, thewatershed had suffered a net loss of ~3.3 x 106m3 of tephra, which was mainlyremobilized towards the deltas by surface runoff and fluvial transport. Intensesedimentation was recorded at the deltas from the beginning of the eruption(0.96cm/year in 2011-2012 at a distal delta environment). Consequently, thelake shoreline was displaced several tenths of meters offshore. Subaqueousmass-movements were identified at the steep delta fronts (14°-21°). One of them(composed mainly by tephra and organic material) occurred on February-May 2012,suggesting an increase of slope instability due to high sedimentation rates oflow-density (0.47g/cm3) and poorly-compacted pyroclastic materials.