Reconstructing the impact in Northern Patagonia of ground shaking and Puyehue-Cordon-Caulle eruption associated with the May 1960 subduction earthquake (Mw 9.5) from lake sediments in Chile and Argentina.

CHAPRON, E.; ARISTEGUI, D.; MULSOW, S.; VILLAROSA, G.; GOMEZ, E. A.; JUVIGNIÉ, E.; PINO, M.; OUTES, V.

Barcelona

Congreso; 4th International Limnogeology Congress (ILIC2007); 2007

Contrasted lacustrine sedimentary basins at both sides of the Andes in Northern Patagonia have been investigated in order to better understand the sedimentary record of major subduction earthquakes from Southern Chile. The case study of 21-22 May 1960 earthquake and induced Puyehue-Cordon-Caulle eruption is here used to establish regional correlations and to evaluate the influence of earthquake-induced landslides in the generation of catastrophic flooding events and violent waves in lakes located at the piedmont. This study combined high-resolution seismic reflection profiles (3.5 kHz), and well-dated short sediment cores from lakes located close to the Puyehue-Cordon-Caulle volcanic complex in Chile (Lago Puyehue) and in Argentina (proglacial lakes Nahuel Huapi and Frias). Radionuclide dating (137Cs) and tephrostratigraphic analysis combined with a multi-proxy study of lacustrine sedimentary facies (grain size, magnetic susceptibility, gamma density, X-ray radiography, total organic carbon) allows the development of detailed age-depth models and the correlation of striking sedimentary events with the impact of May 1960 earthquake. These interpretations are further supported by geomorphic and tephrostratigraphic studies in the catchment areas and by historical chronicles. In Lago Puyehue, the identification of tephra layers (Rininahue 1907, Cordon Caulle 1921-22 and 1960) highlights the development of erosive sedimentary events resulting from ground shaking during the earthquake and from the outburst of earthquake-induced landslide dams in the course of the main tributary of the lake after the end of the eruption. These outbursts triggered a large hyperpycnal flood event of ca. 3 x 106 m3, as identified both on cores and seismic profiles in the basin facing the Golgol delta. More than 150 km from the main epicentre, ground accelerations during the 22nd of May 1960 triggered a large subaqueous landslide offshore San Carlos de Bariloche in Lago Nahuel Huapi. This mass wasting deposit is well identified on seismic profiles and evolved into a mega turbidite in the deep basin. On core, the 1960 slide deposit is cap by a thin tephra layer bearing the typical signature of the 1960 Cordon-Caulle event. This earthquake-induced slide was both favoured by a specific tectonic context and by the construction of a new harbour several month before the event. The subaqueous slide not only destroyed the harbour but also triggered a series of well-documented destructive waves offshore Bariloche. In the narrow proglacial lake Frias, the 1960 event formed an unusual organic rich layer in the deep basin intercalated within glacial varves and several tephra layers.