Philipp Kempf, Jasper Moernaut, Maarten Van Daele, Mario Pino, Roberto Urrutia, Marc De Batist

In CE 1960, Lake Cucao on Chiloé Island in south central Chile was inundated by the tsunami of the Great Chilean Earthquake (Mw 9.5). The area of what is now the lake basin has been submerged since the end of the rapid postglacial sea-level rise and has recorded tsunami inundations in its sediment record since then. This study reconstructs the tsunami history of Lake Cucao. Reflection-seismic profiles and side scan sonar data of the lake reveal a tidal delta with a crosscutting channel, which controls the sedimentary environment in the coast-facing part of Lake Cucao. The convergent pattern of seismic reflections near this channel indicates that tidal currents were active in the lake at least episodically since the formation of a major unconformity with strong reflection amplitude, which records the onset of lacustrine sedimentation. A radiocarbon date at the base of one of the 21 collected sediment cores dates this reflector to ~3800 years BP. Little net vertical displacement (≤ 2 m) in combination with an outlet river channel that can act as a pathway for sediment transport appears to have maintained the sensitivity of Lake Cucao to record tsunami inundation. The sedimentary record contains 15 clastic layers which are interpreted as tsunami deposits. The confidence level on the tsunami interpretation depends on five site-specific criteria, which are: (i) high magnetic susceptibility of the sediment indicating high clastic content, (ii) cross core correlation indicating widespread deposition, (iii) acoustic reflector correlation to the sedimentary record (also indicating widespread deposition), (iv) presence of mud clasts, and (v) age correlation to known paleotsunamis in the area. In this way, eight clastic layers are interpreted as tsunami deposits with a high confidence level, five with a medium confidence level and two with a relatively low confidence level. This study adds a long paleotsunami record on a coastline where extreme tsunamis occur frequently and where long (>2000 years) paleotsunami records are still sparse.