Abstract

A cascading rainfall-landslide-tsunami event occurred on June 29th, 2022, in Todos los Santos Lake, located in southern Chile, affecting the tourist town of Petrohue. The event took place after several days of heavy rain during an extratropical cyclone. Important data were collected during a field survey, including hillslope 3D scans, lake-river bathymetry, orthomosaic photos, and an assessment of damage to public infrastructure. The analysis showed that the landslide had an estimated length, width, and depth of 175 m, 40 m, and 1.5 m, respectively, which resulted in a total volume of 10,500 m(3). The underwater runout distance of the landslide was estimated at 40 m, with a final water depth of 12 m. The initial tsunami wave was observed to be similar to 1 m, and since the distance from the landslide to the town was similar to 500 m, an arrival time of similar to 1 min was observed. Despite the small tsunami amplitudes, the pedestrian bridge of the floating pontoon collapsed due to the flow current and vertical oscillations. The results of the numerical simulation of the tsunami supported the observed data. They showed that the impact of the tsunami was only in the near field and was influenced by the bathymetry, such that refraction and edge waves were observed. The landslide occurred in an area where previous debris flows took place in 2013 and 2015. The main finding of the present research is that the occurrences of this and previous landslides were controlled by the presence of the Liquine-Ofqui fault zone, which generates broad areas of structural damage, with mechanical and chemical weathering significantly reducing rock strength. These observations provide a warning regarding the susceptibility of similar regions to other trigger events such as earthquakes and rainfall. This recent landslide highlights the need for a more comprehensive hazard assessment, for which probabilistic analysis could be focused on large active strike-slip fault systems. It also highlights the importance of community awareness, particularly in areas where tourism and real estate speculation have significantly increased urban development.