Proceedings of the 12th International INQUA meeting on paleoseismology, active tectonic and archaeoseismology

I N T R O D U C T I O N One of the main goals in paleotsunami research is to extend the historical record to reach a better understanding of recurrence variability and thus, improve hazard assessments (Kempf et al., 2020). In relation to other types of coastal archives (e.g., marshes, swales or lowlands), coastal lakes have the advantage of having (i) relatively stable sedimentation rate, improving absolute and relative age estimates; (ii) a non-restrictive accommodation space; (iii) limited post-depositional processes, giving them high preservation potential; and (iv) being less affected by coastal elevation shifts, creating relatively stable sensitivity. However, the barrier that separates these coastal lakes from the ocean needs to have specific geomorphological characteristics to allow tsunamis to reach lake systems while, at the same time, not being affected by other unusual wave events (storms, strong swells) (Kempf et al., 2015). In this study, we present two lakes with organic-rich sediment that are located in the central part of Chiloé Island, south-central Chile (42.7°S), and correlate their sedimentary records with that of Lakes Huelde and Cucao, which have been extensively studied in terms of their tsunami history (Kempf et al., 2015; 2017; 2020). Lake Huelde (11 m depth) and Lake Cucao (25 m depth) are coastal lakes that are approximately 2 km from each other and have different connectivity to the Pacific Ocean – an outflowing river and tidal channel, respectively. They show a continuous record of past tsunamis identifiable by reflection seismic, geochemical and sedimentological data. Additionally, Huillinco Lake (47 m depth and c.11 km to the east of the Pacific Ocean) is connected to Cucao Lake by a wide and over- deep channel with an average depth of approximately 13 m. Huillinco Lake is a meromictic lake that experiences strongwater column stratificationwith freshwater (upper c.7 m) overlying a highly saline and anoxic body of water below c.20 m. Villalobos et al., (2003) hypothesized that this stratification results from seawater inflow during the tsunami produced by the 1960 CE earthquake. Finally, Natri Lake (58 m depth) is c.25 km from the coast line and c.13 km southeast of the Cucao-Huillinco system, at an altitude of 39 m with no marine influence. Chiloé Island, and thus also the considered lakes, is located in the central part of the Valdivia seismotectonic segment (VS), a c.1000 km subduction zone area that can be subdivided into seven sub- segments based on multivariate analysis of geophysical and geodetic data (Molina et al., 2021). This area has c.500 years of written rupture history that began at the time of the Spanish invasion, during which four major megathrust events were documented: 1575, 1737, 1837 and the giant event of 1960 CE. This information allows us to understand and calibrate the upper event deposits contained in sedimentary record. In contrast, information related to paleo-earthquakes in the pre- written history is scarce and spatially distant between sub-segments, hampering the linking of records in the northern and southern parts of the VS. This certainly impacts the certainty and fine-tuning of the regional earthquake history toward the past (Wils et al., 2020). Our aim is to combine published and new sedimentary records obtained from short and long cores from coastal and inland lakes, allowing us to link paleo- rupture, paleo- earthquake andpaleo-tsunami data to fill the knowledge gap about paleoseismicity in the central part of the VS. M E T H O D S With the aim of better characterizing the lake floor morphology and sediment infill, we used a total of 150 kilometres of high-resolution sub-bottom profiles in Lakes Natri, Huillinco and Cucao, as well as the channel that connects the latter two (Fig. 1C). This was made with a GEOPULSE pinger (frequency c.3.5 kHz) with a vertical resolution of 10 to 20 centimetres. Seismic profiles were processed and analysedwith the IHSMarkit Kingdom Suite software. This allowed the selection the location of 36 gravity cores and, for better preservation of the uppermost layers, 10 surface cores with a diameter of 90 mm. Meticulous ongoing analyses of these data from Huillinco Lake allowed us to determine the ideal location for two long piston cores (Fig. 2A) using a UWITEC coring platform (UHP 30450), which covers the upper 11 and 15 m of sediments. To obtain a detailed understanding of the internal structure and properties of the samples, all cores were CT scanned at the Ghent UniversityHospital (Siemens;