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

482 PATA Days 2024 M E T H O D S The studied cores were retrieved in 2013 using a UWITEC hammer-driven piston coring system in two holes in the southeastern part of the deep basin (33°40’38.7’’S; 70°07’56.3’’W). Two parallel long cores were retrieved in each hole (LLE13-1A and LLE13-1B at a water depth of 34.5 m), reaching a maximum depth of ~6 m below the lake floor. All core sections were split, imaged with a line- scan camera and logged with a Geotek Multi-Sensor Core logger to obtain gamma-density, magnetic susceptibility (Bartington MS2E point sensor) and reflectance spectra of the visible light (Konica Minolta spectrophotometer). The cores were also scanned with a Siemens Somatom Definition Flash medical X-ray computed tomography (CT) scanner at the Ghent University Hospital, reaching a voxel size of 0.15x0.15x0.3 mm for the reconstructed volumes. To obtain geochemical composition, the composite core was scanned with an Itrax X-ray Fluorescence (XRF) scanner (CS-45; University of Innsbruck) at a resolution of 500 µm, and locally down to 200 µm. Finally, the grain-size distributions (between 0.04 µm and 2000 µm) were measured using a Malvern Mastersizer 2000 with Hydro 2000G module. The age-depthmodel is being producedby combining the published 210Pb and 137Cs dates (Van Daele et al., 2019) with new radiocarbon dates on terrestrial plant remains, and varve counting (combining line-scan, CT andXRF data). R E S U LT S A N D D I S C U S S I O N In general, Lo Encañado sediments retrieved by the piston coring have a similar facies to that of the early 20th century described by Van Daele et al. (2019). They consist of laminated muds (lamination thickness in the order of millimeters), interrupted by turbidites. Initial results indicate that – similarly to the last century – different types of turbidites can be distinguished, namely co-seismic turbidites (resulting from subaquatic slope failure), flood turbidites and post- seismic turbidites (resulting from slope failures in the catchment). Co-seismic turbidites are brownish in color and sandy at their base. At least 10 turbidites have the appearance of being-co-seismically triggered, two of which related to underlying mass-transport deposits few decimeters thick. The flood and post-seismic turbidites are fine-grained and distinctive for their blue color, which Van Daele et al. (2019) attributed to erosion of freshly exposed, catchment’s unweathered bedrock. The post-seismic turbidites occur right above co-seismic turbidites on a few occasions in the new record, indicating a potential intraplate source for the earthquakes causing these deposits. At a depth of ~4-4.5 m, a series of these turbidites seem unrelated to co-seismic deposits and thus may represent large-scale terrestrial mass- wasting that was unrelated to seismic shaking. However, the interpretation of these deposits requires further analysis Obtaining a trustworthy age model for the Laguna Lo Encañado record is challenging. Radiocarbon dates of fine organic remains provide ages of several thousand years old, which are inconsistent with the dating of the upper meter dated by radionuclides and varve counting (Van Daele et al., 2019), and with radiocarbon dates of few large plant remains. Indeed, calibrating the radiocarbon date of a large plant remain at the base of the record provides a calendar age of 1328 to 1446 CE. The Lo Encañado record thus covers the entire historical period and probably slightly beyond. However, precise radiocarbon dating of the record’s intermediate part is hard due to the radiocarbon plateau covering the historical time. We are therefore counting laminations, which have been shown to be clastic varves at least in the first half of the 20th century (Van Daele et al., 2019). Our preliminary counting on CT scan data indicates that these laminations are indeed varves, as they are broadly consistent with the basal radiocarbon age. The recently obtained XRF data will be used to finalize the varve chronology. Based on the current age constraints and varve chronology, we tentatively match the most prominent co-seismic turbidites in the record to the 1851, 1822, 1751, 1730, 1647 and 1580 historical earthquakes (Beck et al., 1998; Cisternas, 2012; Cisternas et al., 2012), as well as some additional (smaller?) earthquakes. We further have indications of two intraplate earthquakes with a similar imprint to that of the 1945 intraslab and 1958 crustal earthquakes during the century preceding the historical record beginning in 1541 CE, and a large megathrust earthquake around 1400-1450 CE. C O N C L U S I O N S Our analysis of the upper 6 m of the Laguna Lo Encañado sedimentary infill confirms that this lake likely recorded all major known historical earthquakes that hit Santiago de Chile, and a few more. While the presented record is already valuable on its own, given the lake was dammed about 4.5 kyrs ago (Antinao and Gosse, 2009), it may contain a much longer record. If so, this record would provide the seismic recurrence and a better assessment of the hazard to which the country’s capital is subjected. A C K N O W L E D G E M E N T S The presented cores were obtained by the HOLOCHIL project (CGL2012-32501), funded by the Spanish Ministry of Science. The research is further funded by Ghent University BOF grant BOF/ STA/202009/028.