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

Fig. 3: Overview of some of the preliminary results obtained in Lago Rupanco. A) seismic profile rup02_6 obtained in the southern subbasin (see Fig. 2 for location) with projection of the length of short core 1004-1 and the 12 m long core obtained in site 2. B) Histogram-equalized linescan image (left) and CT visualization (right) of two of the short cores, 1004-16 obtained in the northern subbasin and 1004-1, obtained in the southern subbasin (see Fig. 2 for location), including indication of the presumed historic 1960, 1837, 1737 and 1575 turbidites (dark blue), the first two prehistoric seismo-turbidites (light blue), as well as the 1907 and Mil Hojas tephras (red) and radiocarbon sample locations. C) Results of geochemical analysis of the Mil Hojas tephra sampled in the short cores (red diamonds), and comparison to its composition (blue cross) as well as that of the (similar in composition) Puyehue-Cordón Caulle (PCC) 2 tephra (green cross) known from literature (Fontijn et al., 2016), highlighting the correlation confidence level. R E S U LT S & D I S C U S S I O N Seismic stratigraphic analysis shows that the sedimentary infill of Lago Rupanco contains several continuous strong reflectors that can be traced at least throughout each of the lake’s subbasins. Moreover, the deepest parts of each subbasin show clear ponding geometries and onlap terminations at the basin edges, underscoring the likely presence of turbidites (Fig. 3A). Indeed, the short cores in each subbasin reveal turbidites that can be attributed to all main historical megathrust earthquakes along the Valdivia segment (Fig. 3B). This includes the 1960 earthquake as well as the 1575 event, which is deemed similar in rupture extent (Wils et al., 2020). Also the smaller 1837 and 1737 earthquakes (Cisternas et al., 2017) appear to have caused remobilization of hemipelagic slope material in the lake. These age correlations are supported by a tentative event-free sedimentation rate of ~0.4 mm/yr in the northern subbasin and ~0.5 mm/yr in the southern subbasin derived from the geochemically confirmed presence of tephras related to the Riñinahue eruption from 1907 (Fontijn et al., 2014; Van Daele et al., 2015) as well as the Mil Hojas Puyehue-Cordón Caulle eruption (~1200 CE, Alloway et al. (2021)) (Fig. 3C). It is striking that the turbidite thickness of the 1960 and 1575 appears very similar, while the 1837 and 1737 earthquakes, which were considerably smaller in magnitude and rupture extent and produced less intense shaking in the Rupanco area (Lomnitz, 2004), resulted in much thinner turbidites that are not always clearly identifiable in all cores of each subbasin (e.g., Fig. 3B). This shows that the cumulative turbidite thickness (a measure for turbidite volume) per subbasin is a good (relative) measure of local ground shaking intensity. Moernaut et al. (2014) already demonstrated that this trend is valid for several of the piedmont lakes in south-central Chile. Moreover, they demonstrated that cumulative turbidite thickness even scales linearly with increasing intensity. Keeping this in mind, the over 20 thick turbidites that are identifiable on CT scans of the two long cores (some of which were already identified in the short cores, Fig. 3B) likely represent the local 1960-like shaking history of the last 6-8,000 years when assuming a constant sedimentation rate. The ~300 years recurrence time that was identified in the last 2,000 years for 1960-like earthquakes in the region can thus tentatively be considered stable.