Multi-lake turbidite records of Chilean megathrust earthquakes: Refining the use of turbidite volume as paleoseismic proxy

Seismic shaking can remobilise slope sediments in lakes, producing mass wasting deposits as evidence of past earthquakes. Previous studies suggest a link between turbidite volume (i.e., thickness and extent) and shaking intensity as a potential tool for quantitative paleoseismology. In this study we test this relationship using sediment cores from seven lakes along the Valdivia Segment of the Chilean subduction zone, where variable megathrust rupture extents have been inferred. Sediment core correlation and geochronological analyses — including radionuclide dating, tephrochronology, and varve counting — reveal synchronous, multi-basin turbidites, each of which can be linked to a known, historical or prehistoric earthquake during the past ∼1000 years. We find that the turbidite volume scales with the rupture extent of historical earthquakes and show that surficial sediment remobilisation is the primary underlying mechanism in these lakes. However, turbidite thickness measurements and slope core analysis suggest that the varying lithology on slope sequences (e.g. volcanic deposits) modulates the remobilisation depth, reducing turbidite volume independently of shaking strength. In addition, the remobilisation process is strongly controlled by slope angle. For the investigated lakes, confined basins surrounded by slopes of approximately 5° are the ideal natural seismometers, while sediments on steeper slopes (>11°) are likely completely eroded during large earthquakes and not fully recharged between events. Our findings support and refine the application of coseismic turbidite volume as a proxy for shaking strength but highlight the necessity of a better understanding of the surficial remobilisation process for quantitative paleoseismic interpretations based on turbidite thickness.