QUANTITATIVE PALEOSEISMOLOGY FROM CHILEAN LAKES: A SOUTH AMERICAN PERSPECTIVE (Invited Presentation)

Even though in the last decade considerable progress has been made in the field of lacustrine paleoseismology, several challenges remain. For example, (i) how can we differentiate paleo-earthquakes hosted by different fault systems, such as the megathrust versus intraplate faults? And (ii) how can we quantify prehistoric earthquake magnitudes, and determine their rupture locations and extent? Lakes on transects along the Chilean subduction zone that have experienced major historical earthquakes provide insights in the way ahead. Detailed analysis of turbidites triggered by the 1960 and 2010 Chilean earthquakes in south-central Chilean piedmont lakes, revealed that most of the turbidites are formed by remobilization of only a thin veneer (~5 cm) of slope sediments and do not result from disintegration of subaquatic landslides. As a result, the turbidites’ spatial extent and thickness are related to local macroseismic intensities, and a transect of such records allows to estimate magnitudes and rupture extent of past subduction earthquakes in south-central Chile. Also for other types of lakes a positive relationship between turbidite occurrence and macroseismic intensity exists, as a compilation from 17 Chilean lakes shows a linear relationship between both variables. However, when comparing earthquakes from different source fault systems, such as megathrust, intraslab and crustal earthquakes, turbidite-shaking relationships become more complex. In Laguna Lo Encañado (central Chilean Andes) the prolonged shaking of megathrust earthquakes results in more voluminous co-seismic turbidites than those triggered by intraplate earthquakes. On the other hand, the higher source frequency spectrum of intraplate earthquakes causes preferential failure of onshore rocky slopes, as the high frequency waves are less attenuated in rock than in soft sediment. Post-seismic turbidites resulting from these rockslides is – in this setting – thus indicative of an intraplate earthquake.