Distinguishing Intraplate from Megathrust Earthquakes Using Lacustrine Turbidites

Subduction zone seismicity arises from megathrust, crustal and intraslab earthquakes, and understanding the recurrence patterns of each type is crucial for hazard assessments. Lake sediments can record earthquakes from all three seismogenic sources and thereby increase our understanding of potential temporal relations among them. Here we study the turbidite record of Lo Encañado, an Andean lake located in central Chile, just 40 km east of the country´s capital. We show that Lo Encañado turbidites can be attributed to (i) subaquatic slope failure by earthquake shaking (co-seismic), (ii) floods or human impact and (iii) post-seismic catchment response. All historical earthquakes causing shaking intensities >VI have triggered co-seismic turbidites. While megathrust earthquakes triggered the thickest co-seismic turbidites, only the intraplate (intraslab and crustal) earthquakes triggered subaerial slope failures followed by post-seismic turbidites. We argue that this contrasting result is due to different spectra of seismic waves from these earthquake sources: higher-frequency accelerations from intraplate earthquakes are hardly attenuated in rocks around the lake, whereas lower-frequency accelerations from megathrust earthquakes are amplified in soft lake sediment. We test our findings by comparing acceleration response spectra of recent intraslab and megathrust earthquakes along a longitudinal profile, and results suggest that Andean lakes are ideally located to distinguish earthquake sources.