2000-YEAR RECORD OF EIGHT TSUNAMIS AND ASSOCIATED SEISMIC LAND-LEVEL CHANGE, SOUTH-CENTRAL CHILE

Written records of large earthquakes on the subduction zone off south-central Chile are restricted to the last ~500 years. To better understand the long-term spatial variability and segmentation of prehistoric ruptures, we examine coastal sedimentary sequences that preserve sand beds related to tsunami inundation and distinctive, sharp lithologic contacts that signal sudden coseismic vertical deformation. Results of stratigraphic, sedimentologic, and diatom analyses on four riverbank sections from the Tirúa (38.3° S) and Quidico (38.1°S) rivers are consistent with historical accounts of coseismic land-level change and tsunami inundation during five historical earthquakes in AD 2010, 1960, 1835, 1751, and 1575 and reveal three prehistoric earthquakes and accompanying tsunamis in AD 1457-1575, 1443-1547, and 256-461. Deeper, landward slip associated with Maule segment earthquakes results in uplift at our sites (e.g., 2010, 1835, and 1751), while shallower, seaward slip associated with Valdivia segment earthquakes results in no deformation (e.g., 1960) or subsidence (e.g., 1575). Based on the deformation patterns associated with historical earthquakes, we infer that the prehistoric uplift at our sites in AD 1457-1575 and AD 256-461 is the result of deep landward slip, whereas subsidence in AD 1443-1547 is the result of shallower, seaward slip. Statistical analyses comparing sediment characteristics of the 1960 and 2010 tsunami-deposited beds to the characteristics of the previous six sand beds at our sites indicate that the older beds have similar sediment sources and structure to the modern analogues. Our diatom-based reconstructions of coseismic deformation and studies of tsunami deposit sedimentology help address questions about the long-term behavior of the south-central Chilean subduction zone; in particular, they show complex vertical records can be expected at sites where rupture patches abut and overlap.