A 600-year-long stratigraphic record of tsunamis in south-central Chile

The stratigraphy within coastal river valleys in south-central Chile clarifies and extends the region's history of large, earthquakes and accompanying tsunamis. Our site at Quidico (38.1 degrees S, 73.3 degrees W) is located in an overlap zone between ruptures of magnitude 8-9 earthquakes in 1960 and 2010, and, therefore, records tsunamis originating from subduction-zone ruptures north and south of the city of Concepcion. Hand-dug pits and cores in a 3-m-thick sequence of freshwater peat in an abandoned meander (a little-examined depositional environment for tsunami deposits) and exposures along the Quidico River show five sand beds that extend as much as 1.2 km inland. Evidence for deposition of the beds by tsunamis includes tabular sand beds that are laterally extensive (>100 m), well sorted, fine upward, have sharp lower contacts, and contain diatom assemblages dominated by brackish and marine taxa. Using eyewitness accounts of tsunami inundation, Cs-137 analyses, and C-14 dating, we matched the upper four sand beds with historical tsunamis in 2010, 1960, 1835, and 1751. The oldest prehistoric bed dates to 1445-1490 CE and correlates with lacustrine and coastal records of similar-aged earthquakes and tsunamis in south-central Chile.