Mapping and Reconstructing the Paleotsunami Record in Queule, South-Central Chile

Records of past tsunamis help constrain the long-term characteristics of megathrust earthquakes and elucidate the role of sea-level in the preservation of tsunami deposits. Near Queule, south-central Chile (39.3˚S, 73.2˚W), three sand layers interpreted as tsunami deposits are interbedded with dark, organic-rich, silt deposits along a tidal channel landward of a sand spit capped by eolian sand dunes. The uppermost sand layer is attributed to the tsunami from the Mw 9.5 1960 earthquake. The sand layer is widespread, tabular, oxidized, thins landward, and generally occurs at a depth of \u3c20 cm. The spatial distribution of the deposit corresponds closely with the extent of the 1960 tsunami sand on 1961 aerial photographs. We computed numerical simulations of the 1960 tsunami in Queule using the GeoClaw finite-slip hydrodynamic model based on three published earthquake sources. The simulations showed inundation up to 4km inland that overtopped the coastal dunes; agreeing with historical documentation and testimonies of 1960 tsunami survivors. Stratigraphically below the 1960 tsunami deposit are two tabular, landward-thinning sand layers with sharp lower contacts above silty organic-rich layers. Combined radiocarbon ages of seeds, charcoal and wood fragments found at the contacts with the underlying organic layers yielded ages of 5460-5320 and 5990-5910 cal. years BP. These sand layers have similar distribution patterns to the 1960 sand layer, but are finer grained, thinner and less oxidized. Below this sequence of interbedded tsunami sands and silty organic layers there is an abrupt contact underlain by a distinct sequence of four inorganic silt layers alternating with dark brown organic-rich silts, which are older than 6280-6110 cal. years BP. The deepest studied layer is a thick, fine, gray sand. We interpret the gray sand as a submarine environment during the mid-Holocene sea-level high stand, and the alternating inorganic and organic silts as tidal to shallow sub-tidal environments, possibly indicating co-seismic land-level changes. Gradual sea level fall after the deposition of the two paleotsunami sand deposits that changed the geomorphology of the coast and limited the accommodation space necessary to preserve additional overwash sediments could explain the 5000-year hiatus with no evidence of earthquakes or tsunamis. Further research will offer possible explanations for the exception of the 1960 tsunami in breaking this long-term pattern

The giant 1960 tsunami in the context of a 6000-year record of paleotsunamis and coastal evolution in south-central Chile

The tsunami associated with the giant 9.5 Mw 1960 Chile earthquake deposited an extensive sand layer above organic-rich soils near Queule (39.3°S, 73.2°W), south-central Chile. Using the 1960 tsunami deposits, together with eye-witness observations and numerical simulations of tsunami inundation, we tested the tsunami inundation sensitivity of the site to different earthquake slip distributions. Stratigraphically below the 1960 deposit are two additional widespread sand layers interpreted as tsunami deposits with maximum ages of 4960–4520 and 5930–5740 cal BP. This \u3e4500-year gap of tsunami deposits preserved in the stratigraphic record is inconsistent with written and geological records of large tsunamis in south-central Chile in 1575, 1837, and possibly 1737. We explain this discrepancy by: (1) poor preservation of tsunami deposits due to reduced accommodation space from relative sea-level fall during the late Holocene; (2) recently evolved coastal geomorphology that increased sediment availability for tsunami deposit formation in 1960; and/or (3) the possibility that the 1960 tsunami was significantly larger at this particular location than other tsunamis in the past \u3e4500 years. Our research illustrates the complexities of reconstructing a complete stratigraphic record of past tsunamis from a single site for tsunami hazard assessment