MAPPING AND RECONSTRUCTING THE PALEOTSUNAMI RECORD IN A NEW SITE IN QUEULE, SOUTH-CENTRAL CHILE

Records of past tsunamis are a critical component of understanding the long-term behavior of tsunamigenic megathrust earthquake ruptures. We extended the tsunami record in the area of the largest earthquake recorded with modern instrumentation: the Mw 9.5 1960 earthquake in southern Chile. Near the town of Queule (39.3˚S, 73.2˚W), three sand layers interpreted as tsunami deposits are interbedded with dark, organic-rich soils in the marsh and tidal sediments landward of a sand spit capped by eolian sand dunes. The uppermost sand sheet is attributed to the 1960 tsunami; it is widespread, laterally extensive, tabular and generally occurs at a depth of <20 cm. The spatial distribution of this deposit in the field corresponds closely with the extent of the 1960 tsunami sand mapped from 1961 aerial photographs. Stratigraphically below the 1960 sand deposit are two candidate paleotsunami sand deposits with a sharp lower contacts above organic-rich silt units. These older sand layers have similar distribution patterns to the 1960 sand layer, being tabular and thinning landward, but they are finer grained, thinner and less oxidized. Pending radiocarbon dating results will indicate whether these sand sheets were deposited by large historical tsunamis or older events. The ages will be compared with paleoseismic records from other sites in southern Chile to refine our knowledge of the latitudinal extent of those particular paleotsunamis.

We computed numerical simulations of the 1960 tsunami in Queule using the GeoClaw finite-slip hydrodynamic model based on published earthquake source parameters. The simulations showed extensive inland flooding up to 4km inland, overtopping the coastal dunes in all of our study sites. In conjunction with the forward model, we also used an inverse sediment transport model (TSUFLIND) to estimate tsunami flow speed along a transect perpendicular to the coastline. We compared the flow speeds computed from the 1960 tsunami deposit and the previous paleotsunami deposit, assuming similar coastal topography prior to both tsunamis. Ultimately, these results will be used to constrain the characteristics of the associated megathrust ruptures.